**Airborne Magnetic Surveys to Investigate High Temperature Geothermal Reservoirs**

## Supri Soengkono

[35] Goyal K P, Miller C W, Lippmann M J. Effect of measured wellhead parameters and well scaling on the computed downhole conditions in Cerro Prieto wells. In: Pro‐ ceedings of the Sixth Workshop on Geothermal Reservoir Engineering; SGP-TR-50,

[36] Truesdell A H, Fournier R O. Calculation of deep reservoir temperatures from chem‐ istry of boiling hot springs of mixed origin. In: Proceedings of the 2nd United Na‐ tions Symposium on the Development and use of geothermal resources, San

[37] Bethke C M. *Geochemical Reaction Modeling. Concepts and Appli*cations. Oxford Univer‐

Stanford University, CA; 1980. p. 130-138.

Francisco, Abstract, Vol. III; 1975. p. 25.

sity Press; 1996. 397 p.

112 Advances in Geothermal Energy

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/61651

#### **Abstract**

Airborne magnetic survey is an effective geophysical exploration method in terms of cov‐ erage, resolution and cost, particularly for area with restricted or difficult ground access. Research studies in New Zealand have shown airborne magnetic surveys can indicate the regions of high reservoir permeability and thermal up-flow of active geothermal systems. However, the method has not been extensively used in the geothermal investigations, probably because the interpretation of airborne magnetic data has so far been seen as dif‐ ficult and requires a complex quantitative 3D modelling of subsurface magnetisation.

This chapter introduces a new approach to use airborne magnetic survey to investigate high temperature geothermal resources without the need of 3D magnetic modelling. This new approach takes advantage of data processing packages that during the last few years have become accessible through the internet. A simple but comprehensive explanation is given on the physics background of the airborne magnetic surveys. Examples are provid‐ ed from interpretations of real airborne magnetic data from the North Island of New Zea‐ land and the Java Island of Indonesia. This chapter is aimed to provide the readers a sufficient level of knowledge and confidence to organise and/or run investigation of high temperature geothermal reservoirs using airborne magnetic surveys.

**Keywords:** Total force magnetic anomalies, magnetisation of volcanic rocks, hydrother‐ mal alteration and demagnetisation, extent of geothermal reservoirs, reversely magne‐ tised rocks

## **1. Introduction**

Airborne magnetic survey involves measurements of the geomagnetic field (the magnetic field of the earth) from the air using magnetometer installed in an aircraft. The purpose is to detect small changes in the geomagnetic field related to differences in rock magnetisation beneath

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

the survey area. The airborne magnetic survey has been extensively applied in the mineral explorations and as additional tool to support geological mapping projects. It is a very effective geophysical exploration technique in terms of coverage, resolution and cost, particularly in area with difficult or restricted ground access. The airborne magnetic surveys over the Taupo Volcanic Zone (TVZ) in New Zealand, which started with a regional survey dating back more than 60 years ago in the early 1950s [9] and was followed in much later date by a variety of more detailed surveys between 1984 and 2006 [24,14,31], have provided data sets that all are highly consistent to each other. The resolution of the dataset, as expected, depends on the flight line spacing and the survey altitude above the ground. But all the data sets revealed the same features of magnetic anomalies. This consistency clearly shows that airborne magnetic survey is a robust geophysical method. Over geothermal prospect regions worldwide, airborne magnetic data are often already available from some previously conducted surveys by mineral exploration companies searching for epithermal gold deposits or by the government institu‐ tions (Geological Surveys).

Hochstein and Soengkono [14] showed that careful interpretations and three dimensional (3D) quantitative modelling of airborne magnetic anomalies over many geothermal systems in the Taupo Volcanic Zone (TVZ) in New Zealand can provide information on the likely locations of high reservoir permeability and up-flow regions of active geothermal system. They also quoted examples that suggest this would also likely be true for geothermal systems in volcanic settings elsewhere outside New Zealand. However, the interest to use data from airborne magnetic surveys to investigate high temperature geothermal systems has been very slow to develop. A possible reason of this slow development is that the interpretation of magnetic data in geothermal investigations has been considered difficult and the 3D quantitative modelling of the magnetic data can be a complex and problematic task. The changing in geomagnetic inclination at different geographic latitudes causes different pattern of magnetic anomalies over areas which have the same geological structures and lithology but are located at different geographic regions. In addition, unlike the scalar parameter rock density that causes gravity anomalies, the rock magnetisation is a vector. Because of this, the pattern of the magnetic anomalies is complex and more difficult to interpret than the gravity anomalies over the same area. However, with the development of user friendly geophysical processing packages that, since mid-2000s, have become accessible on-line (worldwide), the complexity of magnetic anomalies can now be reduced.

This chapter introduces and explains a new approach to use airborne magnetic data for the investigations of high temperature geothermal resources hosted by volcanic rocks. This new approach is specifically formulated for this chapter based upon the author's experience during the last 30 years in the interpretation and 3D modelling of various airborne magnetic data. It has not been previously published in any papers listed in the reference list of this chapter (Section 7), nor anywhere else. This approach utilises some magnetic data processing techni‐ ques in the computer software that have now become accessible worldwide. The processing techniques are used to directly link the measured airborne magnetic anomalies to the causative source targets. The often complex and difficult 3D modelling of the anomalies would only need to be carried out when it is considered necessary at the final stage of the interpretation, when some further detailed aspects of the magnetic interpretation need to be pursued. The aim of this chapter is to equip readers with some knowledge and confidence to run the investigation of high temperature geothermal reservoirs in volcanic rocks using airborne magnetic data which are already available over target area from some previous surveys, or going to be collected by a new survey specifically aimed to explore the geothermal targets.

The SI unit of magnetic field strength is Tesla, or T (T=Weber/m2 =Vs/m2 ). The unit used for geomagnetic field strength measured during airborne magnetic survey is nT (1 nT = 10-9T), which is equal to the old unit gamma (γ) (1nT = 1γ).

## **2. The geomagnetic field**

the survey area. The airborne magnetic survey has been extensively applied in the mineral explorations and as additional tool to support geological mapping projects. It is a very effective geophysical exploration technique in terms of coverage, resolution and cost, particularly in area with difficult or restricted ground access. The airborne magnetic surveys over the Taupo Volcanic Zone (TVZ) in New Zealand, which started with a regional survey dating back more than 60 years ago in the early 1950s [9] and was followed in much later date by a variety of more detailed surveys between 1984 and 2006 [24,14,31], have provided data sets that all are highly consistent to each other. The resolution of the dataset, as expected, depends on the flight line spacing and the survey altitude above the ground. But all the data sets revealed the same features of magnetic anomalies. This consistency clearly shows that airborne magnetic survey is a robust geophysical method. Over geothermal prospect regions worldwide, airborne magnetic data are often already available from some previously conducted surveys by mineral exploration companies searching for epithermal gold deposits or by the government institu‐

Hochstein and Soengkono [14] showed that careful interpretations and three dimensional (3D) quantitative modelling of airborne magnetic anomalies over many geothermal systems in the Taupo Volcanic Zone (TVZ) in New Zealand can provide information on the likely locations of high reservoir permeability and up-flow regions of active geothermal system. They also quoted examples that suggest this would also likely be true for geothermal systems in volcanic settings elsewhere outside New Zealand. However, the interest to use data from airborne magnetic surveys to investigate high temperature geothermal systems has been very slow to develop. A possible reason of this slow development is that the interpretation of magnetic data in geothermal investigations has been considered difficult and the 3D quantitative modelling of the magnetic data can be a complex and problematic task. The changing in geomagnetic inclination at different geographic latitudes causes different pattern of magnetic anomalies over areas which have the same geological structures and lithology but are located at different geographic regions. In addition, unlike the scalar parameter rock density that causes gravity anomalies, the rock magnetisation is a vector. Because of this, the pattern of the magnetic anomalies is complex and more difficult to interpret than the gravity anomalies over the same area. However, with the development of user friendly geophysical processing packages that, since mid-2000s, have become accessible on-line (worldwide), the complexity of magnetic

This chapter introduces and explains a new approach to use airborne magnetic data for the investigations of high temperature geothermal resources hosted by volcanic rocks. This new approach is specifically formulated for this chapter based upon the author's experience during the last 30 years in the interpretation and 3D modelling of various airborne magnetic data. It has not been previously published in any papers listed in the reference list of this chapter (Section 7), nor anywhere else. This approach utilises some magnetic data processing techni‐ ques in the computer software that have now become accessible worldwide. The processing techniques are used to directly link the measured airborne magnetic anomalies to the causative source targets. The often complex and difficult 3D modelling of the anomalies would only need to be carried out when it is considered necessary at the final stage of the interpretation, when

tions (Geological Surveys).

114 Advances in Geothermal Energy

anomalies can now be reduced.

## **2.1. The normal geomagnetic field**

The normal (undisturbed) geomagnetic field **Bo** can be approached by the effects of a fictitious magnetic dipole at the centre of the earth, orientating at a small angle (about 10˚) to the axis of earth rotation. On the surface of the earth, the *inclination* of the normal geomagnetic field causes by such a dipole varies from +/-90˚ at the magnetic north and south poles, to 0˚ at the magnetic equator (see Figure 1). As the positions of the north and south magnetic (N' and S') and geographic (N and S) poles are not the same, there is also a horizontal *declination* between the magnetic north (the horizontal direction of the earth's magnetic field shown by a compass needle) and the geographic north. The magnetic inclination is considered positive downwards; it is negative in the southern hemisphere (such as in NZ, Australia and Africa). Since the fictitious dipole at the centre of the earth is only quasi static (it has a slow precession around the earth rotational axis), there is a small secular variation of the normal geomagnetic field.

Studies of *remanen*t magnetisation (the permanent magnetisation that is not induced by the present day geomagnetic field) of the rocks of different ages from around the earth show that the orientation of the fictitious dipole at the centre of the earth had flipped many times in the past, when the positions of N' and S' were interchanged (called the geomagnetic reversal). The last geomagnetic reversal occurred about 0.7 Myr ago [19].

The normal geomagnetic field of the earth ranges in strength from about 35,000 nT near the equator to about 60,000 nT near the north and south poles. For examples, in the North Island of New Zealand (about 38˚S latitude) it has the strength of about 54,000 nT whereas in the Java Island of Indonesia (about 7˚S latitude) the geomagnetic field strength is about 44,000 nT. At any point on the surface of the earth, the strength and direction (declination and inclination) of the normal geomagnetic field can be obtained from the internet (by searching for "geomag‐ netic field strength" to get to web sites to do the calculation online). The geomagnetic values are computed using a global model, defined by spherical harmonic coefficients synthesising the quasi static component as well as the secular variation of the earth's magnetic field, and is known as the International Geomagnetic Reference Field (IGRF). This global model is renewed every about 5 years by collaborative effort between magnetic field modellers and the institutes involved in collecting and disseminating magnetic field data from satellites and from observ‐

**Figure 1.** The normal geomagnetic field of the earth.

atories and surveys around the world. Note that the value of the magnetic inclination is not the same as the value of geographical latitude.

## **2.2. Diurnal variations**

The total geomagnetic field measured during a magnetic survey contains small magnetic fields related to local variation of rock magnetisation (this is the target of airborne magnetic survey) and time variant external components from outside the solid earth. The time variant external field includes small diurnal variations (range about 30 nT) of about 24 hours period which correlate with electrical currents in the ionosphere, and a larger transient and erratic disturb‐ ance (range up to 1000 nT) known as magnetic storms which correlate with sunspot activity. The effect of the time variant external fields has to be corrected from the measured airborne magnetic data. If magnetic storm is avoided when conducting the survey, the only significant time variant field affecting the measured geomagnetic field would be that causing the diurnal variations. Near the earth's surface, the magnetic field causing the diurnal variations has an almost constant strength and direction within a 75 km radius. For a magnetic survey it is therefore sufficient to monitor the diurnal variations at a base station within or near the survey area. Data provided by airborne magnetic survey companies contracted to carry out the survey would have been corrected for this diurnal variation.

### **2.3. Magnetic anomalies**

atories and surveys around the world. Note that the value of the magnetic inclination is not

The total geomagnetic field measured during a magnetic survey contains small magnetic fields related to local variation of rock magnetisation (this is the target of airborne magnetic survey) and time variant external components from outside the solid earth. The time variant external field includes small diurnal variations (range about 30 nT) of about 24 hours period which correlate with electrical currents in the ionosphere, and a larger transient and erratic disturb‐ ance (range up to 1000 nT) known as magnetic storms which correlate with sunspot activity. The effect of the time variant external fields has to be corrected from the measured airborne

the same as the value of geographical latitude.

**Figure 1.** The normal geomagnetic field of the earth.

**2.2. Diurnal variations**

116 Advances in Geothermal Energy

Figure 2 shows a schematic vector diagram of the relationship between total geomagnetic field **(Bobs**), the normal (undisturbed) geomagnetic field (**Bo**) and the total sum of local magnetic fields produced by variations of rock magnetisation (**∆B**), at two theoretical measurement points (a) and (b) having two different **∆B** vectors. The strength of total geomagnetic field vector (=**|Bobs|**) is the *only* parameter measured during airborne magnetic survey. As shown in Figure 2, this measured geomagnetic magnetic field **Bobs** vector is the result of vector operation **Bobs** = **Bo** + **∆B**. The sum magnetic field strength caused by anomalous bodies (|∆B|) is usually much smaller (less than 2%) compared to the strength of the normal geomagnetic field (**|Bo|**) (except at a ground location close to an outcrop of very highly magnetic rocks). Hence, the angle between **Bo** and **Bobs** is very small (less than 2˚).

The *total force magnetic anomaly* (termed in this chapter as ∆F) is defined by algebraic (scalar) subtraction of **|Bo|** (the strength "undisturbed" earth's magnetic field) from **|Bobs|**, that is: ∆F = **|Bobs|**-**|Bo|**. As **|Bo|** cannot (are not) directly measured, the value **|Bo|** it is usually taken from computation of the IGRF (see Section 2). The ∆F computed from the measured **|Bobs|** subtracted by the IGRF value contains all the magnetically anomalous mass beneath the measurement point down to very deep level (theoretically it is down to the depth of the fictitious magnetic dipole near the centre of the earth). To approximate ∆F affected only by magnetically anomalous mass beneath the "survey target", one can use the trend of **|Bobs|** across the survey area to determine **|Bo|.** No exact value of such survey target "depth" can be given, but the ∆F values obtained in this way are better for identifying and delineating the survey target. For this reason, computer software that can properly grid airborne magnetic data and compute the trend of gridded data is essential in the interpretation of airborne magnetic surveys. The computation of trends and other processing and plotting of the magnetic data presented in this chapter are all carried out using the computer software Oasis Montaj from the Geosoft Inc. There are some other software packages available in the market (can be purchased online) that can do the same processing and plotting.

If the airborne magnetic data that available for the investigation are already in the form of ∆F obtained by subtracting of the IGRF values (often this is the case with data received from the airborne magnetic survey contractor), removing the trend of such ∆F values over the survey area will provide new (corrected) ∆F values that would represent magnetically anomalous bodies no deeper than the "survey target".

**Figure 2.** Vector diagram of magnetic anomalies. In (a) **∆B** adds to **Bo** and we obtain positive total force magnetic anomaly ∆F. In (b) **∆B** opposes **Bo** and the total force magnetic anomaly is negative.

## **2.4. The patterns of a magnetic anomalies caused by magnetic dipole**

First order effects of observed magnetic anomalies can often be interpreted by simple dipole fields (such field are set up by geological bodies that can be approximated by a magnetic dipole or combination of dipoles). This is especially true for geological bodies that can be approxi‐ mated by a homogeneous sphere, since homogeneous sphere is magnetically equivalent to a single magnetic dipole placed at its centre.

**Figure 3.** Total force magnetic anomalies caused by a magnetic dipole (or a magnetic sphere) at different geomagnetic inclination (different geographic latitudes).

Figure 3 shows diagrams explaining the patterns of total force anomalies (∆F) caused by *positive* magnetic dipoles (orientated in *the same direction* as **Bo**) that are located in: (a) the northern hemisphere, (b) the southern hemisphere, (c) the magnetic *south* pole and (d) the magnetic

**Figure 2.** Vector diagram of magnetic anomalies. In (a) **∆B** adds to **Bo** and we obtain positive total force magnetic

anomaly ∆F. In (b) **∆B** opposes **Bo** and the total force magnetic anomaly is negative.

118 Advances in Geothermal Energy

equator. For the magnetic *north* pole, by using a diagram equivalent to Figure 3 (c) but with a vertical downward direction of the magnetic dipole and **Bo**, it can easily be shown that the pattern of the created total force anomaly (∆F) is exactly the same as that shown in Figure 3 (c). The same can be made also for *negative* magnetic dipoles (orientated *opposite in direction* to **Bo**), which will show the patterns of total force anomalies in Figure 3, but with ∆F values having the opposite sign. Figure 3 also shows that total force magnetic anomalies ∆F created by the *positive* magnetic dipoles at locations away from the magnetic equator are dominantly *positive*. At the south (or north) magnetic pole (Figure 3(c)), the centre of positive anomalies is located directly above the dipole. At the southern hemisphere (Figure 3(a)) the centre of the positive anomaly is shifted to the north of the magnetic dipole and at the northern hemisphere (Figure 3(b)) it is shifted to the south of the magnetic dipole. However, the opposite occurs at the locations close (within about ±10˚) to the magnetic equator. Here, the magnetic anomalies created by a *positive* magnetic dipole become dominantly *negative*! This "counter intuitive" phenomenon can result in a serious error in the interpretations of magnetic anomalies at locations within about ±10˚ magnetic latitude as at some parts of Africa, South America and the southern part of the Philippines.

## **3. Interpretation of magnetic anomalies over geothermal areas**

## **3.1. Magnetic anomalies over the Taupo Volcanic Zone (TVZ) in New Zealand**

The Taupo Volcanic Zone (TVZ) is a region of Quaternary volcanic and geothermal activity in the central North Island of New Zealand. Almost all high temperature geothermal systems in New Zealand are located in this zone. As mentioned in the introduction, a regional airborne magnetic survey was conducted over the TVZ during early 1950s and it was followed much later on by more detailed surveys between 1984 and 2006. The geomagnetic field inclination in the TVZ is about -62.5˚. Because of this high inclination, the relationship between magnetic anomalies and geological features is easier to recognise than in the regions with lower magnetic inclination. The phenomenon that distinct magnetic anomalies are often associated with geothermal reservoirs in the TVZ has been recognised more than 70 years ago by Watson-Munro [35]. However, in the regional map drawn using the data from the early 1950s regional survey (for example, Hunt and Whiteford [16]) the relationship between some geothermal fields and the magnetic anomalies is not obvious and can just be barely seen because of wide flight line spacing (2.5 km) of the survey. The interest in using magnetic surveys as an exploration tool declined in New Zealand in the 1960s when electrical methods were found to be more effective in delineating the lateral extent of conductive reservoir rocks.

The interest was revived when the Geothermal Institute (University of Auckland, New Zealand) started new, lower-level airborne magnetic surveys in 1984. One of the first surveys led to the discovery of a distinct magnetic anomaly over the Mokai geothermal field [20] that was hardly recognisable in the low resolution early map from the early 1950s survey. Between 1984 and late 1999s, the Geothermal Institute expanded the survey to cover an area of about 3000 km2 , covering all geothermal fields in the TVZ. Quantitative interpretation of more than 10 geothermal prospects within the area were undertaken [20,14,12-13,26-27,22-23,28-29,21,25,15]. The quantitative 3D interpretations indicated that airborne magnetic survey can be a useful tool to identify zone of high permeability region of an active geothermal system.

## **3.2. Magnetisation volcanic rocks**

equator. For the magnetic *north* pole, by using a diagram equivalent to Figure 3 (c) but with a vertical downward direction of the magnetic dipole and **Bo**, it can easily be shown that the pattern of the created total force anomaly (∆F) is exactly the same as that shown in Figure 3 (c). The same can be made also for *negative* magnetic dipoles (orientated *opposite in direction* to **Bo**), which will show the patterns of total force anomalies in Figure 3, but with ∆F values having the opposite sign. Figure 3 also shows that total force magnetic anomalies ∆F created by the *positive* magnetic dipoles at locations away from the magnetic equator are dominantly *positive*. At the south (or north) magnetic pole (Figure 3(c)), the centre of positive anomalies is located directly above the dipole. At the southern hemisphere (Figure 3(a)) the centre of the positive anomaly is shifted to the north of the magnetic dipole and at the northern hemisphere (Figure 3(b)) it is shifted to the south of the magnetic dipole. However, the opposite occurs at the locations close (within about ±10˚) to the magnetic equator. Here, the magnetic anomalies created by a *positive* magnetic dipole become dominantly *negative*! This "counter intuitive" phenomenon can result in a serious error in the interpretations of magnetic anomalies at locations within about ±10˚ magnetic latitude as at some parts of Africa, South America and

**3. Interpretation of magnetic anomalies over geothermal areas**

**3.1. Magnetic anomalies over the Taupo Volcanic Zone (TVZ) in New Zealand**

be more effective in delineating the lateral extent of conductive reservoir rocks.

The interest was revived when the Geothermal Institute (University of Auckland, New Zealand) started new, lower-level airborne magnetic surveys in 1984. One of the first surveys led to the discovery of a distinct magnetic anomaly over the Mokai geothermal field [20] that was hardly recognisable in the low resolution early map from the early 1950s survey. Between 1984 and late 1999s, the Geothermal Institute expanded the survey to cover an area of about

, covering all geothermal fields in the TVZ. Quantitative interpretation of more than

The Taupo Volcanic Zone (TVZ) is a region of Quaternary volcanic and geothermal activity in the central North Island of New Zealand. Almost all high temperature geothermal systems in New Zealand are located in this zone. As mentioned in the introduction, a regional airborne magnetic survey was conducted over the TVZ during early 1950s and it was followed much later on by more detailed surveys between 1984 and 2006. The geomagnetic field inclination in the TVZ is about -62.5˚. Because of this high inclination, the relationship between magnetic anomalies and geological features is easier to recognise than in the regions with lower magnetic inclination. The phenomenon that distinct magnetic anomalies are often associated with geothermal reservoirs in the TVZ has been recognised more than 70 years ago by Watson-Munro [35]. However, in the regional map drawn using the data from the early 1950s regional survey (for example, Hunt and Whiteford [16]) the relationship between some geothermal fields and the magnetic anomalies is not obvious and can just be barely seen because of wide flight line spacing (2.5 km) of the survey. The interest in using magnetic surveys as an exploration tool declined in New Zealand in the 1960s when electrical methods were found to

the southern part of the Philippines.

120 Advances in Geothermal Energy

3000 km2

All volcanic rocks were magnetic after their eruption, as a result of their induced magnetisation **(mi** ) and remanent magnetisation (**mr**). Remanent magnetisation is the permanent magnetisa‐ tion of rock which was attained when the rock was formed (or deformed). The main type of remanent magnetisation in volcanic rocks such as lavas and ignimbrites is the thermo remanent magnetisation (TRM) which was attained when the rocks cooled down to below the Curie point of magnetite (about 580 ˚C). It has the same direction as the past **Bo** during the time of cooling. Induced magnetisation is given by the multiplication product of the geomagnetic field magnetising force **H** (A/m) and the magnetic susceptibility κ (a dimensionless parameter) which, in turn, is related mainly to the volume fraction of two primary magnetic minerals, magnetite and titanomagnetite. In the SI unit, the earth's magnetising force **H** can be deter‐ mined from the relationship **Bo** = µ H, where µ is the magnetic permeability of the medium. For non-magnetic medium (such as air) µ = 4 π x10-7 In the magnetic survey, the unit of **Bo** is nT (=10-9T). Thus, H (in A/m) can be obtained from **Bo** (in nT) from H=(1/µ)**Bo**.= (10-9/ (4πx10-7))**Bo** = 7.96x10-4**Bo** ≈ 8x10-4**Bo** Hence, the induced magnetisation is given by the equation: **mi** = κ**H** = κ(8x10-4)**Bo. mi** has the same direction as the present day Bo.

In the TVZ, the strength of the induced magnetisation **(|mi |**) of rhyolites and ignimbrites is of the order of 0.5 A/m [21], pointing to the presence of about 0.8% (by volume) of primary magnetic minerals [17]. Petrology studies by Ewart [8] indicate magnetite values between 0.3 and 0.8% from "point counting". The strength of the remanent magnetisation (**mr|**) of these rocks is significantly greater and lies commonly within the range of 1-4 A/m [21]. The strength of the total magnetisation (**mt** = **mr** + **mi** ) of unaltered, normally magnetised volcanic rocks in the TVZ lies between about 0.5 A/m (tuffs, pumice, and volcanic breccia) and about 2.5 A/m (rhyolite lavas) [14]. The values cited are median values obtained from more than 200 rock samples measured by students at the Geothermal Institute (University of Auckland) between 1988 and 1993. The effect of an overprinted events associated with the natural fluctuation of geomagnetic field (known as the viscous remanent magnetisation, or VRM) in young rocks such as Quaternary volcanic rocks is likely to be small and can be neglected.

## **3.3. Hydrothermal demagnetisation of volcanic rocks**

Hydrothermal alteration usually causes a "de-magnetisation" of initially magnetic reservoir rocks. Hydrothermal demagnetisation causes *negative* magnetisation contrast. Petrology studies show that, in these reservoirs, primary (titano-) magnetite has been replaced by almost non-magnetic minerals such as pyrite, leucoxene, or hematite [32]. In New Zealand some petrology studies in the Geothermal Institute (University of Auckland) showed that in liquiddominated systems (titano-) magnetite appears to be the first mineral replaced during thermal alteration; this also applies to many liquid dominated systems in the Philippines and Indonesia (Prof P.R.L. Browne, pers. comm., 1994). Hydrothermal demagnetisation appears to also occur in exposed rhyolite domed near the margins of some TVZ geothermal systems [20,26]. It seems acidic condensate formed in shallow vapour zones can cause demagnetisation of volcanic rocks forming topographic highs.

The solubility of magnetite increases rapidly in aqueous solutions with decreasing pH values (pH < 6) and at temperatures below 200°C [4]. Magnetite is also unstable in volcanic rocks that are saturated with CO2-rich, steam heated waters with a pH value of <6. These occur, for example, near the top and margins of the Ohaaki reservoir in the TVZ [11]. The waters are corrosive and have caused significant external corrosion to well casings. All cores from the cooler well at Ohaaki which discharged such fluid were nearly non-magnetic. Since cooler, CO2-rich waters at the margin of a gas-rich field like Ohaaki can also occur outside its boun‐ dary, it is possible that non-magnetic rocks can extend beyond the boundary of such a field delineated by resistivity surveys. Magnetite can be stable in the deeper oxidising environment of the natural two-phase system of Olkaria (Kenya) and, in that reservoir, it is less affected by thermal alteration than all other primary phases except quartz (Prof P.R.L Browne, pers. comm. 1994); the same applies to host rocks of the El Tatio outflow in Northern Chile. Because of this, the airborne magnetic survey identifies only the upper part of the geothermal reservoir. Except in the situations where the ground water flows have very strong horizontal component (associated, for example, with steep overall topography) the deep geothermal reservoir would be located beneath the zone of intensive hydrothermal demagnetisation.

Hydrothermal activity can also produce, on a smaller scale, a "'re-magnetisation" when pyrrhotite is deposited, which occurs as a secondary mineral in some New Zealand geothermal reservoirs. Little is known about the formation of this hydrothermal, magnetic trace mineral, which often can be found near rocks containing organic matter. Its magnetic stability range is restricted by its low Curie temperature (about 320°C according to Butler [6]). A study by Browne and Ellis [5] showed that small amounts of pyrrhotite occur in about one third of the wells at Ohaaki geothermal field in the TVZ. Pyrrhotite has also been found in a few wells at two other TVZ geothermal fields, the Wairakei and Waiotapu geothermal fields [33]; it is usually confined to barren fissures, not dispersed. Some 1990s studies by students at the Geothermal Institute (University of Auckland) of cores from 11 wells at Ohaaki geothermal field indicate that the magnetic effect of this alteration mineral (if present) would be small. However, two cores from one well at Ohaaki field at about 800 and 1100 m depths, which showed up with significant magnetisation contain no magnetite but some pyrrhotite. Overall, however, it appeared that "re-magnetisation'" of reservoir rocks caused by pyrrhotite deposi‐ tion is small and can be neglected.

#### **3.4. Reversely magnetised volcanic rocks**

Reversely magnetised volcanic rocks are rocks which have **Mr** pointing opposite to the direction of the present day earth magnetic field. These volcanic rocks have thermo remanent magnetisation (TRM) which opposed the present day geomagnetic field because they were deposited and cooled down to temperature below the Curie point of magnetite (about 580˚C) the time of geomagnetic reversal. As the last reversal of the earth magnetic field occurred about 0.7 Myrs ago (Section 2.1) some older Quaternary volcanic rocks would be reversely magne‐ tised (the Quaternary age ranges from 1.6 Myrs ago to recent). In the TVZ the strength of the reversed **mr** is always greater than that of the induced magnetisation **mi** [28,10]. This phenom‐ enon is probably also true in all other Quaternary volcanic zones elsewhere. As the results, the total magnetisation (**mt** ), which is the rock parameter affecting airborne magnetic anomalies, of the reversely magnetised Quaternary volcanic rocks is opposite in direction to **Bo**. Reversely magnetised rocks have *negative* magnetisation contrast, the same type of magnetisation contrast as that caused by hydrothermal demagnetisation.

## **4. The pattern of total force magnetic anomalies caused by hydrothermal demagnetisation in different geographic locations**

## **4.1. Pattern of the total force magnetic anomalies**

(Prof P.R.L. Browne, pers. comm., 1994). Hydrothermal demagnetisation appears to also occur in exposed rhyolite domed near the margins of some TVZ geothermal systems [20,26]. It seems acidic condensate formed in shallow vapour zones can cause demagnetisation of volcanic rocks

The solubility of magnetite increases rapidly in aqueous solutions with decreasing pH values (pH < 6) and at temperatures below 200°C [4]. Magnetite is also unstable in volcanic rocks that are saturated with CO2-rich, steam heated waters with a pH value of <6. These occur, for example, near the top and margins of the Ohaaki reservoir in the TVZ [11]. The waters are corrosive and have caused significant external corrosion to well casings. All cores from the cooler well at Ohaaki which discharged such fluid were nearly non-magnetic. Since cooler, CO2-rich waters at the margin of a gas-rich field like Ohaaki can also occur outside its boun‐ dary, it is possible that non-magnetic rocks can extend beyond the boundary of such a field delineated by resistivity surveys. Magnetite can be stable in the deeper oxidising environment of the natural two-phase system of Olkaria (Kenya) and, in that reservoir, it is less affected by thermal alteration than all other primary phases except quartz (Prof P.R.L Browne, pers. comm. 1994); the same applies to host rocks of the El Tatio outflow in Northern Chile. Because of this, the airborne magnetic survey identifies only the upper part of the geothermal reservoir. Except in the situations where the ground water flows have very strong horizontal component (associated, for example, with steep overall topography) the deep geothermal reservoir would

Hydrothermal activity can also produce, on a smaller scale, a "'re-magnetisation" when pyrrhotite is deposited, which occurs as a secondary mineral in some New Zealand geothermal reservoirs. Little is known about the formation of this hydrothermal, magnetic trace mineral, which often can be found near rocks containing organic matter. Its magnetic stability range is restricted by its low Curie temperature (about 320°C according to Butler [6]). A study by Browne and Ellis [5] showed that small amounts of pyrrhotite occur in about one third of the wells at Ohaaki geothermal field in the TVZ. Pyrrhotite has also been found in a few wells at two other TVZ geothermal fields, the Wairakei and Waiotapu geothermal fields [33]; it is usually confined to barren fissures, not dispersed. Some 1990s studies by students at the Geothermal Institute (University of Auckland) of cores from 11 wells at Ohaaki geothermal field indicate that the magnetic effect of this alteration mineral (if present) would be small. However, two cores from one well at Ohaaki field at about 800 and 1100 m depths, which showed up with significant magnetisation contain no magnetite but some pyrrhotite. Overall, however, it appeared that "re-magnetisation'" of reservoir rocks caused by pyrrhotite deposi‐

Reversely magnetised volcanic rocks are rocks which have **Mr** pointing opposite to the direction of the present day earth magnetic field. These volcanic rocks have thermo remanent magnetisation (TRM) which opposed the present day geomagnetic field because they were deposited and cooled down to temperature below the Curie point of magnetite (about 580˚C) the time of geomagnetic reversal. As the last reversal of the earth magnetic field occurred about

be located beneath the zone of intensive hydrothermal demagnetisation.

forming topographic highs.

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tion is small and can be neglected.

**3.4. Reversely magnetised volcanic rocks**

To understand the magnetic anomaly pattern caused by geothermal systems at different geographical positions, the total force magnetic anomalies (∆F) of a simplified model geother‐ mal system (contains subsurface hydrothermally demagnetised rocks, a hill and a valley) were computed using 3D quantitative magnetic modelling code written by Soengkono [21] based on the equations of Barnett [2]. The computations were made for the geothermal system sited at different geographical locations; at high, moderate and low magnetic inclinations, at the magnetic poles and along the magnetic equator. The simplified model of geothermal system and its total force magnetic anomalies at the various magnetic inclinations are shown in Figures 4, 5, 6 and 7.

As already mentioned in Section 3.3, the hydrothermally demagnetised rocks have a *negative magnetisation contrast* with respect to the surrounding volcanic rocks. It should be noted that the magnetic model in Figures 4(a), 5(a), 6(a) and 7(a) will be still exactly the same magnetically if the hydrothermally demagnetised rocks body is given a **|Mt |** of -1.7 a/m and the surrounding homogeneous volcanic rocks mass is given a magnetisation **|Mt |** of 0 A/m. This is true because the magnetic effect of a one-dimensional magnetic body (a horizontal magnetic plate, or slab) is everywhere equal zero.

#### *4.1.1. In the high magnetic inclination*

Figure 4 shows the total force anomalies created by the simplified magnetic model at the southern hemisphere at geomagnetic inclination of -60˚ (Figure 4(b)) and at northern hemi‐ sphere at geomagnetic inclination of +60˚ (Figure 4(c)). In both Figures 4(b) and 4(c), the negative magnetisation contrast of the hydrothermally demagnetised rocks (the geothermal reservoir) causes bipolar total force magnetic anomaly with a dominant *magnetic low* (negative anomalies) that are easily recognised on the maps. Although the centre of the magnetic low is shifted from the entre of demagnetised rocks, the magnetic low can still clearly identify the demagnetised body. The extent of the magnetic low can be used almost directly to approximate (roughly delineate) the edges of the demagnetised rocks.

**Figure 4.** Total force magnetic anomalies (∆F) caused by a simplified geothermal system at high geomagnetic inclina‐ tions of -60˚ (southern hemisphere) and +60˚ (northern hemisphere).

## *4.1.2. In the moderate magnetic inclination*

Figure 5 shows the total force anomalies created by the simplified magnetic model at southern hemisphere at geomagnetic inclination of -30˚ (Figure 5(b) and at northern hemisphere at geomagnetic inclination of +30˚ (Figure 5(c)). The bipolarity of the total force magnetic anomalies becomes more pronounced and the strengths (magnitudes) of the positive and negative anomalies are roughly equal. The hydrothermally demagnetised rocks can still be identified, but it becomes difficult to delineate their extent directly from the extent of the magnetic anomalies.

**Figure 5.** Total force magnetic anomalies (∆F) caused by a simplified geothermal system at moderate geomagnetic in‐ clinations of -30˚ (southern hemisphere) and +30˚ (northern hemisphere).

## *4.1.3. In the low magnetic inclination*

**Figure 4.** Total force magnetic anomalies (∆F) caused by a simplified geothermal system at high geomagnetic inclina‐

Figure 5 shows the total force anomalies created by the simplified magnetic model at southern hemisphere at geomagnetic inclination of -30˚ (Figure 5(b) and at northern hemisphere at geomagnetic inclination of +30˚ (Figure 5(c)). The bipolarity of the total force magnetic anomalies becomes more pronounced and the strengths (magnitudes) of the positive and negative anomalies are roughly equal. The hydrothermally demagnetised rocks can still be identified, but it becomes difficult to delineate their extent directly from the extent of the

tions of -60˚ (southern hemisphere) and +60˚ (northern hemisphere).

*4.1.2. In the moderate magnetic inclination*

magnetic anomalies.

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Figure 6 shows the total force anomalies created by the simplified magnetic model at locations close to the magnetic equator, at geomagnetic inclination -10˚ (Figure 6(b)) and at geomagnetic inclination +10˚ (Figure 6(c)). Here, the total force magnetic anomalies pattern caused by the geothermal system is much less bipolar but *positive* anomalies become dominant. Hence, it is important to be always aware to this contradictory phenomenon (*negative* magnetisation contrast creating *positive* anomalies). In this low geomagnetic latitude hydrothermally demagnetised rocks are to be identified from positive anomalies, not negative anomalies. The positive anomalies in Figures 6(a) and 6(b) are clearly recognizable, however the extent of the positive anomaly does not follow the edge of hydrothermally demagnetized rocks very well.

**Figure 6.** Total force magnetic anomalies (∆F) caused by a simplified geothermal system at low geomagnetic inclina‐ tions of -10˚ (southern hemisphere close to magnetic equator) and +10˚ (northern hemisphere close to magnetic equa‐ tor).

## *4.1.4. Along the magnetic equator and in the south/north magnetic poles*

Figure 7 shows the total force anomalies created by the simple model at two extreme locations, along the magnetic equator where the geomagnetic inclination is 0˚ (Figure 7(b) and in the south/north geomagnetic pole where the geomagnetic inclination is either -90˚ or +90˚ (Figure 7(c)). At the magnetic equator, the hydrothermally demagnetised rocks are marked by dominantly positive anomalies (Figures 7(b)). The positive anomalies are easier to recognise than in Figures 6(b) and 6(c), but their extent still does not accurately follow the extent of the hydrothermally demagnetised rocks. At the magnetic pole (Figure 7(c)) the centre of the hydrothermally demagnetised rocks is marked by the centre of strong magnetic low (negative anomalies). Here, it is also easy to trace or delineate the edges of the hydrothermally demag‐ netised rocks (the geothermal reservoir) from the edges of the magnetic low. Hence, the location of geothermal reservoir would be able to be delineated directly if the reservoir is located in the south/north magnetic pole.

positive anomalies in Figures 6(a) and 6(b) are clearly recognizable, however the extent of the positive anomaly does not follow the edge of hydrothermally demagnetized rocks very well.

**Figure 6.** Total force magnetic anomalies (∆F) caused by a simplified geothermal system at low geomagnetic inclina‐ tions of -10˚ (southern hemisphere close to magnetic equator) and +10˚ (northern hemisphere close to magnetic equa‐

Figure 7 shows the total force anomalies created by the simple model at two extreme locations, along the magnetic equator where the geomagnetic inclination is 0˚ (Figure 7(b) and in the south/north geomagnetic pole where the geomagnetic inclination is either -90˚ or +90˚ (Figure

*4.1.4. Along the magnetic equator and in the south/north magnetic poles*

tor).

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**Figure 7.** Total force magnetic anomalies (∆F) caused by a simplified geothermal system geomagnetic inclinations of 0˚ (along the magnetic equator) and +/-60˚ (the magnetic south/north pole).

## **4.2. Delineating the likely extent of geothermal reservoir from total force magnetic anomalies**

The previous section (Section 4.1) has shown that total force magnetic anomalies associated with a geothermal system have different patterns at different geomagnetic inclinations (geographical locations). At high magnetic inclinations (±60˚to ±90˚) a demagnetised geother‐ mal reservoir is marked by negative total force magnetic anomalies. At the magnetic south and north poles (magnetic inclinations of -90˚ and +90˚) the edges of the (theoretical) geothermal reservoir can be delineated directly from the edges of the magnetic low, even when the background volcanic rocks hosting the geothermal reservoir are not magnetically homogene‐ ous. As the location moves away from the magnetic pole(s), it becomes less easy to delineate the geothermal reservoir. At geomagnetic inclinations of around +/- 30˚, it becomes impossible to delineate the geothermal reservoir from the extents of the total force magnetic anomalies. However, it is possible to transform the total force magnetic anomalies into the situation that would be observed if the causative source is located in the magnetic pole, using a standard magnetic operation known as "reduction to pole" (RTP). This operation was first introduced by Baranov [1]. The operation moves centres of anomaly to positions above their sources [3], assuming that the rock total rock magnetisation **(mt** ) is either parallel of directly opposes the direction of **Bo**, which is true for the case of young Quaternary volcanic rocks. However, as the magnetic latitude approaches its equator, the RTP operator becomes unbounded along the direction of magnetic declination and therefore amplifies the noise in this direction to the extent that the resultant RTP field is dominated by linear features aligned with the direction of declination [18]. The solution to this problem has now become available by using specially designed variations of the RTP transform. User friendly software packages that can perform the RTP operation and address the problem of low magnetic inclination are now available online.

Figures 8, 9 show the results of RTP operation using the Oasis Montaj software applied to our theoretical total force anomalies shown in Figures 4, 5 6 and 7.

In Figures 8, the total force magnetic anomalies at geomagnetic inclinations +/-60˚ and +/-30˚ that have been reduced to pole (RTP) can accurately trace the extent of the hydrothermally demagnetised rocks. The hydrothermally demagnetised rocks can also be traced from the total force magnetic anomalies close to the magnetic equator shown in Figure 9, but the tracing (delineation) of their edges becomes slightly more difficult. The opposite transformation of reduction to magnetic equator (RTE) (available in the Oasis Montaj software package) would not help much in solving the problem, as the total force magnetic high at the equator does not accurately follow the edge if the hydrothermally demagnetised rocks either (Figure 7 (b)). In general, the closer the location is to the magnetic equator, the more difficult it is to delineate the source body from the total force magnetic anomalies. A quantitative 3D modelling will help, but conducting this complex and difficult task would be required only if detailed delineation of hydrothermally demagnetised rocks is crucial and absolutely necessary.

**4.2. Delineating the likely extent of geothermal reservoir from total force magnetic**

assuming that the rock total rock magnetisation **(mt**

theoretical total force anomalies shown in Figures 4, 5 6 and 7.

The previous section (Section 4.1) has shown that total force magnetic anomalies associated with a geothermal system have different patterns at different geomagnetic inclinations (geographical locations). At high magnetic inclinations (±60˚to ±90˚) a demagnetised geother‐ mal reservoir is marked by negative total force magnetic anomalies. At the magnetic south and north poles (magnetic inclinations of -90˚ and +90˚) the edges of the (theoretical) geothermal reservoir can be delineated directly from the edges of the magnetic low, even when the background volcanic rocks hosting the geothermal reservoir are not magnetically homogene‐ ous. As the location moves away from the magnetic pole(s), it becomes less easy to delineate the geothermal reservoir. At geomagnetic inclinations of around +/- 30˚, it becomes impossible to delineate the geothermal reservoir from the extents of the total force magnetic anomalies. However, it is possible to transform the total force magnetic anomalies into the situation that would be observed if the causative source is located in the magnetic pole, using a standard magnetic operation known as "reduction to pole" (RTP). This operation was first introduced by Baranov [1]. The operation moves centres of anomaly to positions above their sources [3],

direction of **Bo**, which is true for the case of young Quaternary volcanic rocks. However, as the magnetic latitude approaches its equator, the RTP operator becomes unbounded along the direction of magnetic declination and therefore amplifies the noise in this direction to the extent that the resultant RTP field is dominated by linear features aligned with the direction of declination [18]. The solution to this problem has now become available by using specially designed variations of the RTP transform. User friendly software packages that can perform the RTP operation and address the problem of low magnetic inclination are now available

Figures 8, 9 show the results of RTP operation using the Oasis Montaj software applied to our

In Figures 8, the total force magnetic anomalies at geomagnetic inclinations +/-60˚ and +/-30˚ that have been reduced to pole (RTP) can accurately trace the extent of the hydrothermally demagnetised rocks. The hydrothermally demagnetised rocks can also be traced from the total force magnetic anomalies close to the magnetic equator shown in Figure 9, but the tracing (delineation) of their edges becomes slightly more difficult. The opposite transformation of reduction to magnetic equator (RTE) (available in the Oasis Montaj software package) would not help much in solving the problem, as the total force magnetic high at the equator does not accurately follow the edge if the hydrothermally demagnetised rocks either (Figure 7 (b)). In general, the closer the location is to the magnetic equator, the more difficult it is to delineate the source body from the total force magnetic anomalies. A quantitative 3D modelling will help, but conducting this complex and difficult task would be required only if detailed delineation of hydrothermally demagnetised rocks is crucial and absolutely necessary.

) is either parallel of directly opposes the

**anomalies**

128 Advances in Geothermal Energy

online.

**Figure 8.** Reduction to pole (RTP) of total force magnetic anomalies (∆F) caused by a simplified geothermal system at geomagnetic inclinations of +/-60˚ and +/-30˚.

**Figure 9.** Reduction to pole (RTP) of total force magnetic anomalies (∆F) caused by a simplified geothermal system at geomagnetic inclinations of +/-10˚ (close to the magnetic equator) and 0˚ (along the magnetic equator).

## **5. Examples of airborne magnetic surveys to investigate high temperature geothermal reservoirs**

## **5.1. Introduction**

A new, simple but effective, interpretation approach of airborne magnetic survey for investi‐ gation of high temperature geothermal resources in Quaternary volcanic setting has been introduced and discussed in the previous sections. To gain a more comprehension of its practical aspects, the interpretation approach is applied to real airborne magnetic data from the TVZ in New Zealand (geomagnetic inclination about -65˚) and from the eastern Java Island in Indonesia (geomagnetic inclination about -35˚). These examples of interpretations are presented and discussed in the following sections. Note that all the magnetic interpretations presented are new interpretations, specifically carried out to illustrate the approach introduced previously in this chapter. The gridding, plotting contouring and drawing, trend determina‐ tions and RTP transformations of the anomalies are all carried out using the Oasis Montaj software package.

## **5.2. Examples from TVZ, New Zealand**

### *5.2.1. Wairakei geothermal field*

**Figure 9.** Reduction to pole (RTP) of total force magnetic anomalies (∆F) caused by a simplified geothermal system at

geomagnetic inclinations of +/-10˚ (close to the magnetic equator) and 0˚ (along the magnetic equator).

130 Advances in Geothermal Energy

The high temperature Wairakei geothermal field in the TVZ is the first geothermal field used for electricity generation in New Zealand, and the second in the world after the Larderello geothermal field in Italy. The geothermal system is situated in rather flat topography (Figure 10). The geothermal reservoir was delineated using Schlumberger DC resistivity surveys and the boundary shown in Figure 10 has been slightly refined using information from a few geothermal boreholes. To the southeast of Wairakei is another high temperature field, the Tauhara geothermal field. Both Wairakei and Tauhara reservoir are hosted by Quaternary volcanic rocks. The fields are located at a high magnetic inclination of about -65˚. Figure 11 shows the map of **|Bobs|** over the Wairakei field and the northern part of the Tauhara field obtained from a detailed airborne magnetic survey draped 60 m above ground which was conducted by the gold exploration company Glass Earth NZ Ltd in 2006, and from a previous smaller survey at similar altitude conducted by the GNS Science in 1989. The 1st order trend of **|Bobs|** to estimate **|Bo**| is also shown in Figure 11. Figure 12 presents the total force magnetic anomalies (∆F) map over the area.

The total force magnetic anomalies after the reduction to pole (RTP) transformation are shown in Figure 13. The magnetic anomalies (RTP) shown in this figure are located directly above their causative sources. Prominent magnetic lows are present in the north-western part of the Wairakei field and over the Tauhara field. These magnetic lows represent intensive and/or thick hydrothermal demagnetisation. Less prominent magnetic low can be seen over the fumaroles and steaming ground at Crater of the Moon, representing hydrothermal demag‐ netisation by acidic condensate in the shallow vapour zones. The prominent magnetic low in the north-western part of the Wairakei field suggests that in this area the Wairakei geothermal reservoir extends beyond the field boundary defined by resistivity survey, although the possibility that part of the negative anomalies outside the boundary of the geothermal field are due to by reversely magnetised rocks cannot be completely ruled out.

**Figure 10.** Topographic map of the Wairakei and the northern part of Tauhara geothermal fields in the TVZ (New Zea‐ land) showing the boundary of the fields based on Schlumberger DC resistivity survey.

**Figure 11.** Map of observed total force geomagnetic field strengths **(|Bo|**) over the Wairakei and the northern part of Tauhara geothermal fields in the TVZ (New Zealand). The red contour lines represent **|Bo|** values as defined by the first order trend of all **|Bobs|** across the area.

#### *5.2.2. Mokai geothermal field*

reservoir extends beyond the field boundary defined by resistivity survey, although the possibility that part of the negative anomalies outside the boundary of the geothermal field

**Figure 10.** Topographic map of the Wairakei and the northern part of Tauhara geothermal fields in the TVZ (New Zea‐

land) showing the boundary of the fields based on Schlumberger DC resistivity survey.

are due to by reversely magnetised rocks cannot be completely ruled out.

132 Advances in Geothermal Energy

The high temperature Mokai geothermal field is also located in the Taupo Volcanic Zone, about 20 km west of the Wairakei field. The Mokai geothermal reservoir is also hosted by Quaternary volcanic rocks and has slightly steeper topography than the Wairakei field (Figure 14).

Figure 14 shows the boundary of Mokai geothermal field delineated from Schlumberger DC resistivity surveys, and the surface thermal manifestations of the field. The 60 ohm-m resis‐ tivity contour is extending northeast-north toward the thermal springs in a lower elevation, indicating lateral outflow of geothermal water. Figure 15 shows the map of **|Bobs|** over the Mokai geothermal field obtained from the same detailed survey by Glass Earth Ltd. in 2006

**Figure 12.** Map of total force magnetic anomalies (∆F) over the Wairakei and the northern part of Tauhara geothermal fields in the TVZ (New Zealand). The ∆F values were obtained by subtraction of **|Bo|** determined from 1st order trend of **|Bobs|.**

that covers the Wairakei field. The 1st order trend of **|Bobs|** to estimate **|Bo|** is also shown in the map. The total force magnetic anomalies (∆F) map the Mokai area is shown in Figure 16. A broad magnetic low appears associated with the geothermal field at location slightly shifted to the north. Several other magnetic lows are present outside the Mokai geothermal field to the northwest, southwest, southeast and further to the northeast of the Mokai resistivity boundary.

Reduction to pole (RTP) transformation was applied to the total force anomalies in Figure 16 and the result is presented in Figure 17. In this figure, the magnetic low above the Mokai geothermal field becomes consistent with the resistivity boundary, indicating that hydrother‐

**Figure 13.** The result of reduction to pole (RTP) of the total force magnetic anomalies (∆F) over the Wairakei and the northern part of Tauhara geothermal fields in the TVZ (New Zealand).

that covers the Wairakei field. The 1st order trend of **|Bobs|** to estimate **|Bo|** is also shown in the map. The total force magnetic anomalies (∆F) map the Mokai area is shown in Figure 16. A broad magnetic low appears associated with the geothermal field at location slightly shifted to the north. Several other magnetic lows are present outside the Mokai geothermal field to the northwest, southwest, southeast and further to the northeast of the Mokai resistivity

**Figure 12.** Map of total force magnetic anomalies (∆F) over the Wairakei and the northern part of Tauhara geothermal fields in the TVZ (New Zealand). The ∆F values were obtained by subtraction of **|Bo|** determined from 1st order trend

Reduction to pole (RTP) transformation was applied to the total force anomalies in Figure 16 and the result is presented in Figure 17. In this figure, the magnetic low above the Mokai geothermal field becomes consistent with the resistivity boundary, indicating that hydrother‐

boundary.

of **|Bobs|.**

134 Advances in Geothermal Energy

mally demagnetised rocks are present in the Mokai Reservoir. Moderately low magnetic RTP anomalies mark the western part of Pukemoremoe rhyolite topographic dome, suggesting that acidic condensate formed in shallow vapour zones here has caused hydrothermal demagnet‐ isation. Reversely magnetised rocks are known to be present in this area [28]. Hence, the magnetic lows outside the Mokai geothermal field boundary could represent reversely magnetised rocks, as indicated in Figure 16. The magnetic low caused by the hydrothermally demagnetised rocks inside the Mokai reservoir shows variation in strength. This variation could be caused the variation of intensity and/or thickness of the hydrothermal demagnetisa‐ tion process, which could held clue to the variation of reservoir permeability and/or movement of geothermal water. This could be investigated further by a quantitative 3D modelling of the magnetic anomalies.

**Figure 14.** Topographic map of the Mokai geothermal field in the TVZ (New Zealand) showing the boundary of the field based on Schlumberger DC resistivity survey.

**Figure 15.** Map of observed total force geomagnetic field strengths **(|Bo|**) over the Mokai geothermal field in the TVZ (New Zealand). The red contour lines represent **|Bo|** values as defined by the first order trend of all **|Bobs|** across the area.

**Figure 14.** Topographic map of the Mokai geothermal field in the TVZ (New Zealand) showing the boundary of the

field based on Schlumberger DC resistivity survey.

136 Advances in Geothermal Energy

**Figure 16.** Map of total force magnetic anomalies (∆F) over the Mokai geothermal field in the TVZ (New Zealand). The ∆F values were obtained by subtraction of **|Bo|** determined from 1st order trend of **|Bobs|.**

**Figure 17.** The result of reduction to pole (RTP) of the total force magnetic anomalies (∆F) over the Mokai geothermal field in the TVZ (New Zealand).

**Figure 16.** Map of total force magnetic anomalies (∆F) over the Mokai geothermal field in the TVZ (New Zealand). The

∆F values were obtained by subtraction of **|Bo|** determined from 1st order trend of **|Bobs|.**

138 Advances in Geothermal Energy

#### **5.3. Example from Java Island, Indonesia**

#### *5.3.1. Ijen geothermal field*

The Ijen geothermal field is located in eastern Java Island of Indonesia (see the location map in Figure 18) with a geomagnetic inclination of about -35˚. The airborne magnetic survey over the Ijen geothermal field was carried out in 1990 by Penas-Carson Services Inc. (USA) for the Indonesia Pertamina Geothermal Division. The survey was made at flight elevation of about 1 km above the ground, along west-east flight lines separated by about 0.75-1 km spacing. The Ijen geothermal field is hosted by Quaternary andesitic volcanic rocks which form steep topography around the field. Figure 18 shows the topography of the Ijen area and the locations of surface thermal manifestation of the geothermal field consisting of altered rocks near Telagawaru and a group of thermal springs near Blawan in the north. The "raw" airborne magnetic data were presented by the Penas-Carson Inc., which were obtained by reducing the IGRF variation from the measured geomagnetic field. Such "raw" airborne magnetic data are shown in Figure 19, together with their 1st order trend. Figure 20 shows the total force magnetic anomalies (∆F) over the Ijen area. In this figure the bipolar anomalies associated with Mt Suket, Mt Pendil and Mt Rante become clearly visible. Less clearly shown is the bipolar anomaly over Mt Ijen. Positive anomalies are also seen over the north-eastern Kendeng Ridge. A wide magnetic low is presents over the geothermal field region. The north-eastern part of this magnetic low could be the negative part of the bipolar magnetic anomaly of the north east Kendeng Ridge. Because of the moderate magnetic inclination of the region, the ∆F values shown in Figure 20 are likely to spread widely over the causative sources. A direct interpre‐ tation of this figure can be misleading without the RTP transformation.

The total force anomalies over the Ijen geothermal area after reduction to pole (RTP) transfor‐ mation are shown in Figure 21. The magnetic anomalies (RTP) shown in this figure would be located above their causative sources. The most interesting magnetic RTP anomalies in Figure 21 is the magnetic low that appears to be associated with the outcrop of altered rocks west of Telagawaru. This magnetic low is interpreted in Figure 21 to represent hydrothermally demagnetised rocks. This leads to the inferred model of Ijen geothermal system consisting of a geothermal up-low zone in the Telagawaru – Mt Genteng area and concealed outflow of thermal water towards the thermal springs near Blawan. The magnetic low to the northwest of Kendeng Ridge could represent reversely magnetised Quaternary volcanic rocks. Reversely magnetised rocks are probably also the sources of the three magnetic lows near the southern edge of Figure 21. Positive anomalies marked Mt Suket, Mt Pendil, Mt Rante and Mt Ijen, showing that these mountains are formed by normally magnetised rocks. The magnetic high which follows the north-eastern Kendeng Ridge in Figure 20 has moved in Figure 21 to a new location to be entirely southwest of the ridge. This magnetic high probably represents a subsurface lava body underneath.

**5.3. Example from Java Island, Indonesia**

The Ijen geothermal field is located in eastern Java Island of Indonesia (see the location map in Figure 18) with a geomagnetic inclination of about -35˚. The airborne magnetic survey over the Ijen geothermal field was carried out in 1990 by Penas-Carson Services Inc. (USA) for the Indonesia Pertamina Geothermal Division. The survey was made at flight elevation of about 1 km above the ground, along west-east flight lines separated by about 0.75-1 km spacing. The Ijen geothermal field is hosted by Quaternary andesitic volcanic rocks which form steep topography around the field. Figure 18 shows the topography of the Ijen area and the locations of surface thermal manifestation of the geothermal field consisting of altered rocks near Telagawaru and a group of thermal springs near Blawan in the north. The "raw" airborne magnetic data were presented by the Penas-Carson Inc., which were obtained by reducing the IGRF variation from the measured geomagnetic field. Such "raw" airborne magnetic data are shown in Figure 19, together with their 1st order trend. Figure 20 shows the total force magnetic anomalies (∆F) over the Ijen area. In this figure the bipolar anomalies associated with Mt Suket, Mt Pendil and Mt Rante become clearly visible. Less clearly shown is the bipolar anomaly over Mt Ijen. Positive anomalies are also seen over the north-eastern Kendeng Ridge. A wide magnetic low is presents over the geothermal field region. The north-eastern part of this magnetic low could be the negative part of the bipolar magnetic anomaly of the north east Kendeng Ridge. Because of the moderate magnetic inclination of the region, the ∆F values shown in Figure 20 are likely to spread widely over the causative sources. A direct interpre‐

tation of this figure can be misleading without the RTP transformation.

subsurface lava body underneath.

The total force anomalies over the Ijen geothermal area after reduction to pole (RTP) transfor‐ mation are shown in Figure 21. The magnetic anomalies (RTP) shown in this figure would be located above their causative sources. The most interesting magnetic RTP anomalies in Figure 21 is the magnetic low that appears to be associated with the outcrop of altered rocks west of Telagawaru. This magnetic low is interpreted in Figure 21 to represent hydrothermally demagnetised rocks. This leads to the inferred model of Ijen geothermal system consisting of a geothermal up-low zone in the Telagawaru – Mt Genteng area and concealed outflow of thermal water towards the thermal springs near Blawan. The magnetic low to the northwest of Kendeng Ridge could represent reversely magnetised Quaternary volcanic rocks. Reversely magnetised rocks are probably also the sources of the three magnetic lows near the southern edge of Figure 21. Positive anomalies marked Mt Suket, Mt Pendil, Mt Rante and Mt Ijen, showing that these mountains are formed by normally magnetised rocks. The magnetic high which follows the north-eastern Kendeng Ridge in Figure 20 has moved in Figure 21 to a new location to be entirely southwest of the ridge. This magnetic high probably represents a

*5.3.1. Ijen geothermal field*

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**Figure 18.** Topographic map of the Ijen geothermal field in the Eastern Java (Indonesia) showing the prominent topog‐ raphy of the surrounding Mt. Suket, Mt. Pendil, Mt Rante and Mt Ijen and the surface thermal manifestations (altered rocks and thermal springs) of the field.

**Figure 19.** Map of observed total force "raw" field strengths (**~|Bobs|**) over the Ijen geothermal field in the eastern Java (Indonesia). The red contour lines represent **|Bo|** values as defined by the first order trend of all ~**|Bobs**| across the area.

## **6. Summary and discussion**

This chapter has shown that airborne magnetic data can be very useful in the investigation of high temperature geothermal reservoirs hosted by Quaternary volcanic rocks, particularly in the area with difficult ground access. Data might be already available over the geothermal target area from some previous surveys by mineral exploration companies or government institutions. To carry out a new airborne magnetic survey, many geophysical exploration companies are advertising their service and can be contacted on the internet, some of them can do the survey almost anywhere in the world. Even when a new survey is required, the airborne magnetic survey should still be a cost-effective method to explore and investigate high temperature geothermal resources in Quaternary volcanic setting.

**Figure 20.** Map of total force magnetic anomalies (∆F) over the Ijen geothermal field in the eastern Java (Indonesia). The ∆F values were obtained by subtraction of **|Bo|** determined from 1st order trend of ~**|Bobs|.**

**6. Summary and discussion**

area.

142 Advances in Geothermal Energy

This chapter has shown that airborne magnetic data can be very useful in the investigation of high temperature geothermal reservoirs hosted by Quaternary volcanic rocks, particularly in the area with difficult ground access. Data might be already available over the geothermal target area from some previous surveys by mineral exploration companies or government institutions. To carry out a new airborne magnetic survey, many geophysical exploration companies are advertising their service and can be contacted on the internet, some of them can do the survey almost anywhere in the world. Even when a new survey is required, the airborne magnetic survey should still be a cost-effective method to explore and investigate high

**Figure 19.** Map of observed total force "raw" field strengths (**~|Bobs|**) over the Ijen geothermal field in the eastern Java (Indonesia). The red contour lines represent **|Bo|** values as defined by the first order trend of all ~**|Bobs**| across the

temperature geothermal resources in Quaternary volcanic setting.

Various aspects of the application of airborne magnetic survey for the geothermal investigation are presented and explained in this chapter. The total force magnetic anomalies (∆F) which are the first result obtained from an airborne magnetic survey are explained in considerable details, including how to approximately obtain ∆F values that reflect only the variation of magneti‐ sation no deeper than the "survey target". Simple but effective diagrams to predict total force magnetic anomalies due to a magnetic dipole at different geomagnetic latitudes are intro‐ duced. These diagrams provide a basic knowledge for the application of airborne magnetic survey to investigate a variety of geological features, including high temperature geothermal reservoirs. A demagnetised geothermal reservoir has a negative magnetisation contrast. At high magnetic latitudes (away from the magnetic equator) a demagnetised reservoir is marked by dominantly negative total force magnetic anomalies. However, near the magnetic equator, the opposite occurs that the demagnetised reservoir is marked by dominantly positive

**Figure 21.** The result of reduction to pole (RTP) of the total force magnetic anomalies (∆F) over the Ijen geothermal field in the eastern Java (Indonesia).

anomalies. This confusion can be eliminated by passing the measured total force magnetic anomalies (gridded at regular spacing) through the reduction to pole (RTP) transformation. Any anomalies caused by demagnetised reservoir will become dominantly negative. Further‐ more, the lateral extent of the negative RTP would approximate the lateral extent of the demagnetised reservoir, so the magnetic RTP data can help delineate the geothermal reservoir. Special care must be taken, however, when working close (within about ±10˚latitude) to the magnetic equator, that the software used to perform the RTP transform can run properly for data from low magnetic inclination regions. In general, the farther away the location is from the magnetic equator, the easier it is to delineate the source body from the total force magnetic anomalies.

The occurrence of reversely magnetised rocks can make interpretation of airborne magnetic data for geothermal reservoir becomes more difficult. The reversely magnetised rocks have a similar effect in the airborne magnetic map as the hydrothermally demagnetised rocks. The two can be distinguished from each other only when they occur in prominent topography. Hydrothermally demagnetised rocks will cause no specific total magnetic anomalies whereas the reversely magnetised rocks will appear as negative total force magnetic anomalies over the topography. In any other circumstances it is difficult to distinguish the two from magnetic anomaly map alone. As shown in the examples in Section 5, a geological interpretation is needed to resolve the problem.

The discussions presented in this chapter should equip readers with a sufficient knowledge to confidently organise and run airborne magnetic investigation of high temperature geothermal reservoirs in volcanic setting. The three examples on real airborne magnetic surveys given in the Section 5 could be used as reference for most cases of airborne magnetic investigations of geothermal resources.

## **Author details**

Supri Soengkono\*

Address all correspondence to: s.soengkono@gns.cri.nz

GNS Science, Wairakei Research Centre, Taupo, NewZealand

## **References**

anomalies. This confusion can be eliminated by passing the measured total force magnetic anomalies (gridded at regular spacing) through the reduction to pole (RTP) transformation. Any anomalies caused by demagnetised reservoir will become dominantly negative. Further‐ more, the lateral extent of the negative RTP would approximate the lateral extent of the demagnetised reservoir, so the magnetic RTP data can help delineate the geothermal reservoir. Special care must be taken, however, when working close (within about ±10˚latitude) to the magnetic equator, that the software used to perform the RTP transform can run properly for data from low magnetic inclination regions. In general, the farther away the location is from the magnetic equator, the easier it is to delineate the source body from the total force magnetic

**Figure 21.** The result of reduction to pole (RTP) of the total force magnetic anomalies (∆F) over the Ijen geothermal

anomalies.

field in the eastern Java (Indonesia).

144 Advances in Geothermal Energy


[22] Soengkono, S.: Magnetic anomalies over the Ngatamariki geothermal field. NZ Geo‐ thermal Workshop Proceedings. 1992; 14: 241-246.

[7] Dobrin M.B. and Savit C.H.. Introduction to geophysical prospecting. 4th ed. 1988:

[8] Ewart, A.. Review of mineralogy and chemistry of the acidic rocks of TaupoVolcanic

[9] Gerard, V. B. and Lawrie, J. A.. Aeromagnetic surveys in New Zealand, 1949-1952. Geophysical Memoir 3, Department of Scientific and Industrial Research, Wellington,

[10] Grindley, G. W., Mumme, T. C. and Kohn, B. P.. Stratigraphy, paleomagnetism, geo‐ chronology and structure of silicic volcanic rocks, Waiotapu/Paeroa Range area, New

[11] Hedenquist, J. W. and Stewart, M. K.. Natural CO2-rich steam-heated waters in the Broadlands-Ohaaki geothermal system, New Zealand. Geothermal Resources Coun‐

[12] Henrys, S. A. and van Dijck, M. F. (1987) Structure of concealed rhyolites and dacites in the Broadlands-Ohaaki geothermal field. NZ Geothermal Workshop Proceedings.

[13] Henrys, S. A. and Hochstein, M. P.. Geophysical structure of Broadlands-Ohaaki geo‐

[14] Hochstein, M.P and Soengkono, S.: Magnetic anomalies associated with high temper‐ ature reservoirs in the Taupo Volcanic Zone (New Zealand). Geothermics. 1997; 26: p

[15] Hunt, T.M., Bromley, C.J., Risk, G.F. and Soengkono, S.: Geophysical investigation of

[16] Hunt, T. M. and Whiteford, C. M.. Sheet 5, Rotorua. Magnetic map of New Zealand

[17] Lawton, D. C. and Hochstein, M. P. (1980) Physical properties of titanomagnetite

[18] Li, Y. and Oldenburg, D. W.. Stable reduction to the pole at the magnetic equator.

[19] Mankinen, E.A. and Dalrymple, G.B. (1979) Revised magnetic polarity time scale for the interval 0-5 m.y. BP. Journal of Geophysical Research. 1979; 85: 615-626.

[20] Soengkono, S. (1985) Magnetic study of the Mokai geothermal field. NZ Geothermal

[21] Soengkono, S.. Geophysical studies of the Western Taupo Volcanic Zone. PhD. thesis,

1:250000, Total Force Anomalies. DSIR, Wellington, New Zealand. 1979.

thermal field (New Zealand). Geothermics. 1990; 19: 129-150.

the Wairakei field. Geothermics. 2009; 38: 85-97.

sands. Geophysics. 1980; 45:394-402.

Workshop Proceedings. 1985; 7: 25-30.

The University of Auckland. 1990:350 p.

Geophysics. 2001; 66(2): 571-578.

McGraw-Hill International Editions; 1988. 876 p.

Zealand. Geothermics. 1994; 23:473-499.

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New Zealand; 1955.

146 Advances in Geothermal Energy

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1-24.

Zone, New Zealand. Bulletin Volcanologique. 1966; 29: 147-172.


## **Chapter 6**

## **Geothermal Exploration Methods**

## Essam Aboud

[35] Watson-Munro, C. N. (1938) Reconnaissance survey of the variation of magnetic force in the New Zealand thermal regions. NZ Journal of Science and Technology.

[36] Whiteford, C. M.. Magnetic anomaly map of Central Volcanic region. Geophysics Di‐

1938; B20: 99-115.

148 Advances in Geothermal Energy

vision, DSIR, Wellington. 1979; Report 101.

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/61679

#### **Abstract**

A geothermal reservoir causes inhomogeneity in the physical properties of the subsurface geology due to the high changes in temperature. These physical properties can be ob‐ served by means of varying anomalies from geophysical observations from the surface. These physical properties include electrical conductivity, rock density, magnetic suscepti‐ bility, rock elasticity, and, finally, the temperature. The above mentioned physical prop‐ erties can be detected by surficial geophysical survey. The same parameters can also be measured in wells using "geophysical well logging," providing data that are more accu‐ rate but costly. On the other hand, geochemical exploration assists in gathering informa‐ tion about the subsurface composition of the fluids. This information can be used indirectly to know the temperature, origin, and flow direction, which help in locating subsurface geothermal reservoir. It is clear that, from geophysical and geochemical meth‐ ods, shape, size, structure, depth, and heat sources of the reservoir can be traced and mapped. Thus, the geophysical and geochemical surveys play a key role in geothermal exploration. This chapter will discuss the above mentioned methods in detail presenting some examples from literature review.

**Keywords:** Geothermal exploration, Geophysics, Geochemistry

## **1. Introduction**

Geothermal energy is "heat" contained in the earth's interior. This heat comes from the earth's core continuously outward until it traps in impermeable and fractured layers of the earth's surface. When water is heated by this "heat," hot water or steam can be trapped in permeable and porous rocks under a layer of impermeable rock, forming *geothermal reservoir* or *geothermal system*. Geothermal reservoir can be described schematically as convective water in the upper crust of the earth, which transfers the heat from a heat source to a heat sink [4]. A geothermal reservoir is, in general, composed of three main elements: a *heat source*, a *reservoir*, and a *fluid*, the carrier that transfers the heat. The *heat source* can be either a very high-temperature (> 600

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

°C) magmatic intrusion, which has reached relatively shallow depths (5–10 km), or lowtemperature zones (e.g., volcanic zones). The *reservoir* is a volume of hot permeable rocks through which the circulating fluids extract heat. It is often overlain by a cover of impermeable rocks and connected to a surficial recharge area through which the meteoric waters can replace, or partly replace, the fluids that run from the reservoir through springs or are extracted by boreholes. The *fluid or geothermal fluid* is water, meteoric water, in the liquid or vapor phase, depending on its temperature and pressure. This water often carries with it chemicals and gases such as CO2 and H2S. Figure (1) is a simplified representation of an ideal geothermal reservoir.

**Figure 1.** Schematic diagram of an ideal geothermal system by International Geothermal Association (IGA).

A geothermal reservoir in general causes inhomogeneity in the physical properties of the subsurface geology (e.g., physical properties of rocks) due to the high changes in temperature. These physical properties can be observed by means of varying anomalies from geophysical observations from the surface. These physical properties include electrical conductivity (electrical/electromagnetic (EM) method), rock density (gravity method), magnetic suscepti‐ bility (magnetic method), rock elasticity (seismic method), and, finally, the temperature (thermal survey). The above mentioned physical properties can be detected by surficial geophysical survey. The same parameters can also be measured in wells using "geophysical well logging," providing data that are more accurate but costly. On the other hand, geochem‐ ical exploration assists in gathering information about the subsurface composition of the fluids. This information can be used indirectly to know the temperature, origin, and flow direction, which help in locating subsurface geothermal reservoir. It is clear that, from geophysical and geochemical methods, shape, size, structure, depth, and heat sources of the reservoir can be traced and mapped. Thus, the geophysical and geochemical surveys play a key role in geothermal exploration. This chapter will discuss the above mentioned methods in detail, presenting some examples from literature review.

Geological, geophysical, and geochemical data must be collected and integrated with any available data from previous studies on water, minerals, etc. in the study area and adjacent areas before geothermal exploration program. This information plays an important role in defining the objectives of the geothermal exploration program and could lead to a significant reduction in costs. The classical geothermal exploration program includes the following phases:


°C) magmatic intrusion, which has reached relatively shallow depths (5–10 km), or lowtemperature zones (e.g., volcanic zones). The *reservoir* is a volume of hot permeable rocks through which the circulating fluids extract heat. It is often overlain by a cover of impermeable rocks and connected to a surficial recharge area through which the meteoric waters can replace, or partly replace, the fluids that run from the reservoir through springs or are extracted by boreholes. The *fluid or geothermal fluid* is water, meteoric water, in the liquid or vapor phase, depending on its temperature and pressure. This water often carries with it chemicals and gases such as CO2 and H2S. Figure (1) is a simplified representation of an ideal geothermal

**Figure 1.** Schematic diagram of an ideal geothermal system by International Geothermal Association (IGA).

A geothermal reservoir in general causes inhomogeneity in the physical properties of the subsurface geology (e.g., physical properties of rocks) due to the high changes in temperature. These physical properties can be observed by means of varying anomalies from geophysical observations from the surface. These physical properties include electrical conductivity (electrical/electromagnetic (EM) method), rock density (gravity method), magnetic suscepti‐ bility (magnetic method), rock elasticity (seismic method), and, finally, the temperature (thermal survey). The above mentioned physical properties can be detected by surficial

reservoir.

150 Advances in Geothermal Energy


## **2. Reconnaissance field survey**

Reconnaissance field survey or preliminary survey in geothermal exploration program is considered as the initiation phase on which next phases are defined. This phase involves a work program to assess the available evidence for geothermal potential in a specific area. While some of these evidences are technical (e.g., geological data, thermal manifestation, etc.), others are social (e.g., land access, country regulations, logistics, etc.). The preliminary survey seeks to identify geological settings that might host economically geothermal reservoirs. In practice, the survey essentially involves an "office work" review of geological, hydrological, and/or hot spring/thermal data, drilling data, remote sensing, etc.

The reconnaissance survey phase should also include an assessment of key environmental issues or factors that might affect or be affected by a geothermal project. As with any major infrastructure development, geothermal power plants have their own unique social and environmental impacts and risks that require awareness and management. All these factors can significantly affect the time and cost required to move through the subsequent phases of project. The reconnaissance survey aims to show whether the area of interest has a geological setting or features that may indicate the presence of an economically exploitable geothermal system. Once this is established, the project developer must then determine the feasibility of obtaining concessions over the most promising areas and, if they become productive, how would geothermal power fit with the existing energy infrastructure. Although the reconnais‐ sance survey phase is primarily "desk based," one or more short field visits might greatly assist in confirming the geothermal play type(s), the regional geology, the surface thermal features, and in identifying key environmental and social issues. In general, the basic information collected during the reconnaissance survey phase covers:


Based on the outcomes of the reconnaissance survey, the explorer or developer may decide to proceed to the next step: "exploration phase." Obtaining finance and/or partners to share the risks and expenses of this phase may also be necessary. There may be several potential sites to investigate, which could effectively spread the risk but require higher overall expenditures. Engaging experienced geothermal consultants during the reconnaissance survey phase is one of the keys to identifying and thoroughly assessing relevant background information, identifying possible non-geological issues, and designing an effective forward exploration program. The time required for the reconnaissance survey phase depends on a range of factors. The time may be as short as several months. However, if there are many potential sites to investigate and if environmental approvals and the permit process are complex and finance is difficult to secure, the survey may take a year or longer.

## **3. Geochemical surveys**

Geochemical surveys are used to determine whether the geothermal reservoir is water or vapor dominated, estimate the minimum temperature expected at depth, estimate the homogeneity of the water supply, infer the chemical characteristics of the deep fluid, and determine the source of recharge water. The geochemical survey consists of sampling and chemical and/or isotope analyses of the water and gas from geothermal manifestations (e.g., hot springs, fumaroles) or wells. This survey provides useful data for planning exploration, and its cost is relatively low compared to other more sophisticated methods, such as the geophysical surveys. Geochemical surveys with the use of tracers can also offer information on the direction of movement of subsurface groundwater and of reinjected fluids.

## **3.1. Field and laboratory analysis**

obtaining concessions over the most promising areas and, if they become productive, how would geothermal power fit with the existing energy infrastructure. Although the reconnais‐ sance survey phase is primarily "desk based," one or more short field visits might greatly assist in confirming the geothermal play type(s), the regional geology, the surface thermal features, and in identifying key environmental and social issues. In general, the basic information

**•** Other/additional demands and possibilities for geothermal energy use such as district or

**•** Resource ownership issues (in some countries, geothermal permits are under mining laws; elsewhere, it may be considered a water right under specific geothermal legislation; or a

**•** Information from available literature on any known geothermal systems, including geological, hydrological, and/or hot spring/thermal data and historic exploration data. **•** Information from previous explorations or wells that may have been drilled in the area of

Based on the outcomes of the reconnaissance survey, the explorer or developer may decide to proceed to the next step: "exploration phase." Obtaining finance and/or partners to share the risks and expenses of this phase may also be necessary. There may be several potential sites to investigate, which could effectively spread the risk but require higher overall expenditures. Engaging experienced geothermal consultants during the reconnaissance survey phase is one of the keys to identifying and thoroughly assessing relevant background information, identifying possible non-geological issues, and designing an effective forward exploration program. The time required for the reconnaissance survey phase depends on a range of factors. The time may be as short as several months. However, if there are many potential sites to investigate and if environmental approvals and the permit process are complex and finance

Geochemical surveys are used to determine whether the geothermal reservoir is water or vapor dominated, estimate the minimum temperature expected at depth, estimate the homogeneity

**•** The power market and possible power purchase agreements or feed-in tariff.

**•** Collection and interpretation of available remote sensing or aerial survey data.

**•** Infrastructure issues (roads, water, communication, transmission).

collected during the reconnaissance survey phase covers:

relevant legal framework might not yet exist).

**•** Issues relating to political and financial stability.

is difficult to secure, the survey may take a year or longer.

**3. Geochemical surveys**

**•** Environmental and social issues.

**•** Institutional and regulatory frameworks.

greenhouse heating.

152 Advances in Geothermal Energy

interest.

From the reconnaissance field, comparison between the shape and type of topography, and the thermal emissions that existed can roughly suggest the presence of a high enthalpy system at depth. Then, the chemical composition analysis can definitely suggest whether that area deserves to be further investigated using geophysical methods and eventually through deep drilling. In general, geochemical analysis/application (e.g., geothermometers) in liquid and gas phase gives reliable estimates of temperature of the sources of fluids, whose depth cannot generally be derived by the geochemical prospecting.

For the above reasons, fieldworks for sampling includes thermal water and gas samples, associated either with the springs or as dry emissions, as well as hydrothermal deposits around the sites of investigations. For example, in silicic formations, thermal springs, at the surface, precipitate silica and Fe hydroxides. In some cases, travertine precipitates from thermal springs [7,2], such as those areas located along the Tethys orogenic belt giving rise to huge deposits, and sometimes 100 m thick. During spring's sampling, temperature, pH, electrical conductiv‐ ity, as well as the concentration of HCO3 must be determined in the field (special bottle must be used for silica).

In fact, as silica is an important parameter in geothermal exploration, hot springs precipitate silica after cooling, and thermal spring water is generally stored diluted 1:10, in a separate plastic bottle in order to avoid silica precipitation during sampling. Another important factor in geothermal exploration, ammonia, should be analyzed in the field, using portable spectro‐ photometers. If this is not available, a fraction of the sampled water must be acidified to prevent both oxidation of ammonia and oxidation of other cations, as well as Ca, Fe, etc. precipitation as CaCO3, Fe(OH)3, etc.

Gas sampling is much more important than the springs. With respect to spring waters, which undergo easily dilution and mixing during underground motion, that quite often do not allow to assess the real composition of the original deep hydrothermal solutions, the gas phase rising the crust is less sensitive to dilution and mixing with shallow gases (i.e., atmospheric). Currently, sampling techniques for gases are quite well developed [3]. Air contamination should be avoided during sampling. There are a large number of components that can be measured in both liquid and gas phases. The geothermal prospector's minimum requirement in liquid/gas phase is listed in Table (1).


**Table 1.** Minimum requirements for liquid and gas samples

## **4. Geophysical surveys**

Geophysical surveys are essential tools in geothermal exploration [12]. They allow us to detect rock and fluid properties and the existence of reservoirs and permeability pathways. Geo‐ physical surveys (e.g., seismic, gravity, magnetic, electrical/electromagnetic, and thermal) can be defined as indirect methods. These methods, in fact, are not directly associated with the properties of the hot fluids that are being sought. Rather, they yield information about the attitude and nature of the host rocks. Detailed description of these methods will be discussed in the context.

## **4.1. Seismic survey**

Seismic survey measures the "acoustic impedance" (the product of rock density and seismic velocity). Seismic survey can be divided into two subcategories based on the source of the seismic signal, artificial or natural source (commonly known as passive and active sources). Both the artificial source, such as seismic vibrator (commonly known as vibroseis), and the natural sources, such as earthquakes, volcanic eruptions, or other tectonic activity, generate seismic signal. The surveys can yield important information on the location and orientation of subsurface structures, such as faults and rock discontinuities, which may help to explain the fluid flow.

#### *4.1.1. Seismic Survey Concepts*

A seismic survey is an "active" technique that images boundaries between layers of different acoustic impedance and requires a controlled source of seismic energy, such as seismic vibrators, dynamite explosives, or air guns for marine surveys. The general principle of seismic reflection is to send elastic waves from the source (e.g., seismic signal) into the underground, where each layer reflects a part of the wave's energy and allows the rest to refract through. A number of seismic receivers (geophones) that sense the motion of the ground in which they are placed record the reflected wave field at the surface (Figure 2). Surveys can be designed to image the underground along a profile (2D survey) or within a volume (3D survey).

Processed seismic data are most commonly presented as cross sections or slices (horizontal and vertical) from a seismic cube, with two-way travel time converted to true depth using the

(Source: HarbourDom GmbH, Germany)

**Liquid Gas**

Isotope ratio 18O/16O and D/H 13C/12C ratio in CO2 CH4 and 3

Geophysical surveys are essential tools in geothermal exploration [12]. They allow us to detect rock and fluid properties and the existence of reservoirs and permeability pathways. Geo‐ physical surveys (e.g., seismic, gravity, magnetic, electrical/electromagnetic, and thermal) can be defined as indirect methods. These methods, in fact, are not directly associated with the properties of the hot fluids that are being sought. Rather, they yield information about the attitude and nature of the host rocks. Detailed description of these methods will be discussed

Seismic survey measures the "acoustic impedance" (the product of rock density and seismic velocity). Seismic survey can be divided into two subcategories based on the source of the seismic signal, artificial or natural source (commonly known as passive and active sources). Both the artificial source, such as seismic vibrator (commonly known as vibroseis), and the natural sources, such as earthquakes, volcanic eruptions, or other tectonic activity, generate seismic signal. The surveys can yield important information on the location and orientation of subsurface structures, such as faults and rock discontinuities, which may help to explain the

A seismic survey is an "active" technique that images boundaries between layers of different acoustic impedance and requires a controlled source of seismic energy, such as seismic vibrators, dynamite explosives, or air guns for marine surveys. The general principle of seismic reflection is to send elastic waves from the source (e.g., seismic signal) into the underground, where each layer reflects a part of the wave's energy and allows the rest to refract through. A number of seismic receivers (geophones) that sense the motion of the ground in which they are placed record the reflected wave field at the surface (Figure 2). Surveys can be designed to

Processed seismic data are most commonly presented as cross sections or slices (horizontal and vertical) from a seismic cube, with two-way travel time converted to true depth using the

image the underground along a profile (2D survey) or within a volume (3D survey).

He/4

He ratio

Main component Ca, Mg, Na, K, HCO3, SO4, Cl CO2, N2, H2S, CH4

Minor component SiO2, NH4, B, Br, Sr He, Ar, Ne, H2

**Table 1.** Minimum requirements for liquid and gas samples

**4. Geophysical surveys**

154 Advances in Geothermal Energy

in the context.

fluid flow.

*4.1.1. Seismic Survey Concepts*

**4.1. Seismic survey**

**Figure 2.** Main components of a seismic survey.

seismic velocity model and seismic migration techniques. Interpreted sections typically show the most important seismic reflectors and faults as shown in Figures (3) and (4).

(Source: Erdwärme BayernGmbH & Co. KG)

**Figure 3.** Interpreted seismic reflection cross section with important reflectors highlighted.

(Source: Erdwärme BayernGmbH & Co. KG)

**Figure 4.** Interpreted seismic reflection cross section with interpreted faults highlighted.

#### **4.2. Gravity survey**

Gravity survey in geothermal exploration defines the lateral density variation related to deep magmatic body, which may represent the heat source. These anomalies can be created by different degrees of differentiation of magma or variation in depth of crust–mantle interface which creates also depth variation of isotherms. This survey is simple and easy to be carried out using gravimeter. Once the survey is done, some processing parameters should be taken into consideration. More details about these parameters can be obtained from Seigel [9]. Figure (5) shows an example of gravity surveys in geothermal sites in Japan. It shows that geothermal areas always have low gravity anomalies due to heat sources which change the physical properties in the subsurface rocks.

On the other hand, gravity monitoring surveys in geothermal areas are used to define the change in groundwater level and for subsidence monitoring. Fluid extraction from the ground which is not rapidly replaced causes an increase of pore pressure and hence of density. This effect may arrive at surface and produce a subsidence, whose rate depends on the recharge rate of fluid in the extraction area and the rocks interested by compaction. Repeated gravity monitoring associated with weather monitoring may define the relationship between gravity and precipitation which produces the shallow groundwater level change. When gravity is corrected by this effect, gravity changes show how much of the water mass discharged to the atmosphere is replaced by the natural inflow. The underground hydrological monitoring done by gravity survey is an important indication of the fluid recharge in geothermal systems and the need of reinjection.

**Figure 5.** Gravity anomaly map of four geothermal sites, Japan [5].

## **4.3. Magnetic surveys**

(Source: Erdwärme BayernGmbH & Co. KG)

properties in the subsurface rocks.

**4.2. Gravity survey**

156 Advances in Geothermal Energy

**Figure 4.** Interpreted seismic reflection cross section with interpreted faults highlighted.

Gravity survey in geothermal exploration defines the lateral density variation related to deep magmatic body, which may represent the heat source. These anomalies can be created by different degrees of differentiation of magma or variation in depth of crust–mantle interface which creates also depth variation of isotherms. This survey is simple and easy to be carried out using gravimeter. Once the survey is done, some processing parameters should be taken into consideration. More details about these parameters can be obtained from Seigel [9]. Figure (5) shows an example of gravity surveys in geothermal sites in Japan. It shows that geothermal areas always have low gravity anomalies due to heat sources which change the physical

The earth has a primary magnetic field, which induces a magnetic response in minerals at and near the earth's surface. By detecting spatial changes of the magnetic field, the variations in distribution of magnetic minerals may be deduced and related to geologic structure. In geothermal exploration, each magnetic mineral has a Curie temperature, above which it loses its magnetic properties. This phenomenon is used to detect zones which are magnetically featureless, due to destruction of magnetite in near-surface rocks by hydrothermal alteration. Figure (6) shows an example of Curie depth map of Sinai Peninsula and its relation to heat flow areas [1]. The map shows law Curie depth surface with high heat flow. The usefulness of magnetic surveys in geothermal exploration is controversial. On the other hand, magnetic method can be used to detect the subsurface structure within which the geothermal reservoir is build.

**Figure 6.** Curie depth contour map of Sinai Peninsula deduced from magnetic data. Background is topography relief. It can be recognized that, high heat flows match well with low Curie depths, indicating high geothermal potential [1].

## **4.4. Electrical/electromagnetic surveys**

Most electrical/electromagnetic methods are used to measure the electrical resistivity of the subsurface rocks. Resistivity in the earth is often largely affected by electrical conduction within waters occupying the pore spaces in the rock. Consequently, resistivity varies consid‐ erably with porosity. Temperature and salinity of interstitial fluids tend to be higher in geothermal reservoirs than in the surrounding rocks. Consequently, the resistivity of geother‐ mal reservoirs is generally relatively low.

In electrical methods, current is injected into the earth and the potential difference from which the subsurface resistivity can be obtained is measured. On the other hand, electromagnetic methods are a tool for determining the electrical resistivity distribution in the earth by means of surface measurements of transient electric and magnetic fields. These fields can be naturally or artificially generated. These methods are more suitable for measuring the low resistivities of geothermal reservoirs than the above mentioned electrical resistivity methods. Furthermore, in geothermal areas, the surface resistivity is sometimes so high as to prevent current from entering the ground, and the electromagnetic methods, with a much deeper penetration, help eliminate the screening effect of very resistive surface rocks. Currents of varying frequency are transmitted into the ground, either via the electrodes as in the electrical methods, or by induction loops. Mobile stations measure, at several points, the electrical and magnetic fields created by this transmission. Comparison between these fields enables the resistivities of the underlying formations to be obtained, as a function of the frequency used that is as a function of the depth, as in the magnetotelluric (MT) soundings.

The magnetotelluric (MT) method responds to the earth's electrical resistivity structure [10]. The method involves taking a time series recording of natural, low-frequency, orthogonal electric and magnetic fields at the earth's surface, then interpreting the data in the frequency domain. Natural fluctuations in the earth's magnetic field are generated by lightning, iono‐ spheric resonances, or variations in the solar wind. These fluctuations induce electric currents (or telluric currents) beneath the surface of the earth. The ratio of the electric field to the magnetic field in the induced electromagnetic (EM) wave is a function of the frequency of the signal and the bulk electrical resistivity of the ground. Lower-frequency magnetic fluctuations induce currents through a greater thickness of ground (Figure 7). Recording data over a wide frequency spectrum effectively gives information about a great thickness of ground. Lowerfrequency records (i.e., information about greater depths) require longer collection times.

<sup>(</sup>Source: Harbour‐Dom GmbH, Germany)

Figure (6) shows an example of Curie depth map of Sinai Peninsula and its relation to heat flow areas [1]. The map shows law Curie depth surface with high heat flow. The usefulness of magnetic surveys in geothermal exploration is controversial. On the other hand, magnetic method can be used to detect the subsurface structure within which the geothermal reservoir

**Figure 6.** Curie depth contour map of Sinai Peninsula deduced from magnetic data. Background is topography relief. It can be recognized that, high heat flows match well with low Curie depths, indicating high geothermal potential [1].

Most electrical/electromagnetic methods are used to measure the electrical resistivity of the subsurface rocks. Resistivity in the earth is often largely affected by electrical conduction within waters occupying the pore spaces in the rock. Consequently, resistivity varies consid‐ erably with porosity. Temperature and salinity of interstitial fluids tend to be higher in geothermal reservoirs than in the surrounding rocks. Consequently, the resistivity of geother‐

In electrical methods, current is injected into the earth and the potential difference from which the subsurface resistivity can be obtained is measured. On the other hand, electromagnetic methods are a tool for determining the electrical resistivity distribution in the earth by means of surface measurements of transient electric and magnetic fields. These fields can be naturally

**4.4. Electrical/electromagnetic surveys**

mal reservoirs is generally relatively low.

is build.

158 Advances in Geothermal Energy

**Figure 7.** MT station layout and skin depths for natural electromagnetic waves depending on frequency, Low frequen‐ cies respond to deep structures, high frequencies respond to shallow structures.

The MT method is one of the very few geophysical techniques that can provide information about rock units deeper than about 1 km. This makes it useful for geothermal exploration, where target depths are typically in the range of 1–3 km for convection-dominated geothermal plays and even deeper for conduction-dominated plays. The MT method is particularly useful for convection-dominated plays because it can potentially imagine low resistivity and low permeability smectite clay units that often cap high-enthalpy geothermal reservoirs [6]. For this reason, the MT method is often used to reduce uncertainties about reservoir depth, geometry, and areal extent.

MT surveys can be performed at a regional scale. In these cases, the station spacing may be less than one per square kilometer. It is usually more cost effective to identify a prospective area with other methods and then conduct an MT survey with relatively high station spatial density in that area, with perhaps as many as 10–15 stations per square kilometer.

Magnetotelluric data are normally interpreted through an "inversion" process, whereby a semiautomated algorithm determines the simplest and most likely "apparent resistivity" structure consistent with the collected data. Inversions can also be carried out in 1D, 2D, or 3D, referring to both the spatial distribution of recording stations and the dimensions of the model simultaneously solved. A 1D inversion produces a vertical "sounding" from a single station; a 2D inversion, a profile from a line of stations; and a 3D inversion, a self-consistent block model from an array of stations [11]. Higher-dimensional inversions require significantly greater computing power and time to complete.

Inversions might be carried out by the MT contractor or by an independent third party. Inversion algorithms typically need to be constrained in some way, usually through limiting the allowable number of discrete layers and/or the depths between layers. For this reason, inversion results are subjective because they depend on input from the data processor. The results from 1D, 2D, and 3D inversions can differ significantly from each other for the same set of data, because the models depend on the dimensionality and complexity associated with the magnetotelluric responses. The resolution and accuracy of inversion models in terms of both depth and apparent resistivity decrease with depth.

The results of magnetotelluric inversion are normally presented as apparent resistivity on 1D soundings, 2D profiles (Figure 8), or 3D block (Figure 9).

**Figure 8.** MT cross section profile showing the conductive zone at 13 km [8].

(Source: GNS Science, New Zealand)

plays and even deeper for conduction-dominated plays. The MT method is particularly useful for convection-dominated plays because it can potentially imagine low resistivity and low permeability smectite clay units that often cap high-enthalpy geothermal reservoirs [6]. For this reason, the MT method is often used to reduce uncertainties about reservoir depth,

MT surveys can be performed at a regional scale. In these cases, the station spacing may be less than one per square kilometer. It is usually more cost effective to identify a prospective area with other methods and then conduct an MT survey with relatively high station spatial

Magnetotelluric data are normally interpreted through an "inversion" process, whereby a semiautomated algorithm determines the simplest and most likely "apparent resistivity" structure consistent with the collected data. Inversions can also be carried out in 1D, 2D, or 3D, referring to both the spatial distribution of recording stations and the dimensions of the model simultaneously solved. A 1D inversion produces a vertical "sounding" from a single station; a 2D inversion, a profile from a line of stations; and a 3D inversion, a self-consistent block model from an array of stations [11]. Higher-dimensional inversions require significantly

Inversions might be carried out by the MT contractor or by an independent third party. Inversion algorithms typically need to be constrained in some way, usually through limiting the allowable number of discrete layers and/or the depths between layers. For this reason, inversion results are subjective because they depend on input from the data processor. The results from 1D, 2D, and 3D inversions can differ significantly from each other for the same set of data, because the models depend on the dimensionality and complexity associated with the magnetotelluric responses. The resolution and accuracy of inversion models in terms of

The results of magnetotelluric inversion are normally presented as apparent resistivity on 1D

Resistive Basement

V15 V16 V17 V62 V18 V61 V60 V09 V08

SW NE

0 -2000 -4000 -6000 -8000 -10000 -12000 -14000

ohm.m

0 5 10 15 20 25 30 35

Conductive zone

Resistivity boundary Inferred Fault

density in that area, with perhaps as many as 10–15 stations per square kilometer.

geometry, and areal extent.

160 Advances in Geothermal Energy

greater computing power and time to complete.

both depth and apparent resistivity decrease with depth.

soundings, 2D profiles (Figure 8), or 3D block (Figure 9).

Resistive Basement

**Figure 8.** MT cross section profile showing the conductive zone at 13 km [8].

0 -2000 -4000 -6000 -8000 -10000 -12000 -14000

masl (m) **Figure 9.** MT resistivity block model.

#### **4.5. Thermal measurement surveys**

In geothermal research, the traditional geophysical methods mentioned above are used side by side with more specific techniques. Geothermal prospecting provides information on the thermal conditions of the subsurface, the aerial distribution of the earth's heat flow, and the location and intensity of thermal anomalies. To be more specific, geothermal prospecting allows us:


Heat flow measurements are made by drilling small diameter (4 inches, 10 cm), shallow wells (< 300 m). Generally, heat flow is measured every 10–25 km2 . The geothermal gradient is obtained from temperatures measured with electric thermometers at various depths along a well. Temperature logging is quick and relatively inexpensive. The thermal conductivity of the rocks in the interval in which the gradient has been measured is usually determined by laboratory measurements on core samples. The product of the gradient and conductivity gives the heat flow.

## **5. Exploratory wells**

The final stage of an exploration survey is exploratory well drilling. Usually, the final diameters of these wells are on the order of 8 inches (20 cm) or less, allowing the insertion of special logging tools to measure various parameters from the surface to total depth, and sometimes to carry out fluid production tests. A pump may be lowered into a shallow hot water well some hundreds of meters deep, and compressed air (gas lift) may be injected in deeper hot water wells. Since most geothermal reservoirs are made up of fluid-filled fractures, it is essential that an exploratory well intersects as many fractures as possible. Since natural fractures are related to tectonic activity (folding and faulting), the siting of exploratory wells is greatly dependent on our geologic interpretation of the local structural conditions.

## **6. Conceptual model**

As indicated above, the preliminary survey, exploration, and test drilling phases of a project are all about defining, refining, and testing a "conceptual model" of the geothermal system under investigation; a conceptual model is the schematic representation. A good conceptual model should encapsulate the geological framework, heat source, heat and fluid migration pathways, reservoir characteristics, and surface geothermal features, and should be consistent with all available data and information. The conceptual model is continually refined as each new set of data is collected and assessed, with each refinement adding a new level of detail or confidence to the overall model.

An initial conceptual model should be developed at the earliest stages of the geothermal project. At this time, the model will necessarily be quite crude, perhaps illustrating little more than a generic representation of the expected geothermal play type. The model should then be regularly updated as new data become available to ensure the model respects and remains consistent with all known information. In this way, the most current conceptual model should incorporate all available exploration data. By the end of the exploration phase, the conceptual model should be of sufficient detail to allow an estimate of reservoir depth, temperature, and geometry with sufficient confidence to justify and site wells for the test drilling phase.

The conceptual model can be illustrated with maps, 2D cross sections, or 3D block models. These might be simple free-form drawings at the early stages of a project, but will develop into robust geological models as more information is incorporated. Cross sections should be created at the same scale as the maps that underpin them, preferably with a 1:1 ratio between horizontal and vertical scales. All diagrams should include a representation of the assumed heat source, an estimate of the subsurface temperature distribution (isotherms), some indication of fluid flow directions, and a representation of the expected geothermal reservoir, even if these are only approximate.

#### (Source: GeothermEx Inc., California)

the rocks in the interval in which the gradient has been measured is usually determined by laboratory measurements on core samples. The product of the gradient and conductivity gives

The final stage of an exploration survey is exploratory well drilling. Usually, the final diameters of these wells are on the order of 8 inches (20 cm) or less, allowing the insertion of special logging tools to measure various parameters from the surface to total depth, and sometimes to carry out fluid production tests. A pump may be lowered into a shallow hot water well some hundreds of meters deep, and compressed air (gas lift) may be injected in deeper hot water wells. Since most geothermal reservoirs are made up of fluid-filled fractures, it is essential that an exploratory well intersects as many fractures as possible. Since natural fractures are related to tectonic activity (folding and faulting), the siting of exploratory wells is greatly dependent

As indicated above, the preliminary survey, exploration, and test drilling phases of a project are all about defining, refining, and testing a "conceptual model" of the geothermal system under investigation; a conceptual model is the schematic representation. A good conceptual model should encapsulate the geological framework, heat source, heat and fluid migration pathways, reservoir characteristics, and surface geothermal features, and should be consistent with all available data and information. The conceptual model is continually refined as each new set of data is collected and assessed, with each refinement adding a new level of detail or

An initial conceptual model should be developed at the earliest stages of the geothermal project. At this time, the model will necessarily be quite crude, perhaps illustrating little more than a generic representation of the expected geothermal play type. The model should then be regularly updated as new data become available to ensure the model respects and remains consistent with all known information. In this way, the most current conceptual model should incorporate all available exploration data. By the end of the exploration phase, the conceptual model should be of sufficient detail to allow an estimate of reservoir depth, temperature, and geometry with sufficient confidence to justify and site wells for the test drilling phase.

The conceptual model can be illustrated with maps, 2D cross sections, or 3D block models. These might be simple free-form drawings at the early stages of a project, but will develop into robust geological models as more information is incorporated. Cross sections should be created at the same scale as the maps that underpin them, preferably with a 1:1 ratio between horizontal and vertical scales. All diagrams should include a representation of the assumed heat source, an estimate of the subsurface temperature distribution (isotherms), some indication of fluid

on our geologic interpretation of the local structural conditions.

the heat flow.

162 Advances in Geothermal Energy

**5. Exploratory wells**

**6. Conceptual model**

confidence to the overall model.

**Figure 10.** Flowchart showing project stages with typical data acquired and integrated into the conceptual model.

A good conceptual model provides clear evidence that the explorer has considered and integrated all available data. Nothing in the conceptual model should contradict the data presented elsewhere, unless a clear rationale is provided. The conceptual model demonstrates a justifiable understanding of the geology, temperature, and fluid pathways within the geothermal system. By utilizing the conceptual model, the explorer can select sites for the test drilling phase that maximize the chances for a successful well based on all current data.

All exploration data should be integrated into a conceptual model of the geothermal system under investigation. This model must respect and be consistent with all known information. Figure (10) provides a flowchart of typical data that may be used to build and develop the model. The model needs to be of sufficient detail to allow a first-pass estimate of resource temperature and size and, in the test drilling phase, is used to target deep, full-diameter wells toward particular lithological units and/or structures that are judged most likely to deliver commercial rates of geothermal fluid at commercially viable temperatures.

## **Author details**

Essam Aboud1,2\*

Address all correspondence to: eaboud@gmail.com

1 Geohazards Research Center (GHRC), King Abdulaziz University, Jeddah, Saudi Arabia

2 National Research Institute of Astronomy and Geophysics (NRIAG), Cairo, Egypt

## **References**


neering Stanford University, Stanford, California, January 31-February 2, 2005 SGP-TR-176

[6] Melosh G, Cumming W, Casteel J, Niggemann K, Fairbank B (2010) Seismic Reflec‐ tion Data and Conceptual Models for Geothermal Development in Nevada.

A good conceptual model provides clear evidence that the explorer has considered and integrated all available data. Nothing in the conceptual model should contradict the data presented elsewhere, unless a clear rationale is provided. The conceptual model demonstrates a justifiable understanding of the geology, temperature, and fluid pathways within the geothermal system. By utilizing the conceptual model, the explorer can select sites for the test drilling phase that maximize the chances for a successful well based on all current data.

All exploration data should be integrated into a conceptual model of the geothermal system under investigation. This model must respect and be consistent with all known information. Figure (10) provides a flowchart of typical data that may be used to build and develop the model. The model needs to be of sufficient detail to allow a first-pass estimate of resource temperature and size and, in the test drilling phase, is used to target deep, full-diameter wells toward particular lithological units and/or structures that are judged most likely to deliver

1 Geohazards Research Center (GHRC), King Abdulaziz University, Jeddah, Saudi Arabia

[1] Aboud E, Salem A, Mekkawi M (2011) Curie depth map for Sinai Peninsula, Egypt

[2] Ford TD, Pedley HM (1996) A review of tufa and travertine deposits of the world.

[3] Giggenbach WF (1975) A simple method for the collection and analysis of volcanic

[4] Hochstein MP (1990) Classification and assessment of geothermal resources. In: Dick‐ son, M.H. and Fanelli, M., *eds*., *Small Geothermal Resources: A Guide to Development and*

[5] Koichi Tagomori, Enjang Mustopa, Hisashi Jotaki, Hideki Mizunaga and Keisuke Ushijima (2005) Proceedings, Thirtieth Workshop on Geothermal Reservoir Engi‐

2 National Research Institute of Astronomy and Geophysics (NRIAG), Cairo, Egypt

deduced from the analysis of magnetic data. Tectonophysics 506:46–54

commercial rates of geothermal fluid at commercially viable temperatures.

Address all correspondence to: eaboud@gmail.com

*Earth Sci. Rev*., 41:117–175

gas samples. *Bull. Volcanol.* 36:132–145

*Utilization*, UNITAR, New York, pp. 31—57

**Author details**

164 Advances in Geothermal Energy

Essam Aboud1,2\*

**References**


## *Edited by Basel I. Ismail*

Geothermal energy means the natural heat energy from the Earth. The geothermal resources of the Earth are huge and unlike other conventional and renewable energy sources, geothermal energy has unique features; namely, it is available, stable at all times throughout the year, independent of weather conditions, and has an inherent storage capability. Geothermal energy is also considered to be an environmentally friendly clean energy source that could significantly contribute to the reduction of GHG emissions. The utilization of geothermal energy is usually divided into the part used for electricity generation and the part used for heating applications. Due to its important utilization and future prospects, various interesting topics of research related to geothermal energy are covered in this book. This book is the result of contributions from several researchers and experts worldwide. It is hoped that the book will become a useful source of information and basis for extended research for researchers, academics, policy makers, and practitioners in the area of geothermal energy.

Advances in Geothermal Energy

Advances in

Geothermal Energy

*Edited by Basel I. Ismail*

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