**3.2 Geophysical study**

The geophysical study is comprised of several stages; first, the aeromagnetic information of study area is analyzed. This is done by applying a series of mathematical algorithms (filters) to the aeromagnetic data, which allow highlighting certain

**Figure 4.** *Geological block of the La Dulcita-villa de Ramos area, San Luis Potosí, Mexico.*

characteristics and dismiss others on the study area. The filters applied are known as International Geomagnetic Reference Field (IGRF) [8], which is calculated every 5 years and the immediate inferior should be applied to the date of the aerial survey (e.g., the aeromagnetic flight of our study area was carried out in 1995, the IGRF must be subtracted is that of 1990) [9]. To obtain the intensity values of the total magnetic field (TMF), which are obtained when flying, the contribution of the main dipole is subtracted, which exists in the terrestrial nucleus, thus obtaining the values of the residual magnetic field [RMF, Eq.( 1)].

$$\text{'TMF-IGRF'=RMF} \tag{1}$$

**31**

direct current.

intensity of the electric current:

(Amp) in the electric current.

*<sup>R</sup>* = \_

*<sup>R</sup>* = \_

<sup>ρ</sup> = \_

*The Magnetometry—A Primary Tool in the Prospection of Underground Water*

The orientation of the land magnetic profiles should be as far as possible perpendicular to the structure that is inferred. The processing and analysis of the ground magnetic information is similar to the one made to the aeromagnetic data, a RMF is obtained from the TMF, later RMPF is generated and to this field, the filters of

The magnetic information analysis is up to a one-point simple. This method is based on the fact that the whole Earth behaves like a large magnet that would be in the center of it [17, 18]. For a specific area, it is considered that the magnetic field strength (**H**) is the same, and that the value of the magnetic intensity will be a function of the magnetic susceptibility of the rocks [*k*; Eq. (2)], which is defined as the

This magnetization (I) constitutes the induced magnetization. Additionally, the effects of the remnant magnetization are present. In volcanic and intrusive rocks, this magnetization can be of greater intensity than the induced effects [18, 19]. If we consider that volcanic rocks contain ferromagnesian and is without fractures, it will generate a magnetic response characterized by having an anomaly represented by a magnetic high and a low. That is, the magnetic response has a positive and a negative side. If we make the simile that the rock is a magnet, and fractured it in two parts, we would generate two anomalies, which would have two magnetic highs and lows in a sequence, high-low-high-low magnetic, and so on. If we have a slightly fractured and/or faulty area, it will give us a magnetic response with highs and lows sequenced, with medium frequencies. In contrast, if we have a zone with highly fractured and/or high faulting, it will give us a magnetic response characterized by high frequencies and by a series of sequenced of magnetic highs and lows [16]. These areas are interpreted as zones where two of three of the properties that

identify the aquifers are present, as they have porosity and permeability.

Once the secondary permeability zone in the volcanic rocks has been identified, the next step in the methodology is to prove that the zones associated with humidity, which is achieved with the application of electrical methods; in our case, we use

The electric DC methods are based on Ohm's law [2], which establishes that the resistance is directly proportional to the voltage and inversely proportional to the

> V I

where R (Ohm, Ω) is the electrical resistance, V (Volts) the potential and I

ρL S

The equation in which ρ represents the nature of the conductor and is called

VS

nature and the geometry of the conductor (Eq. 4), in this case the Earth:

resistivity, L is the length in m and S the conductor section in m<sup>2</sup>

isolate the variable, then the following equation is given by:

The previous relationship (Eq. 3) is valid for any electrical circuit, in studies of the underground, the relationship changes since the resistance is a function of the

> , <sup>ρ</sup> = \_ RS L

I = *k***H** (2)

(3)

(4)

IL (5)

. If we replace and

*DOI: http://dx.doi.org/10.5772/intechopen.84322*

derivatives and upward continuation are applied.

capacity of these to acquire magnetization, such that:

Since the magnetic field is a vector (defined by magnitude and direction), the magnetic anomalies in these latitudes are displaced from the sources that produce them. Therefore, this is the reason why other mathematical algorithms must be applied for a filter, which simulates our study area, where the magnetic inclination is 90° and the declination is 0°. This algorithm named Baranov and Naudy [10] is better known as reduction to the magnetic pole field (RMPF) and assures us theoretically that magnetic anomalies will be located in the sources that produce them. The data matrix, thus generated, is the basis for the application of other filters or mathematical algorithms.

In arid volcanic areas, one of the opportunities to locate groundwater is in the confined aquifers on faults. A filter is a mathematical tool to guide us, if we want to know the fracture, faults or the contacts zones in the geological units a filter that provides us with guidance is the Henderson and Zietz [11]. This filter is known as vertical derivatives of first or second degree, because it is going to indicate the areas of high gradients which are normally associated with the geological structures mentioned above. Another filter that has been applied to aeromagnetic information is the Henderson [12] that allows us to change the plane of observation, when we rise, the high frequencies tend to attenuate and highlight the low frequencies, which are associated with the geological structure of the subsoil. This filter is known as the magnetic field upward continuation.

It is possible to interpret the location of the superficial and deep permeability zones with the analysis of the magnetic contour maps through each of these filters [13, 14].

The next step of the methodology, after the aeromagnetic information has been analyzed, is to perform a land magnetometry survey in the areas that have presented some possibility of being associated with fracturing and/or faulting and/or geological contacts [15, 16]. This stage is called anomaly verification.

*The Magnetometry—A Primary Tool in the Prospection of Underground Water DOI: http://dx.doi.org/10.5772/intechopen.84322*

The orientation of the land magnetic profiles should be as far as possible perpendicular to the structure that is inferred. The processing and analysis of the ground magnetic information is similar to the one made to the aeromagnetic data, a RMF is obtained from the TMF, later RMPF is generated and to this field, the filters of derivatives and upward continuation are applied.

The magnetic information analysis is up to a one-point simple. This method is based on the fact that the whole Earth behaves like a large magnet that would be in the center of it [17, 18]. For a specific area, it is considered that the magnetic field strength (**H**) is the same, and that the value of the magnetic intensity will be a function of the magnetic susceptibility of the rocks [*k*; Eq. (2)], which is defined as the capacity of these to acquire magnetization, such that:

$$\mathbf{I} = k \mathbf{H} \tag{2}$$

This magnetization (I) constitutes the induced magnetization. Additionally, the effects of the remnant magnetization are present. In volcanic and intrusive rocks, this magnetization can be of greater intensity than the induced effects [18, 19]. If we consider that volcanic rocks contain ferromagnesian and is without fractures, it will generate a magnetic response characterized by having an anomaly represented by a magnetic high and a low. That is, the magnetic response has a positive and a negative side. If we make the simile that the rock is a magnet, and fractured it in two parts, we would generate two anomalies, which would have two magnetic highs and lows in a sequence, high-low-high-low magnetic, and so on. If we have a slightly fractured and/or faulty area, it will give us a magnetic response with highs and lows sequenced, with medium frequencies. In contrast, if we have a zone with highly fractured and/or high faulting, it will give us a magnetic response characterized by high frequencies and by a series of sequenced of magnetic highs and lows [16]. These areas are interpreted as zones where two of three of the properties that identify the aquifers are present, as they have porosity and permeability.

Once the secondary permeability zone in the volcanic rocks has been identified, the next step in the methodology is to prove that the zones associated with humidity, which is achieved with the application of electrical methods; in our case, we use direct current.

The electric DC methods are based on Ohm's law [2], which establishes that the resistance is directly proportional to the voltage and inversely proportional to the intensity of the electric current:

$$R = \frac{V}{I} \tag{3}$$

where R (Ohm, Ω) is the electrical resistance, V (Volts) the potential and I (Amp) in the electric current.

The previous relationship (Eq. 3) is valid for any electrical circuit, in studies of the underground, the relationship changes since the resistance is a function of the nature and the geometry of the conductor (Eq. 4), in this case the Earth:

$$R = \frac{\rho L}{S}, \quad \rho = \frac{\text{RS}}{\text{L}} \tag{4}$$

The equation in which ρ represents the nature of the conductor and is called resistivity, L is the length in m and S the conductor section in m<sup>2</sup> . If we replace and isolate the variable, then the following equation is given by:

$$\mathbf{p} = \frac{\mathbf{V}\mathbf{S}}{\mathbf{I}\mathbf{L}}\tag{5}$$

*Magnetometers - Fundamentals and Applications of Magnetism*

of the residual magnetic field [RMF, Eq.( 1)].

**Figure 4.**

magnetic field upward continuation.

characteristics and dismiss others on the study area. The filters applied are known as International Geomagnetic Reference Field (IGRF) [8], which is calculated every 5 years and the immediate inferior should be applied to the date of the aerial survey (e.g., the aeromagnetic flight of our study area was carried out in 1995, the IGRF must be subtracted is that of 1990) [9]. To obtain the intensity values of the total magnetic field (TMF), which are obtained when flying, the contribution of the main dipole is subtracted, which exists in the terrestrial nucleus, thus obtaining the values

*Geological block of the La Dulcita-villa de Ramos area, San Luis Potosí, Mexico.*

Since the magnetic field is a vector (defined by magnitude and direction), the magnetic anomalies in these latitudes are displaced from the sources that produce them. Therefore, this is the reason why other mathematical algorithms must be applied for a filter, which simulates our study area, where the magnetic inclination is 90° and the declination is 0°. This algorithm named Baranov and Naudy [10] is better known as reduction to the magnetic pole field (RMPF) and assures us theoretically that magnetic anomalies will be located in the sources that produce them. The data matrix, thus generated, is the basis for the application of other filters or mathematical algorithms. In arid volcanic areas, one of the opportunities to locate groundwater is in the confined aquifers on faults. A filter is a mathematical tool to guide us, if we want to know the fracture, faults or the contacts zones in the geological units a filter that provides us with guidance is the Henderson and Zietz [11]. This filter is known as vertical derivatives of first or second degree, because it is going to indicate the areas of high gradients which are normally associated with the geological structures mentioned above. Another filter that has been applied to aeromagnetic information is the Henderson [12] that allows us to change the plane of observation, when we rise, the high frequencies tend to attenuate and highlight the low frequencies, which are associated with the geological structure of the subsoil. This filter is known as the

It is possible to interpret the location of the superficial and deep permeability zones with the analysis of the magnetic contour maps through each of these filters [13, 14]. The next step of the methodology, after the aeromagnetic information has been

analyzed, is to perform a land magnetometry survey in the areas that have presented some possibility of being associated with fracturing and/or faulting and/or

geological contacts [15, 16]. This stage is called anomaly verification.

TMF − IGRF = RMF (1)

**30**

In geoelectrical exploration, the resistivity of the underground is normally measured with an electrode arrangement of four electrodes, with the electrodes AB being the emission electrodes (current) and MN the potential electrodes.

In this case, the resistivity (Eq. 6) is given by:

$$
\rho \mathbf{a} = \frac{\mathbf{V}}{\mathbf{I}} \times 2\pi \mathbf{K} \tag{6}
$$

where K is the geometric factor (1/AM-1/BM-1/AN+1/BN) of the electrode array, the subscript "a" in the resistivity indicates that the calculated value is apparent.

The resistivity is an inverse property of the electrical conductivity and in exploration, its units is ohm per meter (Ω/m).

In geoelectric exploration, the variation of the resistivity is studied horizontally by means of profiles in which the electrode array is moved as a whole to the different stations. Conversely, equispaced or the vertical variation of the resistivity can be studied by means of vertical electric soundings (VES). At a certain point, for this, the current electrodes (AB) are increasingly opened and the measuring or potential electrodes (MN) are opened only when the measured values are very small (Schlumberger electrode array). In such manner in which data exist in one or two points with different MN opening for the same values of AB, there is an overlap or "cluth" during the measurement of the SEV [1].

A quick way to know the electrical behavior of the underground in a given area is to make profiles of electrical resistivity of two electrode openings, for example, at 200 and 400 m opening of the current electrodes with the Schlumberger electrode array (AB/2 at 100 and 200 m). In this way, we have knowledge of the variation of electrical resistivity in a horizontal direction.

If the resistivity behaves similar to both electrode array separations, it will imply that the entire electrically scanned area is the same. If the resistivity of the profile generated with a larger electrode aperture is higher than that generated at a smaller aperture, it will indicate that at depth the possibility of detecting humidity is zero. On the other hand, if the resistivity is lower at larger electrode aperture, it will have greater chances of detecting humidity. If the profiles of apparent resistivity show an irregular behavior, it will have greater possibilities of detecting a resistive resistance where the resistivity at the largest aperture changes form more resistive to less resistive and indicates that the area under study shows in the underground, where the current circulates more easily and will be an area where the variation of resistivity with depth must be studied, which is done with vertical electric sounding (SEV, [20]).

The VES' must be interpreted qualitatively and quantitatively. Firstly, the morphology of the VES curve must be defined [21] which in order to be associated with humidity, must necessarily have a correlation with the H-type curves (ρ1 > ρ2 < ρ3), which indicates that there is a lower resistivity contrast between the central layer and those that enclose it. The VES curves can also be KH (ρ1 < ρ2 > ρ3 < ρ4), QH (ρ1 > ρ2 > ρ3 < ρ4) or some of the curves that show a portion of type H.

The quantitative interpretation is carried out using commercial software that allows an inversion of the resistivity data [22]. It is convenient to perform a VES in wells where its stratigraphic column is known, in a way that the VES can be calibrated.

Once the previous stages have been carried out, the zones that are chosen for drilling must have a magnetic response which correlates with a fractured zone (permeability) and electrical methods (resistivity) with an area that has a relation with a humid area, represented by a resistive contrast that contains a minimum between two resistivity maxima.

**33**

**Figure 5.**

*Potosí, Mexico.*

*The Magnetometry—A Primary Tool in the Prospection of Underground Water*

The procedure described above has been applied to an area, which is located in the Mesa Central, Mexico, specifically to a rural population called La Dulcita,

The area under study was flown by the Mexican Geological Service, using an Islander aircraft BN2-A21, equipped with a Geometrics G-822 magnetometer, of cesium vapor optical pump, with a sensitivity of 0.25 nT, and an acquisition system of P-101 Picodas data, Automax video camera, 35 mm. A Geometrics G-826A magnetometer was used, with a sensitivity of 1 nT as the base station. Also, Sperry

The course of the flight lines was N-S, with a distance between flight lines of 1000 m

The magnetic field behavior analysis began with the generation of the RMF map (**Figure 5**), which as mentioned in previous paragraphs, is obtained by subtracting the IGRF from the total magnetic field. Based on the RMF, the RMPF was calculated (**Figure 6**). In the W portion of the RMPF, there is a "trend" of magnetic highs (red) that represent the W limit of an area of the graben that exists with a general direction N-S and is characterized on the map with anomalies associated with magnetic lows (blue color). Towards the central portion, two "trends" of magnetic anomalies with direction NE–SW and NNW–SSE are shown that are possibly associated with the geologically multiple intrusive "El Socorro" [7]. The Dulcita area is located on the first step of the graben and alignments (**Figure 7**) with direction N-S and E-W towards its portion W is observed, which can be geologically associated with zones of faults and/or fracturing and/or contacts. The area investigated in

*Map showing the isovalues contour of the residual magnetic field of the Dulcita area, Villa de Ramos, San Luis* 

and a height above ground level of 300 m, the navigation was controlled with an Ashtech GG24 GPS system and the data was subtracted from the IGRF 1990 reference. The total magnetic field intensity in the central portion was 44,858 nT, with an

inclination of 50°43′ and declination of 8°13′ for July 1995.

*DOI: http://dx.doi.org/10.5772/intechopen.84322*

municipality of Villa de Ramos, San Luis Potosí.

**4. Results**

**4.1 Air Magnetometry**

altimeter radar was also used.

*The Magnetometry—A Primary Tool in the Prospection of Underground Water DOI: http://dx.doi.org/10.5772/intechopen.84322*
