5.1. Ground penetrating radar

Ground Penetrating Radar (GPR) as a geophysical technique is relative new and becoming increasingly popular critically understanding the events of the near-surface or shallow subsurface. Davis and Annan [13] viewed the Ground Penetrating Radar (GPR) as a technique of imaging the subsurface at high resolution using electromagnetic waves transmitted at frequencies between 10 to 1000 MHz. GPR could also be viewed as a non-destructive geophysical technique due to its successful geological applications in urban and sensitive environments. Some of these applications include the subsurface mapping of water table soils and rocks structures (e.g. groundwater channels) at high resolutions. It is similar in principle to seismic reflection profiling in however, propagation of radar waves through the subsurface is controlled by electrical properties at high frequencies.

The GPR survey system is made up of three vital components; a transmitter, a receiver directly connected to the antenna and the control unit (Figure 10). The transmitter radiates EM waves into the subsurface that could be refracted, diffracted or primarily reflected depending on the dielectric permittivity and electrical conductivity nature of the subsurface interfaces encountered. Recorded radar data received after the survey is first been observed, analyzed and interpreted by the aid of inbuilt radar processing software like RADPro, Ekko depending on the system type and make. These data are presented in form of radargrams which could either be presented as 2D or 3D subsurface images depending on the combination of the different axes (x, y and z) involved. Interpreting of radargrams is performed by interface mapping which is quite similar to the technique used in interpreting of seismograms. Here each band within on a radargram is presumably classed and identified as a distinct geological horizon;

Figure 10. Flow chart for a typical GPR system (after [13]).

that high frequency EM surveys yield better result for near-surface due to high resolution, however if interested in deeper subsurface investigation (low frequency EM surveys) then we have need a way around the low resolution. In the case of time domain system, secondary magnetic field is measured as a function of time, with early – time measurement being suited best for near-surface information while late- time measurement yields results of the deeper subsurface. It is paramount to note that depth of penetration or investigation and resolution is also been governed by coil configuration; while measurements from coil separations are influenced by electrical properties thus the larger coil separation investigates greater depths

Because Electrical Conductivity is related inversely to Electrical Resistivity, as such discussions relating electrical resistivity to lithology or hydrological properties can be applied in an inverse manner to measurements involving electrical conductivity. Electrical conductivity for example is higher for saturated sediments, clayey materials than for unsaturated sediments and sandy materials respectively. Some examples of investigations involving EM surveys include Sheets and Hendricks [11], who used EM induction methods to estimate soil water content and McNeill [12] that discussed the relation between electrical conductivity and hydrogeological

Ground Penetrating Radar (GPR) as a geophysical technique is relative new and becoming increasingly popular critically understanding the events of the near-surface or shallow subsurface. Davis and Annan [13] viewed the Ground Penetrating Radar (GPR) as a technique of imaging the subsurface at high resolution using electromagnetic waves transmitted at frequencies between 10 to 1000 MHz. GPR could also be viewed as a non-destructive geophysical technique due to its successful geological applications in urban and sensitive environments. Some of these applications include the subsurface mapping of water table soils and rocks structures (e.g. groundwater channels) at high resolutions. It is similar in principle to seismic reflection profiling in however, propagation of radar waves through the subsurface is con-

The GPR survey system is made up of three vital components; a transmitter, a receiver directly connected to the antenna and the control unit (Figure 10). The transmitter radiates EM waves into the subsurface that could be refracted, diffracted or primarily reflected depending on the dielectric permittivity and electrical conductivity nature of the subsurface interfaces encountered. Recorded radar data received after the survey is first been observed, analyzed and interpreted by the aid of inbuilt radar processing software like RADPro, Ekko depending on the system type and make. These data are presented in form of radargrams which could either be presented as 2D or 3D subsurface images depending on the combination of the different axes (x, y and z) involved. Interpreting of radargrams is performed by interface mapping which is quite similar to the technique used in interpreting of seismograms. Here each band within on a radargram is presumably classed and identified as a distinct geological horizon;

while smaller coil separation investigates near-surface.

5. Aquifer characterization – ground waves techniques

parameters of porosity and saturation.

trolled by electrical properties at high frequencies.

5.1. Ground penetrating radar

24 Aquifers - Matrix and Fluids

this would have been correct except for the effects of multiples, interference with previous reflections, noise etc. All this effects on the radargram need to be removed to correctly identify the different geological horizons and geological structures as present within the radargram as such radargram are subjected to varying radar processing operations depending on the aims, objective of the survey been undertaking through the help of inbuilt system radar processing software like RADpro, Pulse Ekko system software etc.

Processing of the radargram could be simplified by processing operations such as dewowing (removal of low frequency components), Gain Control (strengthen weaker events), deconvolution (restores shape of downgoing wave train such that primary events could be recognized more easily), Migration (useful in removing diffraction hyperbolae and restoring dips). The resultant radargram when correlated with the subsurface geology shows varying interfaces, geological structures that might be present (Figure 11a and b). Though GPR has successfully been utilized in unsaturated (non-electrically conductive or highly resistive) and saturated (electrically conductive) environment [14], however performance is higher in unsaturated (non-conductive) than in saturated (conductive) such as non-expanding clay environment such as at Savannah River Site in South Carolina [15].

The depth of penetration or investigation of GPR survey is function of the frequency of the EM waves or radar waves and nature of the subsurface material been investigated as shown in Figure 12 for varying subsurface materials at frequencies ranging between 1 and 500 MHz. If the nature of subsurface material is highly resistive and has low conductivity then we expect a higher depth of penetration however for subsurface materials that are less resistivity and very conductive we expect low depth of penetration. Depth of penetration asides from been dependent on nature of the subsurface material (i.e. resistivity or conductivity nature) is also a function of frequencies which in turn affects resolution of subsurface imagery or radargram. Thus at low frequencies, we expect a greater depth of penetration at the expense of resolution while at high frequencies, we achieve a lower depth of penetration at higher resolution.

boreholes, received by receivers (geophones or hydrophones) and displayed on seismographs (as a combination of waves velocities and attenuation). From these, properties of the subsurface like porosity, hydraulic conductivity, elastic moduli and water saturation which could

Aquifer, Classification and Characterization http://dx.doi.org/10.5772/intechopen.72692 27

Subsurface investigation involving Seismic techniques are categorized into three; Seismic

With Seismic Refraction, the incident ray is refracted along the target boundary before returning to the surface (Figure 13). The arrival times gotten from the refracted energy are displayed as function of distance from the source with their interpretation been made manually using simple software or forward modeling techniques. The relationship between arrival times and distances could be used to obtain velocity information directly. Seismic Refraction techniques are the most

Figure 14. Cross-hole tomography geometry for seismic and radar methods. Sources and receivers are located in separated boreholes, and energy from each source is received by all geophones. Cross-holes acquisition geometries have also

help us better understand the subsurface could be derived.

Figure 13. Major ray paths of P-wave energy (from Burger [2]).

been used with electrical resistivity and EM methods.

Refraction, Cross-hole transmission (tomography) and Seismic Reflection.

Figure 11. (A) Interpretation of a GPR profile image (B) interpretation of the prominent stratigraphic units, structures and faults.

Figure 12. The relationship between probing distance and frequency for different materials (after Cook 1975).

Ground Penetrating Radar (GPR) data have been successful utilized in the hydrogeological investigations to locate the water table and to delineate shallow, unconsolidated aquifers [16].

#### 5.2. Seismic techniques

The use of Seismic techniques in subsurface characterization is based on the propagation of elastic waves generated from a seismic controlled source, propagated through the subsurface,

boreholes, received by receivers (geophones or hydrophones) and displayed on seismographs (as a combination of waves velocities and attenuation). From these, properties of the subsurface like porosity, hydraulic conductivity, elastic moduli and water saturation which could help us better understand the subsurface could be derived.

Subsurface investigation involving Seismic techniques are categorized into three; Seismic Refraction, Cross-hole transmission (tomography) and Seismic Reflection.

With Seismic Refraction, the incident ray is refracted along the target boundary before returning to the surface (Figure 13). The arrival times gotten from the refracted energy are displayed as function of distance from the source with their interpretation been made manually using simple software or forward modeling techniques. The relationship between arrival times and distances could be used to obtain velocity information directly. Seismic Refraction techniques are the most

Figure 13. Major ray paths of P-wave energy (from Burger [2]).

Ground Penetrating Radar (GPR) data have been successful utilized in the hydrogeological investigations to locate the water table and to delineate shallow, unconsolidated aquifers [16].

Figure 12. The relationship between probing distance and frequency for different materials (after Cook 1975).

Figure 11. (A) Interpretation of a GPR profile image (B) interpretation of the prominent stratigraphic units, structures and

The use of Seismic techniques in subsurface characterization is based on the propagation of elastic waves generated from a seismic controlled source, propagated through the subsurface,

5.2. Seismic techniques

faults.

26 Aquifers - Matrix and Fluids

Figure 14. Cross-hole tomography geometry for seismic and radar methods. Sources and receivers are located in separated boreholes, and energy from each source is received by all geophones. Cross-holes acquisition geometries have also been used with electrical resistivity and EM methods.

appropriate for a few shallow (50 m) targets of interest, or where one is interested in identifying gross lateral velocity variations or changes in interface dip [17]. Though Seismic Refraction yields lower resolution than Seismic Reflection and Seismic Cross-hole tomographic, it is however chosen over Reflection as they are inexpensive and help to determining the depth to the water table (buried refractor) and to the top of bedrock, the gross velocity structure, or for locating significant faults. The buried refractor is usually saturated and has a greater velocity than the unsaturated equivalent soil unit and the bedrock surface [18].

Cross-hole tomographic: involves the measurement of the travel times of seismic ray paths between two or more boreholes in order to derive an image of seismic velocity in the interven-

Aquifer, Classification and Characterization http://dx.doi.org/10.5772/intechopen.72692 29

Dielectric Constant: A measure of the separation (polarization) of opposite electrical charges

Effective Conductivity: The coefficient multiplying the expected value of the head gradient to

Electrical Profiling: An electrical survey made at several surface locations using a constant electrode separation distance. Electrical profiles, also called constant spread profiles or resis-

Electrical Resistivity: A measure of the ability of electrical current to flow through materials, measured in Ohm-m. Electrical resistivity is an intrinsic property of a material and is the

Electrical Sounding: An electrical survey made at a single surface location by moving electrodes progressively farther apart. Electrical soundings, also called expanding spread profiles,

Normal Moveout Correction (NMO): Adjusting seismic or radar velocity estimates to flatten

Radargram: A picture of the subsurface profile (graph like) representing a profile length along

Reflection: Energy that bounces back from a surface due to a change in physical properties,

Transmitter: A transmitter is an electronic device used to produce radio waves in order to

Receiver: A receiver is a device used to receive signals and decode signals and transform them

Seismograms: Is an instrument used for measuring earthquake (seismic) signals which could also be adapted to be used for other geophysical investigations. These are held in a very solid

Seismic Refraction: Is a geophysical principle governed by Snell's law. Used in the fields of engineering geology, geotechnical engineering and exploration geophysics, seismic refraction traverses are performed using a seismograph(s) or geophone(s) in an array and an energy

Seismic Reflection: Is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the earth's surface from reflected seismic waves. It's the most common geophysical methodology used for oil and gas exploration which exhibits the highest degree of technical sophistication in terms of both data acquisition and signal processing capabilities.

Unconfined Aquifer: An aquifer that has no overlying confining impermeable layer.

transmit or send data with the aid of an antenna during a geophysical investigation.

into information the computer understands during a geophysical investigation.

tivity profiles, provide information about lateral changes in apparent resistivity.

provide information about apparent electrical resistivity as a function of depth.

the parabolic appearance of reflectors due to offset between sources and receivers.

x-axis and y-axis or A radar image of mineral deposits or a planetary surface.

such as seismic impedance in the case of sound waves resolution.

position either on the bedrock or on a concrete base.

within a material that has been subjected to an external electrical field.

ing ground.

source.

yield the expected value of the flux.

inverse of electrical conductivity.

Cross-hole transmission (tomography) data acquisition is possible using several techniques amongst which includes seismic techniques, electrical resistivity, electromagnetic, radar with seismic being the most common. Majority of cross – hole tomographic seismic data have been collected for research however the those collected over extremely high resolution of up to 0.5 m are better suited for site characterization. Figure 14 shows a typical example of seismic cross-hole survey. The multiple sampling of the intra-wellbore area permits very detailed estimation of the velocity structure [19]. As seismic P-wave velocities can be related to lithological and hydrogeological parameters as discussed above, this extremely high resolution method is ideal for detailed stratigraphic and hydraulic characterization of interwell areas [20].
