4. Aquifer characterization – electrical techniques

In geophysical investigations using electrical techniques, two primary properties of interest been considered are electrical conductivity or dielectric constant.

Electrical techniques make involves profiling and sounding mode of data collection which both involves the use of direct currents or low frequency altering currents passing through the subsurface. While profiling use of the terms profiling involves the inferring of subsurface measurements based on lateral changes in electrical properties over constant subsurface, Sounding infers subsurface measurements of single location as a function of changes in petrophysical properties as function of depth (Figure 5). The use of either of the two modes for data collection is function of the purpose of the investigation.

## 4.1. Electrical resistivity

arrangement of the rock where to change to be that of a rhombohedral (Figure 4B) then, its porosity would reduce to 25.85% as the centers of the eight adjacent spheres form the vertices

Porosity nð Þ¼ Vp

where Vp is the pores of rock or soil sample, Vt is the total volume of pores and solid material.

Transmissivity (T) more simply could be defined as the property of aquifer to transmit water. It could also be defined as the amount of water that can be transmitted horizontally through an aquifer unit by full saturated thickness of the aquifer under a hydraulic gradient of 1 or as the rate at which water of prevailing kinematic viscosity is transmitted through a unit width of

<sup>T</sup> <sup>¼</sup> Kb m<sup>2</sup>

where T is the transmissivity, K is the hydraulic conductivity, b is the saturated thickness of the

Aquifers are characterized by petro-physical properties such as hydraulic conductivity (alternatively called permeability), transmissivity (product of hydraulic conductivity and aquifer thickness) and diffusivity (ratio of transmissivity and storage coefficient). These properties could be examined using geophysical techniques such as Electrical Resistivity, Seismic techniques.

Vt (2)

=day (3)

Mathematically porosity (n) is given by the formula below:

Figure 4. (A) Cubic packing (B) Rhombohedral packing.

of rhombus.

16 Aquifers - Matrix and Fluids

3.3. Transmissivity

aquifer.

aquifer under a unit gradient.

It could be mathematically defined as:

Electrical resistivity (ER) is more frequently been used as compared to other electrical techniques in groundwater investigations of which includes the characterization of aquifers. Electrical Resistivity (ER) involves the introduction of time – varying direct current (DC) or very low frequency (<1 Hz) current into the ground between two current electrodes to generate potential differences as measured at the surface with units of Ohm-meters (Ω-m). A deviation from the norm in the pattern of potential differences expected from homogeneous proffer the necessary information on the form and electrical properties of subsurface inhomogeneities.

A typical Electrical Resistivity (ER) investigation made up of a 2 – electrode system would include 2 – current electrodes and 2 – potential electrodes. As current is been injected into the ground, corresponding potential differences (ΔV) is measured. This measurement coupled with known current (I) and a geometric factor (K) that is a function of the particular electrode configuration, can be used to calculate resistivity (r) following Ohm's law:

$$
\rho = \frac{\Delta V}{I} \text{ K} \tag{4}
$$

The expression in Eq. (4) for a homogeneous ground is also the same applied for heterogeneous ground; however the general term "apparent resistivity (ra)" is substituted for resistivity (r) in

Figure 5. Schematic geological section and associated resistivity and velocity contrasts at interfaces (from Burger [2]. With permission.

Eq. (4). Apparent resistivity (ra) is used here rather than the actual resistivity of the subsurface due to the non-homogeneity nature of the subsurface.

A four – electrode configurations is been used most commonly when it comes to measuring apparent resistivity of the subsurface. The simplest of these configurations is the Wenner configuration (Figure 6a) where the outer two current electrodes C1 and C2, apply a constant current, and the inner two potential electrodes, labeled P1 and P2, measures voltage difference created by this current. The electrode spacing has a fixed value a, and the apparent resistivity of the subsurface sampled by this array could be computed using the equation:

$$
\rho a = \frac{\Delta V}{I} \text{ 2\pi a} \tag{5}
$$

To illustrate, we consider the Wenner array. Profiling involves the lateral movement of the entire array along the surface at fixed distances to obtain apparent resistivity measurements as a function of distance. The values of the measurements are assigned to the geometric center of the electrode array. Interpretation of measurements is usually with its data aimed at location of

Aquifer, Classification and Characterization http://dx.doi.org/10.5772/intechopen.72692 19

Sounding unlike profiling involves gradual and progressively expansion of expansion of the array about a fixed central point with current and potential electrodes being maintained at a relative spacing with depth been a function of electrode spacing and subsurface resistivity contrasts(Figure 7a–c). The dashed lines representing current flow lines in an homogeneous environment while bold lines represents actual current flow in single interface that separates units with different resistivities. Next we look how electrode spacing, current and its' influence on depth of penetration. In Figure 7a, when the electrode spacing is close, it is observed that the current only upper interface (i.e. the interface of lower resistivity). The scenario in Figure 7b is different; as electrode spacing has increased resulting in greater penetration depth and higher apparent resistivity values due to the influence of the lower (higher resistivity) layer. Lastly when the electrodes are farther apart, only substantial current amounts are found to

Figure 7. Effects of electrode spacing and presence of an interface on apparent resistivity measurements. The dashed lines represent current flow lines in the absence of the interface and the solid lines represent actual current flow lines (a–c) as the current electrode spacing is increased, the current lines penetrate deeper and the apparent resistivity measurements are influenced by the lower (more resistive) layer. (d) the qualitative variations in apparent resistivity as a function of

electrode spacing are illustrated by the two-layer sounding curve (from Burger [2]. With permission).

geological structures buried stream channels, aquifers or water bearing formations etc.

flow through the resistivity layer (Figure 7c).

Asides the Wenner array mode of electrode configuration, another commonly used electrode configuration is the Schlumberger array (Figure 6b), where the spacing (MN) between the potential electrodes (P1, P2) is much smaller as compared to the spacing (2 L) between the current electrodes (C1, C2).

The electrode configuration in (Figure 6) represents that of a dipole – dipole array where the potential electrode pair and current electrode is closely spaced, however there exist significant distances between the two sets of electrodes (Figure 6c) Unlike the cases of the Wenner and Schlumberger arrays, where data collected through either profiling or sounding mode depends a lot on the electrode array geometry.

Figure 6. Common electrode configuration used to measure apparent resistivity of the subsurface C1 and C2 are the current electrodes and P1 and P2 are the potential electrodes. (a) Wenner Array (b) Schlumberger array (c) dipole – Dipole array. (from Burger [2]. With permission).

To illustrate, we consider the Wenner array. Profiling involves the lateral movement of the entire array along the surface at fixed distances to obtain apparent resistivity measurements as a function of distance. The values of the measurements are assigned to the geometric center of the electrode array. Interpretation of measurements is usually with its data aimed at location of geological structures buried stream channels, aquifers or water bearing formations etc.

Eq. (4). Apparent resistivity (ra) is used here rather than the actual resistivity of the subsurface

A four – electrode configurations is been used most commonly when it comes to measuring apparent resistivity of the subsurface. The simplest of these configurations is the Wenner configuration (Figure 6a) where the outer two current electrodes C1 and C2, apply a constant current, and the inner two potential electrodes, labeled P1 and P2, measures voltage difference created by this current. The electrode spacing has a fixed value a, and the apparent resistivity

of the subsurface sampled by this array could be computed using the equation:

<sup>r</sup><sup>a</sup> <sup>¼</sup> <sup>Δ</sup><sup>V</sup>

Asides the Wenner array mode of electrode configuration, another commonly used electrode configuration is the Schlumberger array (Figure 6b), where the spacing (MN) between the potential electrodes (P1, P2) is much smaller as compared to the spacing (2 L) between the

The electrode configuration in (Figure 6) represents that of a dipole – dipole array where the potential electrode pair and current electrode is closely spaced, however there exist significant distances between the two sets of electrodes (Figure 6c) Unlike the cases of the Wenner and Schlumberger arrays, where data collected through either profiling or sounding mode

Figure 6. Common electrode configuration used to measure apparent resistivity of the subsurface C1 and C2 are the current electrodes and P1 and P2 are the potential electrodes. (a) Wenner Array (b) Schlumberger array (c) dipole – Dipole

<sup>I</sup> <sup>2</sup>π<sup>a</sup> (5)

due to the non-homogeneity nature of the subsurface.

current electrodes (C1, C2).

18 Aquifers - Matrix and Fluids

array. (from Burger [2]. With permission).

depends a lot on the electrode array geometry.

Sounding unlike profiling involves gradual and progressively expansion of expansion of the array about a fixed central point with current and potential electrodes being maintained at a relative spacing with depth been a function of electrode spacing and subsurface resistivity contrasts(Figure 7a–c). The dashed lines representing current flow lines in an homogeneous environment while bold lines represents actual current flow in single interface that separates units with different resistivities. Next we look how electrode spacing, current and its' influence on depth of penetration. In Figure 7a, when the electrode spacing is close, it is observed that the current only upper interface (i.e. the interface of lower resistivity). The scenario in Figure 7b is different; as electrode spacing has increased resulting in greater penetration depth and higher apparent resistivity values due to the influence of the lower (higher resistivity) layer. Lastly when the electrodes are farther apart, only substantial current amounts are found to flow through the resistivity layer (Figure 7c).

Figure 7. Effects of electrode spacing and presence of an interface on apparent resistivity measurements. The dashed lines represent current flow lines in the absence of the interface and the solid lines represent actual current flow lines (a–c) as the current electrode spacing is increased, the current lines penetrate deeper and the apparent resistivity measurements are influenced by the lower (more resistive) layer. (d) the qualitative variations in apparent resistivity as a function of electrode spacing are illustrated by the two-layer sounding curve (from Burger [2]. With permission).

The curve (Figure 7d) reveals qualitative variations in apparent resistivities which increases with electrode spacing, a, this curve is known as a sounding curve revealing geology of the subsurface with resistivity increasing with depth provided geology is homogeneous. However in the case where the geology is inhomogeneous it results in a complex sounding curve whose interpretation is non-unique. To interpret electrical resistivity sounding data, various curvefitting or computer inversion schemes are used or measured and compared with model computations [1]. A classic example where both modes of data acquisition (profiling and sounding) is been used is in the location of a buried stream channel (Figures 8a and 6a) using Wenner array. The contour map (Figure 8a) produced from resistivity measurements of several profiles collected near San Jose, CA, using an a-spacing (Figure 6a) of 6.1 [1, 2] reveals contours of equal apparent resistivity delineating an approximately east–west trending high apparent

resistivity values. To understand the cause of high apparent resistivity values here, a geological cross-section (BA) was drawn across the map. The geological cross-section (BA) drawn is based on four expanding – spread traverses (soundings), apparent resistivity profile information and information from three boreholes whose locations are indicated on the cross-section. The critical observation of the cross-section shows that the area with high-resistivity as on the apparent resistivity map (Figure 8b) is a zone of gravels and boulders that defines the location

Aside the mapping of subsurface structure and stratigraphy, electrical resistivity measurements could be channeled towards the inferring lithological information and hydrogeological parameters needed for the mapping groundwater. For groundwater mapping, electrical conduction (inverse of electrical resistivity) is considered. Here the interest is the delineation of connected pore spaces, void spaces, interstices, fractures within rocks that are water filled which leads to a reduced resistivity values and high conductivity. However more information is still needed as high conductivity within rock formation or units could be due to a number of things asides water

Common earth materials have wide range of electrical resistivity values revealed in Table 1, however some of these values are known to overlap for different earth materials. Values commonly vary over 12 orders of magnitude and have a maximum range of 24 orders of

• Resistivity is sensitive to moisture content; thus unsaturated sediments usually have

• Granitic bedrock generally has a higher resistivity value than saturated sediments and frequently offers a large apparent resistivity contrast when overlain by these sediments.

Asides the use of sounding curves, empirical formulae have also been adapted in relating measurement of apparent resistivity with hydrological parameters of interest as this relates to aquifers. The empirical formula developed in the laboratory by Archie [4] relates these parameters:

where r<sup>r</sup> is the electrical resistivity of the rock, r<sup>w</sup> is the pore water resistivity, ∅ is the

And n, a, and m are constants {n ≈ 2, 0.6 ≤ a ≤ 1.0, and 1.4 ≤ m ≤ 2.2; Ward [5]}. Though Archie's law was formulated using lithified materials, Jackson et al. [6] posited its' accurate usability for unconsolidated materials also. The equation presented by Eq. (6) is used generally for well log interpretation however if rr, rw, and∅ can be measured separately such that a and m are estimated reasonably then the fractional water saturation could also be inferred using electrical surveys [5]. This concept was utilized by Pfeifer and Anderson [7] to observe and monitor the

In conclusion, it could be said that the complexities that exist in the interpretation of sounding curves and the non-unique solution it gives, suggests the suitability of surface resistivity in

migration of tracer-spiked water through the subsurface using resistivity array.

<sup>r</sup><sup>r</sup> <sup>¼</sup> <sup>a</sup>∅�<sup>m</sup> <sup>S</sup>�<sup>n</sup> <sup>r</sup><sup>w</sup> (6)

Aquifer, Classification and Characterization http://dx.doi.org/10.5772/intechopen.72692 21

some of which includes presence of clay minerals, contamination plumes etc.

magnitude [3]. The following statements as regarding electrical resistivity holds;

• Sandy materials generally have higher resistivity values than clayey materials

higher resistivity values than saturated sediments.

fractional porosity, S is the fractional water saturation.

of a buried stream channel (subsurface structure).

Figure 8. Resistivity survey used to delineate lateral and vertical variations in subsurface stratigraphy. (a) Contour map produced from resistivity measurements, (b) a geologic cross-section (BA) revealing high-resistivity trend in a zone of gravel and boulders that define the location of a buried stream channel (from Ref. [1] application of surface geophysics to groundwater investigations).

resistivity values. To understand the cause of high apparent resistivity values here, a geological cross-section (BA) was drawn across the map. The geological cross-section (BA) drawn is based on four expanding – spread traverses (soundings), apparent resistivity profile information and information from three boreholes whose locations are indicated on the cross-section. The critical observation of the cross-section shows that the area with high-resistivity as on the apparent resistivity map (Figure 8b) is a zone of gravels and boulders that defines the location of a buried stream channel (subsurface structure).

The curve (Figure 7d) reveals qualitative variations in apparent resistivities which increases with electrode spacing, a, this curve is known as a sounding curve revealing geology of the subsurface with resistivity increasing with depth provided geology is homogeneous. However in the case where the geology is inhomogeneous it results in a complex sounding curve whose interpretation is non-unique. To interpret electrical resistivity sounding data, various curvefitting or computer inversion schemes are used or measured and compared with model computations [1]. A classic example where both modes of data acquisition (profiling and sounding) is been used is in the location of a buried stream channel (Figures 8a and 6a) using Wenner array. The contour map (Figure 8a) produced from resistivity measurements of several profiles collected near San Jose, CA, using an a-spacing (Figure 6a) of 6.1 [1, 2] reveals contours of equal apparent resistivity delineating an approximately east–west trending high apparent

Figure 8. Resistivity survey used to delineate lateral and vertical variations in subsurface stratigraphy. (a) Contour map produced from resistivity measurements, (b) a geologic cross-section (BA) revealing high-resistivity trend in a zone of gravel and boulders that define the location of a buried stream channel (from Ref. [1] application of surface geophysics to

groundwater investigations).

20 Aquifers - Matrix and Fluids

Aside the mapping of subsurface structure and stratigraphy, electrical resistivity measurements could be channeled towards the inferring lithological information and hydrogeological parameters needed for the mapping groundwater. For groundwater mapping, electrical conduction (inverse of electrical resistivity) is considered. Here the interest is the delineation of connected pore spaces, void spaces, interstices, fractures within rocks that are water filled which leads to a reduced resistivity values and high conductivity. However more information is still needed as high conductivity within rock formation or units could be due to a number of things asides water some of which includes presence of clay minerals, contamination plumes etc.

Common earth materials have wide range of electrical resistivity values revealed in Table 1, however some of these values are known to overlap for different earth materials. Values commonly vary over 12 orders of magnitude and have a maximum range of 24 orders of magnitude [3]. The following statements as regarding electrical resistivity holds;


Asides the use of sounding curves, empirical formulae have also been adapted in relating measurement of apparent resistivity with hydrological parameters of interest as this relates to aquifers. The empirical formula developed in the laboratory by Archie [4] relates these parameters:

$$
\rho\_r = a \boxtimes^{-m} S^{-n} \rho\_w \tag{6}
$$

where r<sup>r</sup> is the electrical resistivity of the rock, r<sup>w</sup> is the pore water resistivity, ∅ is the fractional porosity, S is the fractional water saturation.

And n, a, and m are constants {n ≈ 2, 0.6 ≤ a ≤ 1.0, and 1.4 ≤ m ≤ 2.2; Ward [5]}. Though Archie's law was formulated using lithified materials, Jackson et al. [6] posited its' accurate usability for unconsolidated materials also. The equation presented by Eq. (6) is used generally for well log interpretation however if rr, rw, and∅ can be measured separately such that a and m are estimated reasonably then the fractional water saturation could also be inferred using electrical surveys [5]. This concept was utilized by Pfeifer and Anderson [7] to observe and monitor the migration of tracer-spiked water through the subsurface using resistivity array.

In conclusion, it could be said that the complexities that exist in the interpretation of sounding curves and the non-unique solution it gives, suggests the suitability of surface resistivity in


of the fields generated, such as amplitude, orientation and phase shift can be measured by the receiver coil and compared with those of the primary field as such information about the presence of subsurface conductors, or subsurface electrical conductivity distribution can be

Aquifer, Classification and Characterization http://dx.doi.org/10.5772/intechopen.72692 23

It's paramount to recall that electrical conductivity is an inverse of electrical resistivity; as such electrical conductivity measurements made using electromagnetic methods is also dependent on subsurface texture, porosity, presence of clay minerals, moisture content and the electrical resistivity of the pore fluid presence. The acquisition of EM data requires less time, achieving greater depth of investigation than resistivity techniques. However, the equipment used are expensive and the methods used to qualitatively interpret data from EM surveys is complicated than those used in resistivity methods. This is because a conductive subsurface environment is essential to set up a secondary field measured with inductive EM methods (Figure 9). Electromagnetic methods as a tool for geophysical investigation and exploration is most suited for the detection of water—bearing formation (aquifers) and high – conductive subsurface

Instrumentation could take in varying forms; but mainly consist of a source and receiver or receiver units. The source (transmitter) transmits time-varying magnetic fields with the receiver measuring components of the total (primary and secondary) field, magnetic field, sometimes the electric field and the necessary electronic circuitry to process, store and display signals [9, 10]. Data obtained from electromagnetic surveys, like their resistivity counterpart can be collected in profile and sounding mode with their information been presented as maps or pseudo-section to give a better picture of the subsurface. Acquisition, resolution and depth of investigation from this survey are been governed by mostly by conditions of the subsurface and domain of measurement. EM surveys are divided into two domain system of measurement namely; frequency and time domain system. For frequency domain EM systems, we have the transmitter classed as either high or low frequency transmitters; high transmitter frequencies permits high- resolution investigation of subsurface conductors at near-surface or shallow depths while lower transmitter frequencies allows for deeper depth of investigation at the expense of resolution. This implies

inferred.

target such as salt water saturated sediments.

Figure 9. Electromagnetic induction technique (from Ref. [9]).

Table 1. Resistivity and dielectric constants for typical near-surface materials (data from Ref. [13]).

determined subsurface geology. Also due to its sensitivity to parameters like moisture content it's been termed a useful tool in hydrological investigations as reviewed by Ward [5], Van Nostrand and Cook [8].

## 4.2. Electromagnetic induction

Electromagnetic (EM) techniques as tool for geophysical exploration has dramatically increased in recent years served as a useful tool for groundwater and environmental site assessment. It involves the propagation of continuous-wave or transient electromagnetic fields in and over the earth through resulting in the generation of time-varying magnetic field. For any of such surveys to be carried out three components are essential; a transmitters, receivers, buried conductors or conductive subsurface. These three form a trio of electric-circuit coupled by an EM induction with currents been introduced into the ground directly or through inductive means by the transmitters.

The Primary field travels from the transmitter coil to the receiver coil via paths above and below the surface. Where a homogenous subsurface is detected no difference is observed between the fields propagated above, below and within the surface other than a slight reduction in amplitude. However, the interaction of the time-varying field with a conductive subsurface induces eddy currents, which gives rise to a secondary magnetic field (Figure 9). The attributes

Figure 9. Electromagnetic induction technique (from Ref. [9]).

determined subsurface geology. Also due to its sensitivity to parameters like moisture content it's been termed a useful tool in hydrological investigations as reviewed by Ward [5], Van

Table 1. Resistivity and dielectric constants for typical near-surface materials (data from Ref. [13]).

Electromagnetic (EM) techniques as tool for geophysical exploration has dramatically increased in recent years served as a useful tool for groundwater and environmental site assessment. It involves the propagation of continuous-wave or transient electromagnetic fields in and over the earth through resulting in the generation of time-varying magnetic field. For any of such surveys to be carried out three components are essential; a transmitters, receivers, buried conductors or conductive subsurface. These three form a trio of electric-circuit coupled by an EM induction with currents been introduced into the ground directly or through inductive means by

The Primary field travels from the transmitter coil to the receiver coil via paths above and below the surface. Where a homogenous subsurface is detected no difference is observed between the fields propagated above, below and within the surface other than a slight reduction in amplitude. However, the interaction of the time-varying field with a conductive subsurface induces eddy currents, which gives rise to a secondary magnetic field (Figure 9). The attributes

Nostrand and Cook [8].

22 Aquifers - Matrix and Fluids

the transmitters.

4.2. Electromagnetic induction

of the fields generated, such as amplitude, orientation and phase shift can be measured by the receiver coil and compared with those of the primary field as such information about the presence of subsurface conductors, or subsurface electrical conductivity distribution can be inferred.

It's paramount to recall that electrical conductivity is an inverse of electrical resistivity; as such electrical conductivity measurements made using electromagnetic methods is also dependent on subsurface texture, porosity, presence of clay minerals, moisture content and the electrical resistivity of the pore fluid presence. The acquisition of EM data requires less time, achieving greater depth of investigation than resistivity techniques. However, the equipment used are expensive and the methods used to qualitatively interpret data from EM surveys is complicated than those used in resistivity methods. This is because a conductive subsurface environment is essential to set up a secondary field measured with inductive EM methods (Figure 9). Electromagnetic methods as a tool for geophysical investigation and exploration is most suited for the detection of water—bearing formation (aquifers) and high – conductive subsurface target such as salt water saturated sediments.

Instrumentation could take in varying forms; but mainly consist of a source and receiver or receiver units. The source (transmitter) transmits time-varying magnetic fields with the receiver measuring components of the total (primary and secondary) field, magnetic field, sometimes the electric field and the necessary electronic circuitry to process, store and display signals [9, 10]. Data obtained from electromagnetic surveys, like their resistivity counterpart can be collected in profile and sounding mode with their information been presented as maps or pseudo-section to give a better picture of the subsurface. Acquisition, resolution and depth of investigation from this survey are been governed by mostly by conditions of the subsurface and domain of measurement.

EM surveys are divided into two domain system of measurement namely; frequency and time domain system. For frequency domain EM systems, we have the transmitter classed as either high or low frequency transmitters; high transmitter frequencies permits high- resolution investigation of subsurface conductors at near-surface or shallow depths while lower transmitter frequencies allows for deeper depth of investigation at the expense of resolution. This implies 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 while smaller coil separation investigates near-surface.

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 parameters of porosity and saturation.

> 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

Aquifer, Classification and Characterization http://dx.doi.org/10.5772/intechopen.72692 25

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

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.

software like RADpro, Pulse Ekko system software etc.

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

South Carolina [15].
