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Identifying Roadbed Underlain by Expansive Clays (Determine Clay Content in Swelling Spoils)

Roads constructed over areas of clay are subject to potential deformation due to volume changes of the clay. Such volume changes result from changes in the moisture content of the clay. Geophysical methods can be used to identify areas of clay.

Clays are very small-grained hydrous aluminosilicates with the phyllosilicate structure. Clays are classified as hydrosilicates, which means that they are formed from the chemical decomposition of pre-existing silicate minerals. Most clays result from the product of weathering and sedimentation, but they are also formed by hydrothermal activities. Clays can occur as part of a soil structure or as independent layers and lenses. They are also commonly found in glacial till, where glacial action has ground the rocks and boulders into fine particles.

Clay occurs in soil, shales, and as independent layers and is formed from weathered pre-existing minerals. It also occurs as weathered products from rocks such as granite, where the feldspars weather to form clay minerals. It is commonly found in glacial till, where glacial action has ground the rocks and boulders into fine particles.

Clay particle sizes range in size from 0.002 mm to 0.001 mm diameter for quartz, feldspar, mica, iron, and aluminum oxides. The finer parts (less than 0.001 mm in diameter) are colloidal and consist mainly of layer silicates with smaller amounts of iron and aluminum oxides. These consist of microscopically fine particles exhibiting a sheet-like structure. Their crystalline structure is such that, because of crystal imperfections, the surface appears negatively ged. During the formation of clay through weathering, positive ges (cations) are adsorbed to the surface. These cations are loosely held to the surface and can subsequently be exchanged for other cations or essentially go into solution if the clay is mixed with water. For this reason, they are called exchangeable ions, and the cation exchange capacity (CEC) of clay is a measure of the number of cations that are required to neutralize the clay particle as a whole.

Any fine-grained mineral, including quartz, has a cation exchange capacity. However, clay particles are very small, and because the surface area per unit volume is very large, a large number of cations are adsorbed. In addition to influencing the conductivity of clay, these adsorbed cations can also contribute significantly to the electrical conductivity in soils containing clay, which then becomes a function of the clay content. Cation Exchange Capacities of some common clay types are presented in Table 8.

Table 8. Cation Exchange Capacities (CEC) of common clay types.

Clay typeCation Exchange Capacity
Kaolinite3 - 15
Chlorite10 - 40
Illite10 - 40
Montmorillonite80 - 150
Vermiculite100 - 150

This table shows that kaolinite has the least influence on conductivity, and vermiculite has the greatest influence. However, other factors influence conductivity values, including the degree of saturation and the salinity of the saturating fluid.

Because of its electrical structure, clay minerals often exhibit an effect called membrane polarization. This is a distribution of ions around the clay pores. When a DC potential is applied across the pore, negative ions accumulate at one end and leave the other end. Because of this polarization distribution, current flow is impeded. When the DC potential is switched off, the ions take a finite amount of time to return to their original positions. This effect can be measured and may be used to help identify clays.

From the above discussion, two basic parameters can be used to map clays and possibly estimate the clay content of soils and other geologic layers-electrical conductivity and membrane polarization. The appropriate geophysical methods that can be used to measure these quantities are electrical conductivity (or resistivity) and Induced Polarization to measure membrane polarization.

The resistivities of rocks vary from 1 to 30 ohm-m for some clays and shales to over 1,000 ohm-m for limestone, intrusive rocks such as granites, and some metamorphic rocks. However, in sedimentary rocks, which often have much larger porosities than metamorphic and intrusive rocks, resistivities generally range from 10 to 1,000 ohm-m. In these rocks, the porosity and salinity of the water in the rock pores are also significant factors influencing resistivity. Although clay is specifically discussed in this web manual, other rock types such as shales may also be detrimental to roadbed stability. Since both clay and shale usually have low resistivities, this parameter may be an important predictor of potential roadbed problems.

Clays can swell as well as have low shear strength. Swelling is usually caused by moisture within the clay. Fine-grained material such as clay can hold a significant amount of water. Moisture can be acquired from surface runoff or, because of the small pores size in clays and the resulting strong capillary forces, moisture can be drawn from material beneath them. Shear strength of clays are significantly influenced by moisture content, decreasing with increased moisture. Furthermore, clays with high swelling potential are susceptible to extreme volume changes as moisture content changes. Roads constructed over areas of soft clay will result in poorly performing pavement systems and often result in a subgrade failure. Subgrade failures may be visibly observed as pavement deformations over problem areas. Thus, there is a need to be able to identify where clays occur along roadways in order that remedial action may be taken before the problem becomes serious.

Dipping clay/shale layer.

Figure 106. Dipping clay/shale layer.

Clay in alluvium.

Figure 107. Clay in alluvium.

Clays may occur in many different geological contexts. Figures 106 and 107 show two common geologic conditions in which clay may occur.

In figure 106, the road is constructed over dipping strata. One of the stratum is a shale/clay layer that has caused the roadbed to deform. In figure 107, the clay layer is within the alluvium and has allowed the roadbed to become depressed. Geophysical methods can be used to help locate clay in both of these cases, although resistivity or conductivity soundings may be needed to distinguish between them.

Determining clay content from geophysical methods may be difficult. However, if this is to be attempted, then the true resistivity of the layer being imaged has to be obtained. Since the instruments measure the bulk resistivity of the ground, the true resistivity of any layer has to be determined by interpretation. To do this, it is also important to understand the geology and structural setting of the low resistivity occurrence. The use of different geophysical methods may be required, first to locate the conductive area, and second to evaluate its true conductivity (or resistivity) and geologic setting.


Conductivity Measurements

Basic Concept: Ground conductivity methods can rapidly locate conductive areas in the upper few meters of the ground surface. These measurements are recorded using several instruments that use electromagnetic methods. The choice of which instrument to use generally depends on the depth of investigation desired. Instruments commonly used include the EM38, EM31, EM34 (Geonics Ltd, Canada), and GEM2 (Geophex, USA). These instruments transmit sinusoidal electromagnetic waves at frequencies up to about 10 kHz (EM38, EM31, EM34) and between 330 Hz and 24 kHz for the GEM2.

The EM38 is designed to measure soil conductivities and has a maximum depth of investigation of about 1.5 m. The EM31 has a depth of investigation to about 6 m, and the EM34 has a maximum depth of investigation to about 60 m. The depth of investigation of the GEM2 is advertised to be 30 to 50 m in resistive terrain (>1000 ohm-m) and 20 to 30 m in conductive terrain (<100 ohm-m).

Electromagnetic instruments that measure ground conductivity use two coplanar coils, one for the transmitter and the other for the receiver. The transmitter coil produces an electromagnetic field, oscillating at several kHz, that produces secondary currents in conductive material in the ground. The amplitude of these secondary currents depends on the conductivity of the material. These secondary currents then produce secondary electromagnetic fields that are recorded by the receiver coil.

The EM34, EM31, and EM38 instruments can be used with the planes of the coils held parallel to the ground (vertical dipole mode) or perpendicular to the ground (horizontal dipole mode). The investigation depth is greatest in the vertical dipole mode and least in the horizontal dipole mode. In addition, in the vertical dipole mode, the conductivity readings are insensitive to near- surface conductivity changes, whereas in the horizontal dipole mode, the readings are significantly influenced by near-surface conductivity changes.

The EM38 instrument used for shallow surveys measures the conductivity of soil and is shown in figure 103. Figure 183 shows the EM31 instrument being used in vertical dipole mode. Figure 197 shows the EM34 being used in the horizontal dipole mode. The GEM2 instrument is shown in figure 191. This instrument uses a wide range of frequencies to obtain conductivities at different depths.

EM38 instrument being used in vertical dipole mode

Figure 103. EM38 instrument being used in vertical dipole mode. (Geonics, Ltd.)

The EM31 MK2 instrument. (Geonis, Ltd.)

Figure 183. The EM31 MK2 instrument. (Geonis, Ltd.)

The EM34 being used in horizontal dipole mode

Figure 197. The EM34 being used in horizontal dipole mode. (Geonics, Ltd.)

The GEM2 electromagnetic instrument.

Figure 191. The GEM2 electromagnetic instrument. (Geophex Inc.)

If conductivity values are required for depths beyond the 6 m available with the EM31, the EM34 can be used. This instrument provides three different separations of the two coils to provide six different depths of investigation (three coil separations and two modes for each separation). However, as the depth of investigation increases, the lateral resolution decreases. The EM34, unlike the EM31, requires two people to operate. Data recording is also much slower than with the EM31 since the automatic timed mode available with the EM31 is not available with the EM34. All of these instruments measure the bulk electrical conductivity of the ground. Since the conductivity of several different layers may be included in the conductivity measure, the correct technical term for the measured conductivity is apparent conductivity. When discussing data from these instruments in this web manual, conductivity or measured conductivity is understood to mean apparent conductivity.

Data Acquisition: Surveys mapping roadbed clays are likely to require depths of investigation up to about 5 to 10 m and need to be performed over long distances. Ideally, the system should be towed by a vehicle traveling at reasonable speeds compatible with the data recording parameters of the instrument. Since the EM34 requires two people to operate and is not capable of automatic time based recording, this instrument is not generally suited to this application and will not be discussed further.

The EM38 and EM31 can record data on a time basis, usually taking readings every second or half second. Thus, they can be assembled on a trailer, coupled with GPS, and towed at a speed of a few kilometers per hour while recording data. Different depths of investigation can be obtained by using the vertical and horizontal dipole modes.

Data Processing: If conductivity readings are taken with both the EM38 and EM31 at each of the dipole modes top provide different investigation depths, the data can be inverted to produce a conductivity cross section along the traverse. The coordinates can be assigned to the conductivity data points using the GPS data.

Data Interpretation: The results of a conductivity survey along a road in New Mexico are shown in figure 108. The purpose of the survey was to see if conductivity data could be recorded along a road efficiently and to correlate the data with soil type and clay content. For this survey, data were recorded with one of the instruments mounted on a trailer constructed primarily from non-conductive materials. The trailer was towed by an All Terrain Vehicle (ATV). A GPS receiver was also mounted on the trailer to provide position information. Data were recorded automatically at half-second intervals with the EM31 and EM38, using both the vertical and horizontal dipole modes. In addition, data from the EM31 were recorded at two different instrument heights above the ground, giving two different penetration depths. Since data can be recorded from only one instrument using one configuration at a time, several passes along the road had to be made; one for each instrument and parameter (dipole mode and height). These procedures provided up to six conductivity values from different depths of investigation.

Conductivity data along a road and the interpretation. (Blackhawk GeoServices, Inc.)

Figure 108. Conductivity data along a road and the interpretation. (Blackhawk GeoServices, Inc.)

Having obtained conductivity measurements at a number of different depths at each recording location, the data were modeled and provided the interpreted vertical distribution of conductivity with depth. This interpretation is shown in the lower plot in figure 108. The upper plot shows the ground conductivity measured with the EM38 at a depth of 0.75 m below the road surface.

Advantages: Conductivity data is fairly easy and efficient to record using the instruments described in the previous section. These instruments can be used in different modes providing different depths of investigation and can be tailored to focus on maximizing depth (Vertical dipole mode) or maximizing the influence of the near surface conductivity (horizontal dipole mode).

Limitations: Ground conductivity measurements are taken using a coil that transmits electromagnetic (EM) energy and induces electrical currents to flow in conductive objects. If metal objects are within the zone of influence of the transmitter, these will influence the data. Such objects might include any above-ground metal, including fences and buried objects such as pipelines, culverts, and utilities. If the instrument is being towed by a vehicle, it is important to make sure that the influence of the vehicle is minimal.

For the survey in New Mexico, several passes along the road had to be made to record the data with all of the desired recording parameters. This significantly reduced the overall productivity of the survey. However, there are newer instruments being developed that have three receiver coils placed at different distances from the transmitter coil that provide three different investigation depths at each reading location. The use of this instrument should significantly improve productivity.

Resistivity Methods

The EM38 and/or the EM31 can be used to map conductivity along roadways of interest and determine zones having high conductivities that may indicate clay. Although this may be sufficient in order to locate areas of concern, if structural remediation is required, some understanding of the geological setting may be helpful. It is also not possible to estimate the clay content from these data without knowing the true conductivity of the zones of interest or correlation with laboratory analysis or drill results.

The true conductivity of a conductive zone may be determined using TDEM or resistivity sounding methods. In the case illustrated in figure 107, the sounding should indicate a thin clay layer with more resistive layers both above and below the clay layer. In the case shown in figure 106, the sounding should indicate high conductivity values for a considerable depth.

It may still be difficult to estimate absolute values of the clay content, although the relative clay content between different anomalies may be possible, with the higher conductivity layers suggesting higher clay content. If the absolute clay content at one anomaly location can be determined from drill results or other methods, it may be possible to estimate the clay content of other anomalies.

Resistivity Soundings

Basic Concept: Resistivity soundings are used to obtain the vertical distribution of resistivity in the ground. The result is an interpretation showing the depths and resistivities of the layers imaged by the sounding.

Resistivity soundings are recorded using a number of electrode arrays, some of which are illustrated in figure 90. These arrays are used for different types of resistivity surveys. The Schlumberger array is often used for resistivity soundings, as is the Wenner array. The Pole-pole array provides the best signal, but is cumbersome because of the long wires required for the remote electrodes and is rarely used. The Dipole-dipole array was originally used mostly by the mining industry for induced polarization surveys. Readings were taken using several different separations of the voltage and current dipoles, providing measurements of the variation of resistivity with depth. Long lines of data were recorded requiring many readings. This array has now become common for resistivity surveys using the automated resistivity systems. If more signal (voltage) is needed than can be provided with the Dipole-dipole array, the Pole-dipole array can be used.

Electrode arrays used to measure resistivity.

Figure 90. Electrode arrays used to measure resistivity.

Figure 91 shows the Schlumberger electrode array being used to record resistivity data (figure 91a). Resistivity sounding data obtained using the Schlumberger array are called Schlumberger soundings. Using data from several electrode spacings, a graph of measured resistivity against electrode spacing is plotted (figure 91b). This graph can be interpreted to provide the depths and resistivities of the layers indicated in the sounding curve. In the case illustrated, at small values of AB/2 (electrode spacings), the measured resistivity approaches that of the overburden (figure 9b). The measured resistivity values then fall as the conductive clay layer influences the data. Finally, the measured resistivity values begin to increase as the electrode spacings become large enough to allow current to penetrate beneath the clay layer into more resistive material.

Electrode array for (a) measuring the resistivity of the ground, and (b) a resistivity sounding curve.

Figure 91. Electrode array for (a) measuring the resistivity of the ground, and (b) a resistivity sounding curve.

Data Acquisition: Resistivity soundings are conducted by planting the electrodes in the ground, first at the smallest electrode spacing required, and taking a resistivity reading. The electrode spacing is then increased, and the procedure is repeated. It is advisable to plot the measured resistivity values against electrode spacing in the field while recording the data, thereby producing a resistivity sounding curve. If a conductive clay layer is expected measurements are taken until the measured resistivity first falls and then begins to increase, showing that the conductive layer has been fully imaged.

Data Processing: Processing the data amounts to simply removing any obviously bad data points.

Data Interpretation: The data are then input to a computer program that produces a layered depth and resistivity model whose resistivity curve matches that of the field data. This process starts with the interpreter inputting a preliminary resistivity/depth model. The program calculates the resistivity curve from the model and evaluates the differences between the resistivity field data and the resistivity data from the model. It then adjusts the model and calculates a new resistivity curve. This process is repeated until the resistivity data from the model match that from the field data. This process is called inversion.

Advantages: Resistivity soundings are a good method for obtaining the vertical distribution of resistivity to depths of about 50 meters.

Limitations: The method is somewhat cumbersome in that four electrodes have to be inserted into the ground for each resistivity measurement. If the ground is hard or covered with concrete or asphalt, placing the electrodes can be quite time consuming. In addition, in dry ground, water may have to be poured onto the electrodes to improve the electrical contact between the electrode and the soil.

The interpretation assumes that the geological layers are homogeneous and horizontal and have a large lateral extent compared to their depth. In the case illustrated in figure 107, the clay layer has a limited lateral extent, and the interpreted depth and resistivity of the clay layer will not be correct.

Since this method uses electrodes to put current into the ground, any local metal objects above ground that provide a low resistance path between these objects and the ground, such as a fence, will also become influenced by the electrode array used to take the resistivity measurements. They will then produce voltages that will be detected by the electrode array and reduce the interpretability of the data.

Automated Resistivity Systems

Basic Concept: Resistivity measurements can be taken using recently developed instruments that use addressable electrodes and can be recognized by the control unit. These systems are called Automated Resistivity Systems. With these systems, the required electrodes are planted prior to any data recording, and the wires between all of the electrodes to the recorder are also connected. The recorder is then programmed with the electrode array and other data recording parameters and instructed to take the m easurements. One such system is called a Sting/Swift system and is illustrated in figure 201.

Sting-Swift Resistivity Measuring System. (Advanced Geosciences, Inc.)

Figure 201. Sting-Swift Resistivity Measuring System. (Advanced Geosciences, Inc.)

Data Acquisition: This system is able to provide detailed resistivity data. However, because of the labor required to plant the electrodes and lay out the cables, this system is probably not suited for reconnaissance exploration of large areas. If large areas are to be explored, other methods should be used; this technique should be used for detailed analysis of interesting anomalies. Conductivity methods, discussed previously, may be appropriate for reconnaissance.

The automated resistivity instrument records data from a specified number of electrodes, depending on the instrument used. If longer lines of data are needed, the system is moved along the line as required, and the data recording process repeated.

Data Processing: Bad data points have to be removed. The data are then plotted as pseudosections, as illustrated in figure 153. These plots present data from the small electrode spacings near the ground surface line. Data from the larger electrode spacings are plotted some distance vertically beneath the ground surface line, simulating a plot of resistivity against depth along the traverse and creating a plot called pseudosection.

Measure (apparent) resistivity pseudosection.

Figure 153. Measure (apparent) resistivity pseudosection. (Advanced Geosciences, Inc.)

Figure 109 shows pseudosection data that have been interpreted to give a plot of resistivity versus interpreted depth along the traverse.

Resistivity data plotted to form a pseudosection. (Blackhawk GeoServices, Inc.)

Figure 109. Resistivity data plotted to form a depth vs resistivity section. (Advanced Geosciences, Inc.)

Data Interpretation: Interpretation is done using inversion software. This software iteratively modifies a preliminary resistivity/depth model until the calculated model data fit the field data.

Advantages: The method records a significant amount of data, and since data recording is automated, this is done quite efficiently. Because of the amount of data, detailed interpretations are possible.

Limitations: Since electrodes have to be inserted into the ground, the method is difficult to use in areas where the surface of the ground is hard, such as concrete or asphalt- covered areas. Also, if the ground is dry, water may need to be poured on the electrodes to increase the electrical contact between the electrodes and ground. The data are taken along a line, and lateral variations in resistivity normal to the line are not accounted for. However, recording lines parallel to each other can rectify this problem, although this may not be feasible for some roadway surveys. In addition, three-dimensional surveys can also be recorded.

Cultural features such as power lines, fences, and other metal features may provide false anomalies. These cultural features will have to be noted in the field and accounted for in the interpretation.

Time Domain Electromagnetic Soundings

Basic Concept: Time Domain Electromagnetic (TDEM) soundings provide another method to obtain vertical distribution of the resistivity of the ground in addition to resistivity soundings. This method is particularly well suited to mapping conductive layers. To a significant degree, this method has now uperseded the resistivity sounding method since it requires less work for a given investigation depth and generally provides more precise depth estimates. However, resistivity soundings are still useful for shallow investigations or when resistive targets are sought. Figure 120 provides a conceptual drawing of the TDEM method showing the transmitter and receiver and the current and voltage waveforms.

Time Domain Electromagnetic Sounding.

Figure 120. Time Domain Electromagnetic Sounding.

TDEM soundings are an electromagnetic method used to provide the vertical distribution of resistivity within the ground. A square loop of wire, whose side length is about half of the depth of investigation, is laid on the ground surface. This is the transmitter loop. A receiver coil is placed in the center of the transmitter loop. Electrical current is passed through the transmitter loop and then quickly turned off. This sudden change in the transmitter current causes secondary currents to be generated in the ground. Currents in conductive layers decay slower than those in resistive layers. The relation between the decay amplitude and time and layer conductivity and depth is quite complex. Generally, longer decay times relate to deeper depths.

The voltage measured by the receiver coil does not decay instantly to zero when the current is turned off but ontinues to decay for some time because of the decaying secondary electrical currents in the ground. The voltage measured by the receiver is then converted to resistivity. A plot is made of the measured resistivity against the time after the transmitter current has turned off (delay time), as illustrated in figure 121. At short delay times, the measured resistivity approaches that of the near-surface layer, whereas at longer delay times, the measured resistivity approaches that of the deeper layers.

A Time Domain Electromagnetic Sounding curve.

Figure 121. A Time Domain Electromagnetic Sounding curve.

Figure 121 illustrates the curve that would be obtained over three-layered ground where the near-surface layer is resistive. This is followed by a layer having a much lower resistivity (higher conductivity) and causes the measured resistivity values to decrease. The third layer is again resistive, as shown by the increasing measured resistivity values. This sounding curve illustrates the type of curve that may be expected over a clay layer between resistive layers.

Data Acquisition: The field layout for the equipment has been described previously. Switched current is passed through the transmitter loop, and the resulting voltage is measured by the receiver coil. The switching and measuring procedure is repeated many times, allowing the resulting voltages to be stacked and improving the signal-to-noise ratio. This procedure is repeated at different sounding locations until the area of interest has been covered.

Data Processing: Bad data points are removed and a sounding curve is plotted for each location showing the measured resistivity against decay time.

Data Interpretation: This sounding curve is interpreted using computer software to provide a model showing the layer resistivities and thickness. The interpreter inputs a preliminary model into this software program that calculates the sounding curve for this model. It then adjusts the model and calculates a new sounding curve that better fits the field data. This process is repeated until a satisfactory fit is obtained between the model and the field data. This process is called inversion.

Advantages: TDEM soundings are an efficient method for investigating the vertical distribution of ground resistivity. For conductive layers the interpretation is generally more accurate than that for resistivity soundings.

Limitations: Generally, the method is better suited to mapping conductive formations rather than resistive formations. However, since the desired target is clay, which is usually conductive, this is not a serious limitation.

The TDEM method is not suited to mapping targets at very shallow depths. This is particularly so when the resistivity of these layers is high. Generally, in a conductive environment, depths less than 2 m cannot be resolved.

The transmitter loop radiates electromagnetic fields in all directions. Metal objects on the ground surface are influenced by these fields and will influence the data. Such objects include fences, power lines, and other "cultural" features at the survey site. Since the transmitter loop has to have a side length about half of the investigation depth, surveys done along roadways may be constrained due to the limited space available.

Induced Polarization

Basic Concept: Induced Polarization (IP) is an electrical method that measures the change in the measured resistivity of the ground with frequency. All of the electrode arrays (figure 90) used to measure resistivity can also be used to measure IP. In fact, when measuring IP, resistivity is also measured.

Electrode arrays used to measure resistivity.

Figure 90. Electrode arrays used to measure resistivity.

Two methods are used to obtain IP data: time domain and frequency domain. In time domain, a constant current is passed into the ground for a short time (seconds) using two of the four electrodes and then rapidly switched off. During this off time, the remaining two electrodes measure the resulting voltage. If an IP effect is present, the voltage across these electrodes will not suddenly return to zero as the current is turned off, but will decay to zero over a period of time, usually within a few seconds. The electrode array, current and voltage waveforms are illustrated in figure 110. Although any electrode array can be used, the Dipole-dipole array, as illustrated in figure 110, is probably the most commonly used array. Simple IP measurements usually integrate the IP voltage over a specified time, say T1 and T2 (figure 110), providing a single number that is a measure of the IP response, often called chargeability.

Induced Polarization waveform.

Figure 110. Induced Polarization waveform.

In frequency domain, the transmitter current waveform is sinusoidal. IP measurements are taken at different transmitter frequencies, producing a variation of the IP effect with frequency called Spectral IP (SIP) when more than two frequencies are measured. If spectral IP values are required from data obtained in time domain, the current and voltage signals have to be converted to frequency domain, which is done using the Fourier transform.

IP phenomena occur over clays, metallic minerals, and graphite zones. It is also observed near fences and other metallic surface features. The IP effect resulting from clay is discussed in the introduction to this section and involves Membrane Polarization.

Data Acquisition: Induced Polarization surveys are conducted much like resistivity surveys. Readings are taken at discrete stations to form lines of data crossing the area of interest. The automated resistivity systems discussed previously can also be used to record IP data. Figure 151 illustrates one of these systems, which can record up to 10 channels simultaneously.

Induced Polarization instrument.

Figure 151. Induced Polarization instrument. (Iris Instruments, Inc.)

Spectral IP data provide more interpretable information than the simple IP measurements discussed above and are reported to be more definitive for interpretation of clays. However, this is still being researched and is rarely used as a production method

Data Processing: Both the resistivity and IP data are usually plotted to form a pseudosection described previously. A typical chargeability pseudosection is shown in figure 152. Figure 153 shows the associated resistivity section.

Chareability pseudosection. (Terraplus, Inc.)

Figure 152. Chareability pseudosection. (Terraplus, Inc.)

Measure (apparent) resistivity pseudosection. (Terraplus, Inc.)

Figure 153. Measure (apparent) resistivity pseudosection. (Terraplus, Inc.)

Although these data are from a shallow survey using small electrode spacings, it does illustrate the method, presentation techniques, and the interpretation. Surveys to much greater depths can be performed using larger electrode spacings.

Data Interpretation: The data is interpreted using software that can produce a model that fits the field data. Although the literature often refers to the use of the IP method to locate clays, no published information was found showing a correlation between the magnitude of the IP or SIP response to clay type or content. The spectral IP response to clays may provide more interpretable information than the conventional IP response. Investigating clay content using the SIP method appears to have promise and would benefit from additional research.

Since resistivity measurements are also obtained when recording IP data, these two parameters can be used to determine the occurrence of clay and possibly map the clay content.

Advantages: The IP method is reported to respond to clay providing a higher IP response in their presence. If this is so, this may be the only method that, in the right environment, may provide a unique clay response.

Limitations: The IP method is quite labor intensive and requires electrodes to be inserted into the ground. Generally, because the method requires more electrical current than the resistivity method, the contact resistance (resistance of the electrode to ground) of the electrodes usually has to be lower than that for an equivalent resistivity survey. Thus, the electrodes may have to be inserted to greater depths or saturated with water to improve the electrical contact with the ground. If the ground surface is hard (concrete or asphalt), additional work has to be done to insert the electrodes. If spectral IP measurements are required, additional care must be taken since the desired voltages may be very small, depending on the specifications of the recording parameters.

Any grounded metal structures, for example metal fences, near the electrodes in use will influence the data, as will buried metal features such as pipelines.