Electrical Case Study:
A private individual purchased 20 acres in Montana in hill country overlooking the Missouri River on which he wants to build his retirement home. The land in this area is dry with the water table located approximately 300 feet below surface and numerous bentonite clay lenses are present in the subsurface. Bentonite is a specific kind of clay with the unique property that it expands when mixed with water. The drillers that operate in this area use reverse circulation water drilling rigs. These drills use water to carry the rock chips to the surface so the drill can continue to penetrate the subsurface. When these drills encounter significant amounts of bentonite, the water causes the clay to expand and the drill bit is frozen in place. If this occurs the driller must pull up his drill stem and find another location to drill the well. With drillers charging between $20 and $25/foot, the cost of hitting a bentonite lens can be expensive. The landowner selected two spots on which he would like to build his house and wanted to determine if he could drill a well at either of these two locations without hitting bentonite. I was contracted by him to conduct a small geotechnical survey to locate the depth to water table and any possible bentonite lenses at the two sites.
The field survey used is called a Schlumberger sounding survey. In this survey, two potential (voltage) electrodes are placed at the center of the profile line. Two current electrodes are placed at the ends of the profile line. The survey begins with the current electrodes each located one meter from the center. Current is induced into the ground using a transmitter and the voltage across the potential electrodes is measured using a receiver. The current electrodes are then systematically moved further out to 1.5, 2, 3, 4, 5, 7, 10, 15, 20, 30, 40, 50, 70, 100, 150, 200, and 300 meters and the readings are repeated. Each time the current electrodes are moved further out, the array looks deeper into the earth. Once all the data have been collected, the geophysicist models what layers of conductivity would cause the voltages to change for each sounding. Clay is conductive and has very low resistivities (~5 ohm-m). I expected the clayey soil found at the survey site would have resistivities between 20 and 100 ohm-m. Ground water generally increases the conductivity of whatever soil in which it is present. The water table should cause the background resistivities to decrease from 20 to 50 ohm-m. Any bentonite clay lenses within the soil should show up as very low resistive layers of approximately 5 ohm-m.
While conducting the resistivity survey you can also measure an effect called induced polarization (IP). Certain elements have a large number of free electrons on their surface. The two most notable examples are sulfides and clays. When the elements are placed in an electric field, the free electrons tend to orient themselves to the field. As the electrons move to orient themselves, they create a current which causes a voltage difference which can be measured. Since no sulfides were expected at the survey location, any IP effect measured could be attributed to clay lenses. Any large amounts of clay present should have IP chargeabilities in excess of 10 or 15.
Site A: Modeled Layers (red) with observed (blue) and modeled data (green)
The profile line for Site A was oriented to follow a relatively narrow ridge because any large changes in elevation can cause spurious responses in the resistivity sounding. The sounding curve is plotted in log-log format. The curved shows the observed data in blue, the layered model used in red, and the model curve in green. The Y axis shows the resistivity values between 10 and 100 ohm-m and the X axis shows the current electrode separation distance (and depth) from 1 to 300 meters. I modeled the curve using four layers. The layers are:
0.0 m to 0.5 m: 150 ohm-m, probably rock
0.5 m to 1.5 m: 15 ohm-m, very clayey soil
1.5 m to 81.5 m: 45 ohm-m, somewhat clayey soil
81.5 m to infinite: 30 ohm-m, water table in somewhat clayey soil
These values seem reasonable for the soil type present in the area and provide a reasonably good fit to the observed data. The high near surface resistivity was probably caused by a large rock. I had a lot of difficulty driving the current electrodes very deep into the ground near the center of the profile line. There does not appear to be any clay lenses present in this area except for the thin lens very near the surface (0.5 m to 1.5 m). The water table is modeled at a depth of 81.5 m (267 feet). The largest IP chargeability obtained during the sounding was 6. The low chargeability values also support the conclusion no significant clay lenses are present.
Site B: Modeled layers (red) with observed (blue) and modeled data (green)
The profile line for Site B was oriented to avoid crossing a road and to minimize changes in topographic elevation. The sounding curve is plotted in log-log format with the resistivity plotted on the Y axis and the current electrode separation distance (depth) plotted on the X axis. The resistivity values range from 10 to 100 ohm-m and the separation distance values range from 1 to 250 meters. I modeled the curve using two layers. They are:
0.0 m to 80.0 m: 75 ohm-m, slightly clayey soil
80.0 m to infinite: 25 ohm-m, water table in slightly clayey soil
This curve was much easier to model than the one for Site A because there were not any near surface contrasts. This area seems to be somewhat less clayey than Site A because of the higher resistivities. The IP chargeability values were also lower with the maximum value being only 3.
I consulted well logs for adjacent property owners' water wells to check the validity of the geophysical results. All of the logs in the area indicated clayey soil with occasional clay lenses. The water table depths for the neighbors' wells are approximately the same as my modeled water table depths. I informed my client that both sites appeared to be good locations for drilling a well since no clay lenses were detected. I recommended Site B slightly over Site A because the soil did not seem as clayey. This should improve the pumping rate he can achieve from his well. At the time of writing, the landowner has not yet drilled his well.
Results presented with permission of client.