Northeastern Section - 36th Annual Meeting (March 12-14, 2001)

Transient electromagnetic (TEM) soundings have long been used to map depth to bedrock, and stratigraphic layering (corresponding to hydrogeologic units) in the subsurface. We investigated the use of TEM to map the thickness of the Gray Delta in an abandoned sand pit in Gray, Maine. The site was chosen because sufficient drilling and geophysical information was available to provide a check on the resolution of our method. Our study entailed analyzing the resolution improvement in calibrating the TEM receiver coils and incorporating these results in a numerical model of the thicknesses and resistivities of the geoelectric section. TEM data calibration is deemed necessary due to the finite bandwidth of both the receiver and receiver coils. Interpretation involves correlating these derived geoelectrical layers with geologic/hydrogeologic units.

A known uniform magnetic field can be created in the center of a solenoid. We constructed a solenoid three meters long, with a radius of 0.65 m, which was big enough for the TEM field coils to fit inside. Data procedures were tested in a miniature version in which scale-model coils, used for physical scale modeling in the University of Southern Maine geophysical laboratory, were calibrated in a small solenoid. An HP35665 Dynamic Signal Analyzer was used to transmit a sweep of frequencies from 10 Hz to 51.4 kHz through the windings of the solenoid. This instrument then collected voltages across a known resistor, providing the transmitted current, and voltages from the TEM receiver coil inside the solenoid, yielding the coil's response. Corrections were then made numerically to field TEM data collected in Gray.

The Gray delta is a glaciofluvial outwash delta, comprised of fine- to medium-grained unconsolidated sands. Bedrock beneath the delta is a granite of the Sebago Lake pluton. TEM results showed that the delta has a resistivity of about 400 ohm-m and is approximately 30 m thick correlating well with seismic refraction data and information from nearby wells.