Electromagnetic Depth Sounder

An advanced electromagnetic technique for deducing depths of water tables as well as the conductivities of subsurface strata is described. Field data have been collected in the vicinity of Dale Dry Lake, in southern California, and our interpretation thereof is in very good agreement with geologic well data. The technique consists essentially of adapting a curve-matching process to the "two-coil tilt angle method." The variables used were tilt angle as a function of frequency with coil-coil spacing as the parameter. Water table depths at 10 and 30 feet, respectively, have been uniquely deduced as well as the conductivities of the ground-water and upper-layer overburden. In addition, the interpretation scheme facilitates the establishment of the most sensitive operating parameters for deducing a given depth of groundwater. For at least the two-layer case, our preliminary investigation has demonstrated that the depth to a conductvitity discontinuity can be uniquely determined by electromagnetic means.