Schlumberger soundings near Medicine Lake, California

Abstract

The use of direct current resistivity soundings to explore the geothermal potential of the Medicine Lake area in northern California proved to be challenging because of high contact resistances and winding roads. Deep Schlumberger soundings were made by expanding current electrode spacings along the winding roads. Measured apparent resistivities were corrected using the geometric factor for the exact array geometry instead of a linear array geometry. For horizontally stratified, laterally homogeneous media, the apparent resistivities measured with a nonlinear Schlumberger array are equal to those measured with a linear Schlumberger array provided that (a) distances from the current electrodes to the center of the array are equal, and (b) the proper geometric factor is used to calculate the apparent resistivity. Corrected sounding data were interpreted using an automatic interpretation method. Forty-two maps of interpreted resistivity were calculated for depths extending from 20 to 1000 m. Computer animation of these 42 maps revealed that (a) certain subtle anomalies migrate laterally with depth and can be traced to their origin, (b) an extensive volume of low-resistivity material underlies the survey area, and (c) the three areas (east of Bullseye Lake, southwest of Glass Mountain, and northwest of Medicine Lake) may be favorable geothermal targets. Six interpreted resistivity maps and three cross-sections illustrate the above findings.