Time‐domain electromagnetic soundings at the Nevada Test Site, Nevada

Abstract

Structural discontinuities and variations in the resistivity of near‐surface rocks often seriously distort dc resistivity and frequency‐domain electromagnetic (FDEM) depth sounding curves. Reliable interpretation of such curves using one‐dimensional (1-D) models is difficult or impossible. Short‐offset time‐domain electromagnetic (TDEM) sounding methods offer a number of advantages over other common geoelectrical sounding methods when working in laterally heterogeneous areas. In order to test the TDEM method in a geologically complex region, measurements were made on the east flank of Yucca Mountain at the Nevada Test Site (NTS). Coincident, offset coincident, single, and central loop configurations with square transmitting loops, either 305 or 152 m on a side, were used. Measured transient voltages were transformed into apparent resistivity values and then inverted in terms of 1-D models, Good fits to all of the offset coincident and single loop data were obtained using three‐layer models. In most of the area, two well‐defined interfaces were mapped, one which corresponds closely to a contact between stratigraphic units at a depth of about 400 m and another which corresponds to a transition from relatively unaltered to altered volcanic rocks at a depth of about 1000 m. In comparison with the results of a dipole‐dipole resistivity survey, the results of the TDEM survey emphasize changes in the geoelectrical section with depth. Nonetheless, discontinuities in the layering mapped with the TDEM method delineated major faults or fault zones along the survey traverse. Schlumberger resistivity soundings expanded to an AB/2 of 1220 m detect only the first interface mapped by the TDEM method. Schlumberger and large‐offset, frequency‐domain, sounding curves appear to be more distorted by lateral variations than the TDEM sounding curves.