Abstract
Lawrence Berkeley Laboratory (LBL) conducted controlled-source electromagnetic (EM) surveys at three geothermal prospects in northern Nevada to test and demonstrate the applicability of LBL's EM-60 system to geothermal exploration. During the late summer and early fall of 1979, over 40 soundings were made in Panther Canyon (Grass Valley), near Winnemucca; Soda Lakes, near Fallon; and McCoy, west of Austin.The EM-60 is a frequency-domain system using three-component magnetic detection. For the surveys reported here, we applied + or -65 A to a 100-m diameter four-turn horizontal loop, generating a dipole moment >10 6 mks over the frequency range 10 (super -3) to 10 3 Hz. With such a source loop and a remote magnetic reference for noise cancellation, we made soundings at transmitter-receiver separations of up to 4 km over the frequency range of 0.05 to 500 Hz. This has yielded a maximum depth of exploration to 2 km or more, adequate for the purposes of the geothermal investigations. Recorded spectra are interpreted by means of simple apparent-resistivity calculations made in the field and by layered-model inversions computed in the laboratory. The EM interpretations are then compared with other available geologic/geophysical data sets for the purpose of combined interpretation and method evaluation. Experience with the EM-60 system in Nevada has shown it to be an efficient and possibly more cost-effective alternative to dc resistivity and magnetotellurics for geothermal exploration. An average of two soundings per field day for depths of exploration up to 2 km was obtained routinely.Results from EM-60 work at Panther Canyon compare very favorably with earlier dipole-dipole resistivity surveys. Both methods outlined an irregularly shaped, buried conductive body associated with a region of high heat flow, but the feature was delineated by means of the EM-60 in just over half the field time required for the dipole-dipole resistivity survey. At Soda Lakes, 13 high-quality EM soundings were obtained from two transmitters in six field days under ideal field conditions. With the EM-60 data, we were able to map the depth to and inclination of a buried conductive body associated with an area of high subsurface temperatures. In this case, the EM results confirmed an earlier MT survey interpretation and gave additional detailed near-surface information. At the remote and mountainous McCoy site, data interpretation was complicated because of the rugged terrain. By modifying existing interpretive software, we were able to calculate the effects of tilted-source dipoles and elevation differences on soundings and thus interpret data. The EM soundings detected a conductive zone at a depth of 200 m at the south end of the prospect, where a nearby drillhole had encountered water at 100 degrees C at the same depth. In addition, EM soundings at McCoy provided information on a deep conductor below 2 km which has yet to be drilled.
