Audio-magnetotelluric methods in reconnaissance geothermal exploration




An audio-magnetotelluric (AMT) system has been developed by the U.S. Geological Survey for low-cost reconnaissance exploration of geothermal regions. This is an electromagnetic sounding technique in which the scalar or Cagniard resistivity is computed at 12 frequencies logarithmically spaced from 7.5 to 18 600 Hz. Our system uses natural source fields except at the two upper frequencies of 10 200 and 18 600 Hz where artificial VLF sources are available. As a reconnaissance technique we use AMT surveys in conjunction with regional gravity, magnetic, and telluric surveys. The exploration depth is a function of the resistivities of the lithologic section, but typically ranges from the surface to 0.2 km in low-resistivity areas and to greater than 2 km in high-resistivity regions. Results of the initial reconnaissance AMT surveys provide a rational basis for deciding on the extent of costlier follow-up surveys. As part of the U.S. Geological Survey geothermal program, surveys were conducted in Long Valley and Surprise Valley, California; the Vale, Ore-Weiser, Idaho region; and Bruneau-Grand View, Raft River, and Island Park regions of Idaho. AMT surveys in five additional known geothermal resource areas (KGRA's) have been scheduled for completion by May 1975. In the Raft River and Bruneau-Grand View regions and Long Valley, follow-up electrical surveys substantiated the effectiveness of the AMT technique for reconnaissance surveying.

Additional Publication Details

Publication type:
Conference Paper
Publication Subtype:
Conference Paper
Audio-magnetotelluric methods in reconnaissance geothermal exploration
Year Published:
U.S. Energy Research and Development Administration
Publisher location:
Washington, D.C.
Contributing office(s):
Energy Resources Program
6 p.
Larger Work Type:
Larger Work Subtype:
Conference publication
Larger Work Title:
Proceedings: second United Nations Symposium on the development and use of geothermal resources, San Francisco, California, USA, 20-29 May 1975
First page:
Last page: