Audio-magnetotelluric methods in reconnaissance geothermal exploration

About three years ago, at the initiation of a major effort by the U.S. Geological Survey into studies of geothermal systems, a decision was made to evaluate the audio-magnetotelluric, AMT, technique for this application. At that time we had had no real experience with the method nor any equipment. Some prior work by Lebel [1971], and Slankis and others [1972], suggested that field operations might be slow and difficult reducing the advantages of the method. A cautious approach was used in developing equipment and field techniques until some experience was gained with the method. As no commercial equipment was available, design and fabrication of the AMT equipment to cover the frequency range 8 Hz to 18,600 Hz was done in-house. Because of the nature of the method, our approach has been to use AMT surveys for reconnaissance exploration and to follow up with other electrical methods in interesting areas. In this paper some of the results of our research are shown to illustrate the effectiveness of the technique.