Audiomagnetotelluric (AMT) surveys have been made in more than 40 geothermal areas in the western U.S. The majority of the measurements was made in the Basin and Range and Columbia Plateau physiographic provinces.Complete spectral analyses made in conjunction with the field surveys illustrate the impulsive nature of the natural signals which appear to overshadow the Schumann resonances at the low frequencies. The spectrum below 200 Hz is fairly flat and the signal strength is sufficient for operation throughout the year. Above 200 Hz and centered near 2 kHz, an absorptive band limits the signal strength making data acquisition difficult if not impossible in the mid-band region, particularly during the winter months. The spectra also show that power line harmonics may be very strong to about 1000 Hz and can be 30 to 50 dB above the natural signal level.Histograms of the measured apparent resistivities at 7.5 and 27 Hz for all measurement locations within the Basin and Range and Columbia Plateau physiographic provinces indicate a narrow distribution for the Basin and Range, while a broader distribution characterizes the Columbia Plateau. Comparisons between the average resistivities and Known Geothermal Resource Areas (KGRA) show that low average resistivities appear to correlate with regions containing a number of large volume geothermal systems and not with high heat flow.

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