No abstract

Gravity gradients delineate uncharacteristically straight, north-south trending graben faults in the Rio Grande rift west of Questa caldera, and gravity and audiomagnetotelluric (AMT) data show north-south trending low-density and high-resistivity zones from the caldera southward. A gravity inversion technique was used to isolate the gravity anomaly of the caldera and related intrusive rocks from the complicated Bouguer gravity field. The residual gravity anomaly together with AMT data reported in a companion paper by C. L. Long seem to delineate the subcaldera batholith. Geophysical models indicate this to be a north-south trending dikelike or beamlike body about 25 km long, 5 km wide, and > 4 km thick, thickness estimate being limited by the penetration depth of the AMT soundings. The north-south trend is not consistent with the regional pattern of early Miocene southwest directed extension and instead may reflect passive control by preexisting north-south strike-slip faults.

An audio-magnetotelluric (AMT) system has been devel oped by the U.S. Geological Survey for low-cost reconnais sagce exploration of geothermal regions. This is an electro magnetic sounding technique in which the scalar or Cagniard resistivity is computed at 12 frequencies logarithmically spaced from 7.~ to J8 600 Hz. Our system uses natural source fields except at the two upper frequencies of 10200 and 18600 Hz where artificial VLF sources are available. As a reconnaissance technique we use AMT surveys in conjunction with regional gravity, magnetic. and tell uric surveys. The exploration depth is a function of the resistiv ities 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 pro gram, 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 comple tion 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.

Audiomagnetotelluric (AMT) soundings at Questa caldera were made in the course of a mineral resource study of the Columbine-Hondo and Latir Peak Wilderness areas, New Mexico, in the fall of 1980 and 1981. This regional electrical survey covered an area of approximately 518 km 2 and consisted of 92 AMT stations at an average spacing of approximately 3 km. An AMT apparent resistivity map and one-dimensional models delineate several major lithologic and structural features. A large north-south resistivity high is interpreted to outline a largely concealed batholith 20 km long, 2-6 km wide, and >4 km thick. Two high-resistivity east-west trends coincide with the mapped margins of the Questa caldera, indicating the electrically contrasting rocks at its boundary. Apparent resistivity lows coincide with areas of hydrothermal alteration known to contain stockwork molybdenum deposits.

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 resistivitica at 7.5 and 27 Hz for all measurement locations within the Basin and Range and Columbia Plateau phyaiographic provinces indicate a narnjw distribution for the Basin and Range, w*hile 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,.