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Electromagnetic induction data using distant field sources, mostly of natural origin, in the frequency range of 4.5-27,000 Hz are analyzed to depict the geoelectric structure in an area of volcanic-rock cover located in southeastern Arizona between the Mbrenci and Safford porphyry copper deposits. The data for each station consist of scalar electromagnetic measurements at descrete frequencies for two-orthogonal magnetic and electric field pairs. Observations spaced about 5-km apart indicate resistivities in the range of 100-700 ohm-m for the unweathered Tertiary volcanic rocks to a depth of 200 to 500 m. Beneath this zone the data indicate resistivities in the range of 10-100 ohm-m that suggest the existence of an older volcanic rock unit. The less resistive unit appears to be displaced upward beneath Turtle Mountain, an area bounded to the northeast and southwest by mapped Basin and Range faults, and bounded to the southeast by an unmapped fault of older origin that trends northeast. Lateral changes in the resistivity of the two main geoelectric layers result in lowered resistivity in an area of known hotsprings near the confluence of the Gila and San Francisco Rivers, as well as along a north-south trending zone located on the east flank of Turtle Mountain, about 5-km (3-mi) west-northwest of the hot springs. This second anomaly is at a probable depth of 400-500 m and is interpreted to indicate a buried fault or fracture zone. Geoelectric structure of the Gila-San Francisco Wilderness area, Graham and Greenlee Counties, Arizona from audio-magnetotelluric data by Douglas P. Klein and Michael J. Baer Introduction Twenty-one audio-magnetotelluric (AMI) soundings (4.5-27,000 Hz) were made in the northern Peloncillo and Gila Mountains of southeastern Arizona. Nineteen of these stations are located in the Gila-San Francisco wilderness area, which also incorporates the Gillard geothermal area (Figs. 1 and 2). These nineteen soundings are spaced at intervals of about 5 km (3 mi) and provide a reconnaissance picture of the geoelectrical structure to interpretated depths of about 1,000 ra (3,280 ft). The majority of the observations were made as part of the wilderness mineral evaluation program of the U.S. Geological Survey (USGS). The data presented also include five stations (50-54) obtained during a study of the Gillard and Clifton geothermal areas (Klein and others, 1980). Stations 53 and 54 are located about 13 km (8 mi) northeast of the Gila-San Francisco area. The data from these last two stations do not contribute to the interpretative discussion of this report but are included in the Appendices because they have been subjected to more complete processing than previously reported. AMT data consist of recordings of natural magnetic field variations and their corresponding induced electric fields in the Earth at descrete frequencies in the range of a few Hz to tens of kHz. The principles of the AMT method correspond to those of the magnetotelluric (MT) method (Cagniard, 1953, Vozoff, 1972, Vozoff and others, 1963) but the signals employed are at higher frequency and originate mainly from distant atmospheric electric disturbances rather than from ionospheric and magnetospheric phenomena. Strangway and others (1973) have considered the potential and limitations of the AMT method in mineral exploration. Examples of the character of the telluric signals employed in the AMT method, along with a summary of results obtained at 800 stations distributed mainly in geothermal prospects in the northern Basin and Range Province, is provided by Hoover and others (1978). Particular case histories of the AMT method in geothermal and mineral environments have been described by Strangway and Kozier, 1979, Hoover and others, 1976, Hoover and Long, 1976. This study supplements previous geological and geophysical descriptions of the Gila-San Francisco study area that provide considerable detail on its geological setting and mineral potential (Richter and Lawrence, 1981, Richter and others, 1982, Klein, 1983). The study area straddles the juncture of the Gila River and San Francisco River (fig. 1). Topographic elevations in the area range from a low of 975 m (3,200 ft) where the Gila River flows into the Fig. 1 Map showing location of Gila-San Francisco wilderness study area (shaded) in Graham and Greenlee Counties, Arizona (from Richter and others, 1981).

Geoelectric structure of the Gila-San Francisco Wilderness Area, Graham and Greenlee Counties, Arizona, from audio-magnetotelluric data