Deep electrical investigations in the Long Valley Geothermal Area, California

Abstract

Direct current resistivity and time domain electromagnetic techniques were used to study the electrical structure of the Long Valley geothermal area, A resistivity map was compiled from 375 total field resistivity measurements. Two significant zones of low resistivity were detected, one near Casa Diablo Hot Springs and one surrounding the Cashbaugh Ranch—Whitmore Hot Springs area. These anomalies and other parts of the caldera were investigated in detail with 49 Schlumberger dc soundings and 13 transient electromagnetic soundings. An extensive conductive zone of 1‐ to 10‐Ωm resistivity was found to be the cause of the total field resistivity lows. Drill hole information indicates that the shallow parts of the conductive zone in the eastern part of the caldera contain water of only 73°C and consist of highly zeolitized tuffs and ashes in the places that were tested. A deeper zone near Whitmore Hot Springs is somewhat more promising in potential for hot water, but owing to the extensive alteration prevalent in the caldera the presence of hot water cannot be definitely assumed. The resistivity results indicate that most of the past hydrothermal activity, and probably most of the present activity, is controlled by fracture systems related to regional Sierran faulting.

Magnetotelluric transect of Long Valley caldera: Resistivity cross‐section, structural implications, and the limits of a 2-D analysis

“…It is the major locus of the roughly 1.5 km vertical displacement separating the uplifted resurgent dome from the downdropped east moat, and also constitutes a primary control for the post-caldera moat rhyolites, voluminous hydrothermal alteration, and hot spring activity (Bailey et aI., 1976;Bailey, 1987). Much of our low-resistivity layer here presumably corresponds to the areally extensive low resistivity mapped by Stanley et al (1976). The drilling logs of Smith and Rex (1977) in Republic Geothermal hole 66-29 (Figure 1) confirm that the low resistivity results from alteration and porosity in post-caldera sediments and Early Rhyolite tuffs and the uppermost unwelded Bishop Tuff.…”

“…Reconnaissance bipole-bipole apparent resistivities (Stanley et al, 1976) show the greatest upper crustal conductances in the east moat of the caldera, and in a confined area to the south of the resurgent dome, but with a general north northwest preferred trend to the contours. Analysis combining long-period (20 s) telluric ellipses and gravity data by Hermance et al (1988) revealed north northwest-elongate conductive bodies in both the east and west moats of the caldera, as well as the prior confined conductor in the south.…”

Electrical Resistivity Imaging technique to delineate coal seam barrier thickness and demarcate water filled voids

“…According to Verma et al (1983) several workers have used different techniques of surface resistivity prospecting to study geological problems (Logn (1954), Van Nostrand and Cook (1966), Hallof (1966), Zohdy (1969), Zohdy et al (1973), Singh and Jha (1972), Jain et al (1973), Stanley et al (1976) and Patella (1978), Maillol et al (1999)). However, the response of coal seams on account of Hyderabad, India has taken up the work to carry out experimental geophysical studies to find out the water filled voids and the possibility of finding the barrier thickness of coal seams in the underground coal mining area.…”

Northern Nevada geothermal exploration strategy analysis

“…There is evidence from the amplitude of the long-period vertical magnetic (24 hour) variation and the magnetotelluric depth soundings that an anomalously shallow conductor occurs beneath the Basin and Range ' . (Hermance and Pedersen, 1976 A conductive zone, also determined by means of MT surveys (Stanley et al, 1976), was found at depths of 4-7 km in the Carson Sink area of Nevada.…”