Magnetotellurics At Long Valley Caldera Geothermal Area (Nordquist, 1987)

GEOTHERMAL ENERGY

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Exploration Activity: Magnetotellurics At Long Valley Caldera Geothermal Area (Nordquist, 1987)

Exploration Activity Details
Location	Long Valley Caldera Geothermal Area
Exploration Technique	Magnetotellurics
Activity Date	- 1986
Usefulness	useful
DOE-funding	Unknown

Exploration Basis

Unocal Geothermal Division released the results from 158 time-domain electromagnetic (TDEM) sounding surveys in 1986, in addition to results from 77 magnetotelluric (MT) stations collected in conjunction with Chevron Resources. Data from these surveys have been published by Nordquist (1987) and reinterpreted by Park and Torres-Verdin (1988). These geophysical surveys were designed to assess the Long Valley hydrothermal system and to identify possible deep geothermal drilling targets beneath the western portion of the caldera.

Notes

The Unocal MT data outline a broad region with < 50 ohm-m invariant apparent resistivity at 1 Hz, and four regions with < 20 ohm-m resistivity within the caldera. Lithologic logs from wells drilled into these lower resistivity regions revealed strong correlations between intervals of extensive hydrothermal clay alteration and intervals of low resistivity in the wellbores. Reinterpretation of the Unocal and Chevron data sets have identified similar regions that exhibit low resistivity at shallow depth. Two-dimensional MT modeling of both the Unocal and public-domain (see additional activity entries) data sets across the caldera's west moat revealed a U-shaped low resistivity region with limbs elongated to the northeast and northwest of the (Unocal) Mammoth-1 well at Casa Diablo. This low resistivity region is unusually deep, extending into the pre-caldera basement to the northwest, and is roughly aligned with the projected location of the Laurel-Convict fault within the caldera. The intervals of well RDO-8 that host high temperatures and hydrothermal alteration also correlate with this deep region of low resistivity, reinforcing the association between low resistivity, extensive clay alteration, and active hydrothermal circulation. Intervals of low resistivity near well 44-16 are confined to geothermal reservoirs within the early rhyolite sequence and Bishop Tuff. Resistivity in the vicinity of Mammoth Mountain is similar to the resistivity structure encountered around well 44-16, but this interpretation is less certain due to sparse data coverage in the southwestern portion of the caldera. The MT data, together with data from wells drilled into and adjacent to the northwestern limb of the low resistivity region, support the interpretation that thermal waters ascend along deep basement faults and migrate into volcanic caldera-fill rock units beneath the caldera's west moat. However, the resistivity data obtained through the MT surveys do not eliminate Mammoth Mountain as a possible upflow zone within the present-day hydrothermal system.

References

• Gregg A. Nordquist (1987) Mapping the Hydrothermal System Beneath the Western Moat of Long Valley Caldera Using Magnetotelluric and Time-domain Electromagnetic Measurements

• Stephen K. Park, Carlos Torres-Verdin (1988) A Systematic Approach to the Interpretation of Magnetotelluric Data in Volcanic Environments with Applications to the Quest for Magma in Long Valley, California

• Michael L. Sorey, Gene A. Suemnicht, Neil C. Sturchio, Gregg A. Nordquist (December 1991) New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits