Magnetotellurics At Long Valley Caldera Geothermal Area (Hermance, Et Al., 1988)
Exploration Activity: Magnetotellurics At Long Valley Caldera Geothermal Area (Hermance, Et Al., 1988)
|Exploration Activity Details|
|Location||Long Valley Caldera Geothermal Area|
|Activity Date||1988 - 1991|
Public-domain magnetotelluric (MT) surveys published in 1988 and 1991 contribute additional information to the already extensive electrical data set for the Long Valley region available from Unocal Geothermal Division and Chevron Resources.
Regions of low resistivity were also identified during the 1988 public-domain MT investigations. Two-dimensional MT modeling of both the Unocal and public-domain 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.
- John F. Hermance, Warren M. Slocum, Gregory A. Neumann (1988) The Long Valley/Mono Basin Volcanic Complex: A Preliminary Magnetotelluric and Magnetic Variation Interpretation
- Philip E. Wannamaker, P.M. Wright, Zhou Zi-xing, Li Xing-bin, Zhao Jing-xiang (1991) Magnetotelluric Transect of Long Valley Caldera: Resistivity Cross_Section, Structural Implications, and the Limits of a 2-D Analysis
- 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