Conceptual Model At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2003)

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Exploration Activity: Conceptual Model At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2003)

Exploration Activity Details
Location Long Valley Caldera Geothermal Area
Exploration Technique Conceptual Model
Activity Date 2003 - 2003

Usefulness useful
DOE-funding Unknown

Exploration Basis
Farrar et al. (2003) provide a comprehensive conceptual model of the different stages of hydrothermal activity, flow, and recharge in the Long Valley caldera groundwater system based on detailed integration of results from pump tests, fluid level monitoring, temperature logging, and fluid sampling/analysis of the LVEW with information obtained from other wells drilled on or near the Resurgent Dome.
Temperature data for five wells drilled on the Resurgent Dome were considered, including new data from the recently completed LVEW borehole. Data from these wells were compared against temperature data from five wells drilled into the hydrothermal system in the west and south moat zones. Temperature profiles in several wells showed peak temperatures between about 120-220°C at relatively shallow depths above ~1000 m depth that decline to the east, associated with eastward and southward lateral flow of thermal water through the system. These occurred in:

  • The wells located on the Resurgent Dome east and southeast of LVEW (wells 13-21 and 13-26)
  • The west moat (wells RDO-8 and 44-16)
  • The south moat (Unocal Mammoth-1 well at Casa Diablo).

This thermal regime differs significantly from the dominantly conductive temperature gradients averaging about 35 °C/km, observed in the Bishop Tuff in the upper interval of LVEW, and throughout the Clay Pit-1 well. This contrasts sharply with the temperature profile in the underlying metamorphic basement, in which temperatures in LVEW become isothermal below 2000 m depth. This isothermal temperature interval is inferred to be related to a major permeable fracture zone intersected by LVEW at 2600 m depth that contains a 103°C fluid, which must be flowing more-or-less vertically outside the wellbore to produce constant temperatures observed across the ~600 m depth interval. The authors of the original study postulate that this fracture zone penetrated by LVEW may be the Eastern Graben Fault that bounds the eastern side of the caldera's medial graben.[1] The Eastern Graben Fault is a steeply dipping normal fault thought to represent an extension of the northwest-trending Hilton Creek Fault, a major frontal fault of the Sierra Nevada Mountains to the south.Other, shallower wells drilled on or around the Resurgent Dome (wells Unocal Mammoth-1, 13-21, and 13-26) also exhibit isothermal intervals in the Bishop Tuff at about 100°C approaching the bottom of the wellbores. Of these wells, only (Unocal) Mammoth-1 penetrates metamorphic basement, in which the isothermal interval continues to the bottom of the well, similar to LVEW.


Additional References
  1. Christopher D. Farrar,Michael L. Sorey,Evelyn Roeloffs,Devin L. Galloway,James F. Howle,Ronald Jacobson. 2003. Inferences On The Hydrothermal System Beneath The Resurgent Dome In Long Valley Caldera, East-Central California, USA, From Recent Pumping Tests And Geochemical Sampling. Journal of Volcanology and Geothermal Research. 127(3-4):305-328.