Thermal Gradient Holes At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2003)
Exploration Activity: Thermal Gradient Holes At Long Valley Caldera Geothermal Area (Farrar, Et Al., 2003)
|Exploration Activity Details|
|Location||Long Valley Caldera Geothermal Area|
|Exploration Technique||Thermal Gradient Holes|
|Activity Date||1998 - 2002|
The study integrates detailed 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 to provide a comprehensive conceptual model of the different stages of hydrothermal activity, flow, and recharge in the Long Valley caldera groundwater system.
Temperature data for five wells drilled on the resurgent dome are considered, including new data from the recently completed LVEW borehole. Data from these wells are compared against temperature data from five wells drilled into the hydrothermal system in the west and south moat zones. Temperature profiles in wells located on the resurgent dome east and southeast of LVEW (wells 13-21 and 13-26), in the west moat (wells RD08 and 44-16), and in the south moat (Unocal Mammoth-1 well at Casa Diablo) 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. 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 relate to a major permeable fracture zone intersected by LVEW at 2600 m depth that contains a 103°C fluid, which must be ascending outside the wellbore to produce constant temperatures observed across the ~600 m depth interval. The authors postulate that this fracture zone may be a deep intersection between LVEW and the eastern graben fault (EGF) that bounds the eastern side of the caldera's medial graben. The EGF 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.
- 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