Geophysical Delineation of the Crater Bench, Utah, Geothermal System

摘要

Crater Bench, the site of Utah's most active thermal spring system is situated in west-central Utah within the Basin and Range province of western North America. With flow estimates of 5400 and 8400 lpm and temperatures of up to 87 °C, minimum thermal output is estimated at 20 MW. However, the geothermal system is poorly understood with little constraint on subsurface extent and capacity. We report on geophysical surveys leading to a drilling program aimed at correcting these deficiencies. Previous work done in the area of Crater Bench includes aeromagnetic, dipole-dipole resistivity and limited gravity surveys. In the summer of 2010, we added 50 magnetotelluric (MT) stations and 88 new gravity stations. 2D MT modeling shows a lens-shaped conductor 300 to 500 m thick overlying an area of low resistivity (10 to 60 ohm-m) directly below the Crater Bench basalt flows as well as depth-to-basement estimates of 1.3 to 3.6 km. The complete Bouguer anomaly map indicates a 20 km by 10 km mass excess centered at Abraham Hot Springs (AHS) on the eastern margin of Crater Bench. Results of 2D gravity modeling provide depth-to-basement estimates of 1.5 km (above the gravity high) to 3.4 km (adjacent gravity-low areas). Modeling results of MT and gravity data correlate well and the prevalent deep structure is interpreted as a horst block, that may facilitate an upflow zone for the hot springs. A volcanic heat source is not indicated by MT modeling though it may still be a minor component in a deep-circulation geothermal system. Our predictions will be tested by drilling at the site in the summer of 2011. Geophysical surveys are an effective, practical and non-invasive approach to delineating subsurface controls and characterizing geothermal systems.