Geothermal resources in southwestern Utah: Gravity and magnetotelluric investigations

Recent geothermal studies on sedimentary basins in Western Utah suggest the possibility of significant geothermal reservoirs at depths of 3 to 5 km. This research focuses on 3 areas (Crater Bench, Pavant Butte, and Thermo), located within sedimentary basins having high geothermal potential. New geophysical data which include 364 gravity stations and 247 magnetotelluric (MT) stations collected during the summers of 2010 to 2012 have been used to augment historical gravity, electromagnetic, and borehole data where coverage is poor or insufficient. Two-dimensional gravity and MT models were created for these study areas in order to gain insight on the subsurface structural controls and to understand better the geothermal systems and potential of each study area. At Crater Bench, gravity and MT models show overall basalt flow thicknesses of 60 to 160 m and inferred depth-to-basement estimates of 1.3 to 3.6 km and a buried horst structure which is interpreted to be the structural control of the hot springs fluid flow. In the Pavant Butte study area, gravity and MT models display an elongate, mostly two-dimensional basin with a corridor about 15 km wide having 2 km of sediments on top of basement rock and in some areas reaching depths up to 3 km. Deep conductive bodies observed in this area hint at the presence of hot, saline fluids throughout the basin. Thermo displays intersecting gravity low trends of 4 to 10 mGal amplitude which intersect adjacent to the surface manifestation of the hot spring system and are interpreted as the structural control. Gravity and MT models indicate shallow depth-to-basement values (200 m) near the hot springs and up to 2 km to the southwest accompanied by low resistivities. Geothermal waters are old; water chemistry supports the conceptual model of waters migrating from the southwest basin up deeply penetrating faults and fractures to produce the hot springs. This geophysically-based study has added to the understanding of these potential geothermal systems.