Geophysical Signatures of the Milford, Utah FORGE Site

Abstract

Geophysical data have been utilized in a preliminary characterization of the Milford FORGE site, located in southeastern Utah. The site is situated over Tertiary-Quaternary granitic intrusions and Precambrian gneiss that crop out in the Mineral Mountains and are at 3.1 km depth in the Acord-1 well near the center of Milford Valley 10 km to the west. Modeling of the 20 mGal gravity low, caused by basin fill overlying the granite, was accomplished using a decreasing density contrast with increasing depth based on Acord-1 geophysical logs. 2D models show the granite gently dipping to the west beneath the surficial fan deposits on the western edge of the Mineral Mountains. This dip increases farther out, forming a more localized basin beneath the center of the valley. At the proposed deep drilling site, the granite is at about 400 m depth. A residual, reduced-to-pole aeromagnetic map is featureless near the proposed site due to the burial depth of the relatively magnetic granite. Long-wavelength, magnetic-high anomalies west of the site may be originating from buried Tertiary andesite which occurs in the Acord-1 well. The andesite crops out in the Beaver Lake Mountains 18 km to the west. Several magnetotelluric soundings close to the site show that the basin fill sediments have a resistivity of about 2-3 ohm-meters (Ωm) due to both their clay content and to the warm geothermal groundwater they host.