Geothermal Exploration Program, Hill Air Force Base, Davis and Weber Counties, Utah
Abstract
This report discusses results obtained from a program designed to locate a low- or moderate- temperature geothermal resource that might exist beneath Hill Air Force Base (AFB), Ogden, Utah. These studies were done on behalf of the U.S. Department of Energy by the University of Utah, Department of Geology and Geophysics and the University of Utah Research Institute, Earth Science Laboratory Division, as part of a cooperative agreement between the Departments of Energy and Defense.
A phased exploration program was conducted at Hill AFB. Published geological, geochemical, and geophysical reports on the area were examined, regional exploration was conducted, and two thermal gradient holes were drilled.
Hill AFB is situated on alluvial deposits of the Weber River delta, and is adjacent to the Wasatch Mountains. The alluvial deposits contain extensive near-surface cold-water aquifers that could mask deeper thermal fluids. Bedrock in the area is part of the so-called "overthrust belt" of Utah, Wyoming, Idaho adn Montana. The high-angle Wasatch fault separates the valley from the mountains. Faultst along which water could circulate to depth, be heated in the normal thermal gradient of the earth, and rise to the near surface, formed the primary exploration target at Hill AFB. Stratigraphic aquifers, that might contain thermal water leaked from faults, formed a secondary exploration target.
Mercy analyses of soils, done at Ogden Hot Springs to test this technique at a geothermal occurence near Hill AFB, indicated a strong but laterally restricted anomaly associated with hydrothermal activity. The small areal extent of the anomaly and extensive disturbance of soils on Hill AFB indicated that use of the soil mercury technique would be unlikely to contribute to target definition on the Base. Other chemical analyses of thermal and non-thermal waters near and on the Base failed to demonstrate a near-surface thermal component in the fluids.
Detailed gravity profiles, interpreted with constant and variable density models, suggest that the minimum depth to bedrock ranges from 0.46km (1500 ft) at the east side of the base of 2.29 km (7500 ft) on the west side of the base.
Nearly 15 line miles of reflection seismic surveys indicate the presence of north-south-trending faults on the base and confirm the depths to bedrock interpreted from the gravity data.
Two thermal gradient holes were drilled. Both holes showed the effects of the near-surface, cold-water aquifers. A hole at the east edge of the base was drilled to 390m (1280 ft), and had an observed bottom hole temperature of 13 degrees C (55 degrees F). A hole near the south gate was drilled to 996m (3269 ft), and had an observed bottom hole temperature of 40 degrees C (104 degrees F). These temperatures are cooler than would be expected in a normal Basin and Range province environment in the absence of cold water overflow in aquifers recharged by the Weber river.
This exploration program demonstrates that thermal waters are not present in the shallow subsurface beneath Hill AFB. Options remaining for utilization of the thermal content of the groundwater include ground-water heat pumps or deeper testing of zones with unknown temperature, water quality, and productivity.