USING GRAVITY AND MAGNETICS TO DELINEATE STRUCTURAL CONTROLS ON GEOTHERMAL FLUIDS, NORTHERN CACHE VALLEY, IDAHO

The Northern Cache Valley (NCV) of southeastern Idaho is a north-south trending Basin and Range graben that is receiving renewed interest for its geothermal potential. Several geophysical and hydrological studies of the NCV occurred during the 1970’s motivated by the presence of thermal wells and springs in the area, culminating with geothermal exploration wells drilled by Sunedco Energy Development. The test borehole temperatures were deemed too low for power production using technology available at that time (test hole temperatures < 120^o C). In January of 2014, a water well drilled to 79 meters, encountered Na-Cl-HCO₃ water with a measured bottom hole temperature of 104^o C. Traditional magnesium corrected Na-K-Ca geothermometry of water from that well estimated the temperature of the thermal reservoir to be 204° C. Inspired by this revived interest, new studies utilizing updated geochemical and geophysical techniques are currently under way. Present understanding of the NCV geothermal system suggests that fluid flow is associated with a fault(s) adjacent to Little Mountain – a small secondary horst complex within the primary Cache Valley graben. The existing data from the area is relatively sparse and is not suitable for pinpointing the location of faults thought to carry thermal water from depth to the near surface. To accurately characterize the plumbing of the geothermal system, lines of closely spaced potential field (gravity and magnetic) measurements were performed across the suspected location of the Little Mountain bounding faults. Analysis of these data in conjunction with existing hydrogeologic and geochemistry data will aid in determining fault locations carrying thermal fluid to shallow depths. Future work proposed to better characterize the geothermal system includes thermal imaging, utilizing an unmanned aerial vehicle (UAV), to detect elevated ground surface temperatures thought to exist based on preferential snow melt in the area.