POTENTIAL-FIELD (GRAVITY AND MAGNETIC) METHODS APPLIED TO GEOTHERMAL SYSTEMS: THE UMATILLA INDIAN RESERVATION AND OTHER SYSTEMS OF THE WESTERN U.S (Invited Presentation)

Hydrothermal systems require natural permeability to provide pathways for fluid flow. Thus, characterizing the structural framework of a geothermal system is critical to understand and ultimately exploit geothermal resources. Potential-field methods offer a fast, inexpensive, and effective means of mapping and modeling subsurface structure and geometry, and are often employed to explore for and characterize areas potentially favorable for geothermal resources.

Our potential-field products include map-based interpretations, using filtering and derivative techniques, and models of crustal lithology, determined with forward and inverse methods. Through these approaches, we estimate the depth, thickness, and geometry of concealed units. When combined with other information (mapping, seismic, MT, borehole, etc.), we can constrain the geometry of reservoirs, map structural grain, and identify faults and fracture zones that facilitate circulation of geothermal fluids.

A recent study of the Umatilla Indian Reservation in NE Oregon provides an example. In 2017, we collected a high-resolution aeromagnetic survey, 1300 gravity stations, 69 MT stations, and extensive LiDAR data to investigate the area’s geothermal resource potential. The region is underlain by pre-Cenozoic basement, Cenozoic mafic extrusive and intrusive units, and Quaternary sediments; rocks that that span a range of properties well suited for potential-field studies. Geologic features (e.g., faults and flow margins) that juxtapose rocks with large contrasts in rock properties cause distinctive gravity and magnetic anomalies that permit us to extend available geologic mapping and better understand underlying tectonic and magmatic structure.

The newly acquired data reveal the study area sits in a complex zone of intersecting structural features that reflect deep crustal discontinuities, dike swarms, and faults – many of which have little to no surface expression. Offsets in anomalies across geophysically-inferred faults indicate sense of motion and provide estimates of accumulated horizontal offset along faults. The geophysical expression of laterally extensive faults, that extend well beyond their mapped extents, reveal structures that may be important targets for accessing deep-seated hydrothermal fluids.