Interpretations of a 3D Seismic Volume, Hawthorne Geothermal Field, Nevada

Hawthorne, Nevada is located in the Walker Lake Domain of the Great Basin, a region in the western United States known for extensional tectonics and the high temperature gradients necessary for geothermal power production. Geothermal heat sources include magmatic types and extensional. The extensional type is more common for Nevada, where near-surface thermal gradients come from a thinned crust instead of from volcanism. Extensional systems often do not exhibit surface indicators such as springs or fumaroles; rather, the thermal fluids remain capped below the surface in "blind" systems requiring the need for geophysical exploration. Heterogeneous compositions and seismic velocities common to geothermal systems create particular seismic imaging difficulties because simplifying assumptions about velocity gradients cannot be made. A 3d seismic volume collected by the Navy Geothermal Programs Office on the Hawthorne Ammunitions Depot represents a rare opportunity to examine the range of geologic interpretations that can exist on seismic data in the Great Basin. Strong reflection events within the volume project to a ~20 degree dip, allowing the possibility of a low angle normal fault; while bedding offsets could be interpreted as a series of steep basinward step faults. Synclines in vertical sections correspond to concentric circles in horizontal sections, not only raising questions about the possibility of migration processing artifacts, but also present similarities to sill intrusions as seen in marine 3d data. This paper explores the seismic evidence for a range of structural interpretations.