Imaging Shallow Outflow Alteration to Locate Productive Faults in Ormat’s Brady’s and Desert Peak Fields Using CSAMT

Abstract

This paper details the results and interpretations from a 2017 CSAMT (controlled-source audio-magnetotelluric) survey collected at Ormat’s Brady’s and Desert Peak geothermal fields. The goals of the survey were to 1) image low-resistivity smectite-clay alteration caused by outflow of thermal fluids discharging along structures, 2) correlate alteration with structures using geologic mapping, subsurface data, and gravity/magnetic geophysical surveys, 3) integrate these structures into a geological model with the goal of optimizing the existing wellfield configuration. The CSAMT survey consisted of seven lines using 50m receiver dipoles and five lines using 100m dipoles for a total of 50km of survey line. Two transmitter locations were used during the survey, with survey lines spaced an average of 1km apart. Ormat’s geophysicists processed the data using 1D and 2D inversions to investigate the resistivity distribution over the survey area. In both geothermal fields, the CSAMT survey identifies low-resistivity anomalies primarily confined to the Truckee River Formation. Depth-slices through the model domain show how alteration occurs at the shallowest depths near productive faults, and moves basin-ward once entrained in shallower flow paths. High concentrations of ions found in Desert Peak geothermal fluids make the distinction of smectite-clay alteration less clear along the outflow path.