Near-Surface Geophysical Characterization of Holocene Faults Conducive to Geothermal Flow Near Pyramid Lake, Nevada
Dudley, Colton; Dorsey, Alison; Opdyke, Paul; Naphin, Dustin; Ramos, Marlon; Louie, John; Schwering, Paul; and Pullammanappallil, Satish
Linear deposits of calcium carbonate tufa columns mark recent faults that cut 11 ka Lake Lahontan sediments at Astor Pass and Anderson Bay, north of Pyramid Lake, Nevada. Throughout the Great Basin, faults appear to control the location of geothermal resources by providing pathways for fluid migration. Reservoir-depth (greater than 1 km) seismic imaging at Astor Pass reveals a fault that projects to one of the lines of tufa columns at the surface. The presence of the tufa deposits suggests this fault carried warm geothermal waters through the lakebed clay sediments in recent time. The warm fluids deposited the tufa when they hit cold Lake Lahontan water at the lakebed. Lake Lahontan covered this location 11 ka to a depth of at least 60 m. In collaboration with the Pyramid Lake Paiute Tribe, an Applied Geophysics class at UNR, and Nevada Seismological Lab student workers investigated the near-surface geophysical characteristics of two proposed faults. The survey at Anderson Bay includes high-resolution magnetic surveys, six near-surface refraction microtremor arrays, and one P-wave seismic reflection and refraction array. The magnetic results show a northwest and southeast trending anomaly that differs 500 nT over distances as small as 100 m. The refraction microtremor results show that most shear velocities increase progressively toward the Lake shoreline, though one array shows a 57% decrease toward the shoreline. The 30 m depth-averaged shear velocities that decrease toward the shoreline indicate possible fault presence. The P-velocity refraction results indicate an area of low velocity within two deeply dipping structures, emphasizing the high magnetic gradient across the anomaly. Seismic reflection shows one reflective layer at 60 m depth, which is interpreted as depth to bedrock. Additionally, the survey at and near the tufa columns at Astor Pass comprises near-surface seismic reflection and refraction, electrical resistivity tomography, near-surface refraction microtremor arrays, nine near-surface direct-current resistivity soundings, magnetic surveys, and gravity surveys. The refraction microtremor results show shear velocities near tufa and faults to be marginally lower, compared to Vs away from the faults. Overall, the 30-m depth-averaged shear velocities are low, less than 300 m/s, consistent with the lakebed clay deposits. These results indicate that no seismically fast (> 500 m/s) tufa deposits are present below the surface at or near the tufa columns. Vs30 averages were for example 274 ± 13 m/s on the fault, 287 ± 2 m/s at 150 m east of the fault, and 290 ± 15 m/s at 150 m west of the fault. The P-velocity refraction optimization results similarly indicate a lack of high-velocity tufa buried below the surface in the Lahontan sediments, reinforcing the idea that all tufa was deposited above the lakebed surface. The seismic results provide a negative test of the hypothesis that deposition of the lakebeds in the Quaternary buried and preserved older tufa columns within the section. Near-surface Wenner arrays with a-spacings up to 30 m show a higher resistivity near the faults, and tufa, than away from the faults. Resistivity averages within a few meters of the surface were 33 ± 17 ohm-m on the fault, 13 ± 3 ohm-m east of the fault, and 9 ± 3 ohm-m west of the fault. It is possible that the geothermal waters are fresher, and more resistive, than waters held in the lakebed clays. Water samples from more than 1 km depth in exploration wells have a TDS of 2500 p.p.m., nearly drinking-water quality. The relatively resistive water, perhaps localized by greater permeability along the fault, could explain the higher resistivity measured near the fault. The results show that there is no high-velocity, high-resistivity tufa along the faults below the surface, so we are unable to use buried tufa to locate the faults that may promote geothermal upwelling in this area. We further hypothesize that as sedimentation buried the tufa during the Quaternary, warm geothermal waters re-dissolved it, and re-precipitated it in the cold lake-bottom water.
AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013