William Schermerhorn | Search USGS

USGS2022

UAS-based tools for mapping and monitoring hydrothermal systems: An example from Mammoth Lakes, California

Unoccupied Aerial Systems (UAS) can accommodate a variety of tools for mapping and monitoring hydrothermal systems (e.g., magnetic, gas, photogrammetry, and thermal infrared [TIR]). These platforms offer increased speed, coverage area, and uniformity compared to ground-based measurements, as well as lower flight height – and therefore higher resolution – than occupied aircraft. We adapted a suite of tools for use with UAS and implemented these methods in a study focused on the area around Shady Rest Park, Mammoth Lakes, California, within the Long Valley Caldera. This location, which contains tree kills, gas vents, soil gas emissions, heated ground, and hydrothermal alteration, is the site of ongoing efforts to monitor changes in the surface expression of the local hydrothermal system. The methods applied in this study include: (1) airborne visible imagery for surficial mapping and the creation of high-resolution digital elevation models; (2) airborne magnetic measurements; (3) airborne TIR imagery; (4) airborne gas emission measurements; and (5) ground-based gravity measurements. We conducted these surveys in May and October of 2021, in part to establish baseline TIR and gas data against which future changes to the hydrothermal system may be assessed. UAS-based magnetic and ground-based gravity data were collected to map subsurface geology and to characterize potential subsurface controls on thermal anomalies and gas emissions. Results of these efforts at mapping and monitoring the hydrothermal system at Mammoth Lakes demonstrate how an integrated UAS- and ground-based approach may be applied more broadly to study other known or potential hydrothermal and volcanic systems. We consider the benefits and limitations of each method, particularly the TIR and gas sensors, which have less well-developed processing techniques in place for UAS applications. By integrating results from several of these different methods, however, the limitations facing each individual approach may be mitigated, and a better understanding of the hydrothermal system may be reached.

California↗

USGS2022

A geophysical characterization of structure and geology of the Northern Granite Springs Valley Geothermal System, Northwestern Nevada

The northern Granite Springs Valley in northwestern Nevada is the focus of recent studies for its potential for hosting undiscovered geothermal resources. Although the area lacks definitive surface manifestations of an active hydrothermal system, previous studies identify this region as having potential for hosting a blind geothermal resource, based on elevated subsurface temperatures and a favorable structural framework of the area. As part of the Nevada Play Fairway Project, we conducted high resolution geophysical surveys to better characterize the valley’s geothermal resources. This included ground magnetic, gravity, magnetotelluric, and rock property studies aimed at mapping and modeling subsurface geology and structure. Various derivative and filtering methods were employed to delineate buried faults and contacts from gravity and magnetic data. A depth to basement gravity inversion reveals that the basin is deepest on the west side of the valley. Flanking the basin to the east is a prominent gravity high interpreted as an intra-basin horst. A new high-resolution ground magnetic survey reveals a prominent elongate NW-trending magnetic high, interpreted as an unexposed subsurface dike swarm situated near the boundary between the basin and horst and confined to basement. Geophysical models help constrain basin fill comprised of Cenozoic sediments and volcanic rocks. These overlie Mesozoic crystalline basement that, in the west, consists of Cretaceous granitic intrusives and, to the east, dominantly Mesozoic metasedimentary rocks. The contact between these basement lithologies is not certain but inferred to coincide with the geophysically mapped dike swarm. This is partly supported by the fact that the dikes, as projected along strike to the northwest, intersect the contact between the Cretaceous intrusions and older Mesozoic basement rocks to the north of the study area. Although the age of the inferred dike swarm is not known, the trend of the anomaly is consistent with some of the Tertiary dikes in the nearby Sahwave Range, suggesting emplacement predated or was coeval with early development of the basin. The coincidence of the geothermal system, horst, dike swarm, and terminating normal fault zone suggests that basin tectonics and hydrothermal activity were influenced by both pre-existing basement structure and recent deformation. This relationship may pertain more generally to other hydrothermal settings throughout the Great Basin. If so, future efforts focused on mapping basement geology and structure may prove important to understanding underlying structural controls on geothermal systems. This work is supporting the next phase of research involving additional 3D geophysical and geologic modeling under the U.S. Department of Energy funded INGENIOUS project. The focus of this new work is on the western flank and structural corners of the horst block, based on evidence from detailed geophysical structural mapping, new shallow temperature data, and detailed 3D geologic and geophysical modeling, all aimed at identifying sites for temperature gradient drilling that may intersect zones with sufficient permeability and temperature to support geothermal development.

Geothermal Resources Council Transactions↗