Geology topics

Jeffrey B. Witter

Publications and source records attributed to Jeffrey B. Witter.

6 recordsLinked to original sources

USGS2023

Three-dimensional geologic map the southeastern Gabbs Valley geothermal area, Nevada

This three-dimensional (3D) geologic map displays the subsurface geology in the upper ~4 kilometers of the Earth’s crust in the southeastern Gabbs Valley geothermal area of west-central Nevada. The 3D map was constructed by integrating the results from detailed geologic mapping, 3D gravity inversion modeling, and potential-field-geophysical studies. This effort was undertaken as part of the Nevada Play Fairway Project, a regional effort to characterize new geothermal resources in the United States. Local data collection and analysis in southeastern Gabbs Valley, Nevada, including the construction of this 3D map, led to the drilling of six temperature gradient wells and identification of previously unknown hydrothermal fluids at 150 meters depth. The measured temperatures, which are as high as 124.9 degrees Celsius, indicate that the southeastern Gabbs Valley hydrothermal system has temperatures that are comparable to geothermal fields that have been developed for electricity generation in the region. We describe the geologic units and structures displayed by the map and discuss the methods used to integrate the geologic and geophysical information into the 3D geologic interpretation. The accompanying map provides horizontal and vertical section views and oblique perspective views from several angles. The digital data for elements of the map, the individual 3D fault surfaces, and stratigraphic surfaces are available from Siler (2022). The accompanying map sheet and video displaying the 3D map are available at https://doi.org/10.3133/sim3498 .

Nevada↗

USGS2019

Three-dimensional geologic map of the southern Carson Sink, Nevada, including the Fallon FORGE area

The three-dimensional (3–D) geologic map characterizes the subsurface in the southern Carson Sink region. We created the 3–D map by integrating the results from seismic-reflection, potential-field-geophysical, and lithologic well-logging investigations completed in and around the Fallon FORGE site as part of the U.S. Department of Energy Frontier Observatory for Research in Geothermal Energy (FORGE) initiative from 2015–2018. The FORGE initiative was part of an effort to develop the technologies, techniques, and knowledge needed to make enhanced geothermal systems a commercially viable electricity-generation option for the United States. Geologic units and structures mapped during the Fallon FORGE study, which particularly focused on the Mesozoic basement, were extrapolated to create the 3–D map of the southern Carson Sink area. The 3–D map area is 10 km wide along the east-west and north-south axes and extends 2.5 km below sea level, ~3.7 km below the land surface. Views of the map include horizontal and vertical sections and oblique perspective views from several angles. We describe the geologic units and structures and discuss the methods used to integrate the geologic and geophysical information in a 3–D geologic interpretation. We provide digital data for elements of the map, such as individual 3–D fault and stratigraphic surfaces and surface-fault traces. Input data are available from various data repositories through cited web links. A brief movie displaying the 3–D map is available at https://doi.org/10.3133/sim3437 .

Nevada↗

USGS2019

Uncertainty and risk evaluation during the exploration stage of geothermal development: A review

Quantifying and representing uncertainty for geothermal systems is often ignored, in practice, during the exploration phase of a geothermal development project. We propose that this occurs potentially because the task seems so formidable. The primary goal of this paper is to initiate a dialogue within the geothermal community about: which geothermal uncertainties should receive the most attention and which uncertainty analysis methods could provide the greatest benefit for the advancement of the geothermal energy industry. Specifically, in this paper, we review uncertainty quantification techniques that are applicable to geothermal exploration. In general, uncertainty associated with data acquisition/processing (i.e., objective uncertainty) is small compared to the uncertainty in interpretational space (i.e., subjective uncertainty) that lies between data points where extrapolation is required. Therefore, it is important to classify, assess, and quantify uncertainty to help select strategies to reduce uncertainty and to better gauge the impact that separate uncertainties have on the overall likelihood of project success. The discipline of geostatistics provides multiple quantitative methods for producing stochastic models which adhere to measured data and spatial correlation. The petroleum industry has successfully used both geostatistics and decision analysis methods to combine diverse and multiple types of uncertainties. We argue that instead of one single and final interpretation of the geothermal system, numerous interpretations may be more indicative of the possible subsurface scenarios, and these different scenarios can be evaluated using decision analyses and value of information methodologies. Finally, we recommend that the potential power generation of a geothermal reservoir should be grounded in the geologic data and modeling for a specific field and their estimated uncertainties.

Geothermics↗

USGS2018

New data yield new geologic insights at the Fallon FORGE site, Carson Sink Region, Nevada

The geologic structure beneath the Fallon Frontier Observatory for Research in Geothermal Energy (FORGE) site represents a record of the Mesozoic through Cenozoic tectonism, volcanism, and sedimentation that has affected the Carson Sink local to Fallon, NV. A robust dataset confirms that the lithologic sequence consists of Quaternary through Miocene sedimentary and volcanic rocks resting non-conformably on Mesozoic crystalline basement. The basement consists of four lithologic units; 1) Triassic-Jurassic low-to-medium grade meta-rhyolites, 2) Jurassic low-to-medium grade quartzites and other metamorphosed marine sedimentary rocks, and 3) Jurassic low-to-medium grade meta-basalt and -basaltic-andesite lavas, all intruded by 4) Jurassic-Cretaceous quartz monzonite. The geologic section dips ~20-25° west, tilting that was accommodated by a predominant system of north-to-north-northeast striking, east and west moderately-to-steeply dipping normal faults. The above, relatively broad-scale characteristics of the geologic framework of the Fallon site have been developed throughout the ~3 year duration of the project. With the collection of new data and re-analysis of existing data in Phase 2B of the Fallon FORGE project, our detailed understanding of the relatively finer-scale aspects of the geologic framework, including aspects of the stratigraphic sequence and the locations and attitudes of individual faults have evolved and an updated 3D geologic map has been developed. Here, we compare the Phase 1 3D geologic map to the Phase 2B 3D geologic map and demonstrate the evolution of our understanding of the geologic framework of the Fallon site and the value of the new data that was collected in Phase 2B in developing this updated framework.

Geothermal Resources Council Transactions↗

USGS2018

2D and 3D potential field mapping and modelling at the Fallon FORGE site, Nevada, USA

Accurate geological characterization of Fallon FORGE is important for preparing the site as an EGS laboratory. As part of this effort, a 3D geologic map was constructed previously from well logs, surface geologic mapping, 2D seismic profiles, interpreted gravity & magnetic maps, and a gravity-inferred basement surface. In this study, we have conducted both 2D and 3D modelling of high-resolution gravity and magnetic data (pre-existing and new) in an effort to further refine and test this 3D geologic map at Fallon. This effort enabled a direct comparison of the 2D and 3D model results. Potential field modelling was guided by rock-property measurements of samples from drill-core and outcrop from the Fallon area. In total, five 2D potential field model profiles, up to 30 km in length, were constructed that extend across the Fallon area. The 3D gravity model volume was 8 km (N-S) x 8 km (E-W) x 4 km (thick). The majority of the 3D gravity model volume had 100 m cubic cells; but cells near the land surface were 1 m thick to adequately capture topography. Overall, the 2D & 3D geophysical modelling largely confirmed the previously constructed 3D geologic map at Fallon for three reasons: 1) lithologic boundaries in the 2D & 3D density models mostly agree with those in the 3D geologic map, 2) the rock properties used in the models lie within the range of independent measurements made on representative rock samples from the region, and 3) the match between the observed and calculated anomalies are largely within the measurement error of the observed fields. In places where the geophysical and geologic models differ, geophysical model results have revealed subsurface structural features that have helped refine geologic interpretations which, in turn, lead to adjustments to the 3D geologic map. In this paper, we present the 2D & 3D geophysical model results and discuss how they were utilized to confirm and refine our 3D geologic understanding of the Fallon FORGE site.

Geothermal Resources Council Transactions↗

USGS2014

Uncertainty and risk evaluation during the exploration stage of geothermal development

Quantifying and representing uncertainty for geothermal systems is often ignored, in practice, during the exploration phase of a geothermal development project. We propose that this occurs potentially because the task seems so formidable. The primary goal of this paper is to initiate a dialogue within the geothermal community about: which geothermal uncertainties should receive the most attention and which uncertainty analysis methods could provide the greatest benefit for the advancement of the geothermal energy industry. In this paper, we discuss uncertainty quantification techniques that are applicable to geothermal exploration. In general, uncertainty associated with data acquisition/processing (i.e., objective uncertainty) is small compared to the uncertainty in interpretational space (i.e., subjective uncertainty) that lies between data points where extrapolation is required. Therefore, it is important to classify, assess, and quantify uncertainty to help select strategies to reduce uncertainty, and to better gauge the impact that separate uncertainties have on the overall likelihood of project success. In addition, geostatistics provides multiple quantitative methods for producing stochastic models which adhere to measured data and spatial correlation. The petroleum industry has successfully used both geostastistics and decision analysis methods to combine diverse and multiple types of uncertainties. We argue that instead of one single and final interpretation of the geothermal system, numerous interpretations may be more indicative of the possible subsurface scenarios, and these different scenarios can be evaluated using decision analyses and value of information methodologies. Lastly, we recommend that the potential power generation of a geothermal reservoir should be grounded in the geologic data and modeling for a specific field and their estimated uncertainties. In this paper, we provide a brief overview of many of these topics while a more complete review has been recently published in Witter et al. (2019).

Conference Paper↗