Geology topics

Craig Hartline

Publications and source records attributed to Craig Hartline.

7 recordsLinked to original sources

USGS2026

Monitoring reservoir processes at The Geysers geothermal field: Joint time-lapse acquisition and cooperative joint inversion of passive seismic and magnetotelluric data

To monitor time-lapse changes in reservoir water and steam concentrations, as well as subsurface flow in the geysers geothermal field in California, we implemented a multi-physics geophysical imaging approach. This approach utilized continuously monitored microseismic events coupled with repeat magnetotelluric (MT) observations. Travel time data from microseismic events recorded at surface stations were collected over an area of approximately 75 km² continuously from 2018 to 2023, alongside three repeat MT surveys conducted in 2021, 2022, and 2023. The resulting seismic and MT data were inverted both separately and jointly to generate time-lapse images of geophysical properties that correspond to temporal changes in water and steam saturation within the reservoir, as well as flow paths and barriers. By correlating the geophysical images with known water injection and steam production volumes, we were able to calibrate the data and gain confidence in the results, which can now be applied throughout the reservoir where borehole data are unavailable. The joint imaging results, combined with reservoir data derived from borehole observations, allow for the interpretation of temporal changes in the reservoir. This enables the operator to optimize reservoir management and their drilling program.

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USGS2025

Summary of results from monitoring the Geysers with continuous passive seismic and repeat magnetotelluric measurements (2021-2023)

Understanding temporal variations in a geothermal field can support operators in decision making that pertains to optimizing production and mitigating hazards. Between 2021 and 2023, The Geysers geothermal field in northern California was monitored with an array of continuous passive seismic sensors and annual repeat magnetotelluric (MT) measurements. Each of these data sets were analyzed and modelled separately to understand the data, sensitivity, and any observable changes. Then, the data were inverted jointly using a crossgradient method to further constrain temporal changes in geophysical properties within the geothermal field. Multiple permutations of annual datasets were used as inputs to the joint inversion. Results demonstrate seismic data constrain smooth inversion of the MT data, and the MT data provide supplementary information about the location of temporal fluid changes. Estimating relative changes in steam saturation for various time intervals of the joint models shows compartmentalized changes in the field, and good spatial correlation with the location of injection wells. These results demonstrate that collecting both passive seismic and MT measurements then modeling them jointly provide complementary information and a relatively inexpensive method for monitoring temporal changes in an active geothermal field that provides results to support operators.

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USGS2023

Cooperative joint inversion of magnetotelluric and microseismic data for imaging the Geysers geothermal field, California, USA

The Geysers geothermal field located in northern California, USA, is the world’s largest electricity-generating geothermal facility. To delineate the spatio-temporal distribution of reservoir steam and recharge water, we have collected microseismic and magnetotelluric (MT) data using a dense array of stations in 2021. The microseismic and MT data have been inverted together using a 3D cooperative joint inversion workflow. The joint inversion exploits a cross-gradient structural constraint because electrical conductivity structures observed in the geothermal field are strongly correlated with

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USGS2023

Magnetotelluric monitoring of the Geysers Steam Field, northern California: Phase 2

An original magnetotelluric (MT) survey collected in 2017 included 42 MT stations mainly in the northwestern part of The Geysers geothermal field in northern California. These data were modeled in 3D and imaged the electrically conductive cover, the electrically resistive steam field, and the electrically resistive Geysers plutonic complex (Peacock et al., 2020; Peacock et al. 2020a). Success of the original survey initiated collaboration between the U.S. Geological Survey and Lawrence Berkeley National Labs to monitor the steam field with MT and passive seismic to create a joint 4D model over the course of three years. This project, funded by the California Energy Commission, began in 2020. Two repeated MT surveys were collected at The Geysers, one in April 2021 (Peacock et al., 2022) and the second in April 2022 that extend further south adding 13 stations to the original 2017 survey. Reported here are observations comparing the 2017 data with the 2022 data. Preliminary results indicate the steam field changed in differently across the steam field, similar to changes observed between the 2017 and 2021 data.

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USGS2022

Repeat magnetotelluric measurements to monitor The Geysers steam field in northern California

The Geysers in northern California is the world’s largest electricity generating steam field. To help understand changes in the steam reservoir, repeat magnetotelluric (MT) measurements are being collected once a year from 2021-2023. These data will be compared and modeled to provide 4-D images of changes within the reservoir. Joint inversion with passive seismic data will be done to further constrain changes observed in the geophysical models. This study describes the first repeat survey and provides comparisons with MT data collected in 2017. In April 2021, 41 of the 42 MT stations collected in 2017 were repeated in addition to 14 new stations in the southern part of the steam field. Calculating residual phase tensors from MT responses between the two surveys shows compartmentalized changes within the steam field. Changes are observed at periods longer than 1 second with the largest changes of up to 30 percent observed at periods of 30 seconds. The residual phase tensors also show good repeatability between the surveys for periods less than 1 second, with changes on the order of 1 percent. To model the data, the preferred 3D resistivity volume that resulted from the inversion of the 2017 data is employed as the starting model for inversion of the new data. The two resulting inversion models are then subtracted to identify areas of change within the reservoir.

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USGS2020

Geophysical characterization of the Northwest Geysers geothermal field, California

The Clear Lake Volcanic Field in northern California is the youngest and northern-most part of a long-lived volcanic system that has produced recent (~10 ka) eruptions. Adjacent to the Clear Lake Volcanic Field is the worlds largest energy producing geothermal field, The Geysers. The hottest part of The Geysers geothermal field is in the northwest where temperatures reach ~400 C at 3 km depth. Low permeability, high thermal gradients, and low steam saturation prescribed development of an enhanced geothermal system (EGS) in the Northwest Geysers to increase energy producing capacity. Though the Northwest Geysers is known to be the hottest part of the field, geophysical methods have failed to adequately image any inferred heat source. This project aims to image the heat source of the Northwest Geysers using newly collected gravity and magnetotelluric (MT) measurements. Gravity data were jointly modeled with existing magnetic data along a two-dimensional profile aligned with an existing geologic cross-section. The key feature of the potential field model is a low-density, low-susceptibility body at 5 km depth (bmsl) under the EGS. MT data were modeled in three-dimensions to characterize subsurface resistivity structure, where the upper 3 km of the resistivity model agrees well with existing data. Lithologic and steam saturation are estimated from modeled resistivity values using existing geophysical data. Below 3 km depth (bmsl), the resistivity model images a possible young intrusion under the EGS. A possible zone of partial melt (<5%) below 7 km depth (bmsl) in the northwestern part of the field is also imaged which extends northeast towards the main part of the Clear Lake Volcanic Field.

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USGS2019

Geophysical Characterization of the heat source in the Northwest Geysers, California

The Geysers, in northern California, is the largest energy producing geothermal field in the world. Looking to expand capacity, the operator Calpine Corporation developed an anomalously hot (~400 °C at 2.5 km depth) part of the field in the northwest Geysers, including testing of an enhanced geothermal systems (EGS). Though the area is anomalously hot, geophysical methods have failed to adequately image any inferred magmatic heat source. Gravity measurements were collected and jointly modeled with existing magnetic data along a two-dimensional profile aligned with an existing geologic cross-section. The key feature of the potential field model is a low-density, low-susceptibility body below the EGS at 5 km depth. Magnetotelluric (MT) measurements were collected around the northwest Geysers and modeled in three-dimensions to characterize subsurface resistivity structure. The resistivity model images an extension of a Quaternary granitic pluton locally known as “the felsite” under the EGS project and a possible zone of partial melt (<10%) below 7 km in the northwestern part of the field.

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