Repeated Magnetotelluric Measurements at the Geysers, California

Abstract

An original magnetotelluric (MT) survey collected in 2017 included 42 MT soundings mainly in the northwestern part of The Geysers 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). Success of this 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. This project, funded by the California Energy Commission, began in 2020. Two repeated magnetotelluric (MT) surveys have been collected at The Geysers, one in April, 2021 and another in April, 2022. Both surveys extend further south within the steam field than the 2017 survey and thus require a larger modeling grid. The model from 2017 is interpolated onto the larger grid and used as a starting model to model the 2021 and 2022 data. The 2022 data are also modeled using the results of the 2021 data as a starting model. Reported here are the observations from those models which indicate the steam field became more resistive between 2017 and 2021. Between 2017 and 2022 the steam field is slightly more resistive in 2022. Between 2021 and 2022 minimal changes are observed.