Understanding temporal variations in a geothermal field is important for operators to optimize production and mitigate hazards. Between 2021 and 2023, The Geysers geothermal field in northern California was monitored with an array of continuous passive seismic instruments and annual repeat magnetotelluric (MT) measurements. Each of these data sets were analyzed and modelled separately to understand the data, sensitivity, and observable changes. Then, the data were inverted jointly using a cross-gradient 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 the seismic data further constrains the smooth inversion of the MT data and the MT data provides supplementary information about where fluid content has changed. Estimating relative changes in steam saturation for various time intervals of the joint models shows compartmentalized changes in the field, and good comparison with location of injection wells. These results demonstrate that these two datasets complement each other, provide results that could be used by an operator, and provide a relatively cheap method for monitoring temporal changes in an active geothermal field.

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