Annual repeat magnetotelluric data for monitoring The Geysers Geothermal Field, California (2021-2023)

The Geysers geothermal field in northern California is currently the largest energy producing steam field in the world. To better understand temporal subsurface changes in the steam field a geophysical monitoring project including continuous passive seimsic and repeat magnetotelluric measurements was funded by the California Energy Commission and led by Lawrence Berkeley National Lab. As one task of the project, the U.S. Geological Survey collected magnetotelluric data annually between 2021-2023 at approximately the same 50 locations depending on accessibility. The raw time series and processed transfer functions from annual repeat surveys in 2021 (55 stations), 2022 (50 stations), and 2023 (52 stations) are included in this data release. Data are formatted as MTH5 files (Peacock et al., 2022, https://doi.org/10.1016/j.cageo.2022.105102), where each HDF5 file includes all the data for a single repeated station including the time series sampled at field sampling rates and a combined sample rate of 1 sample per second, and transfer function for each year data was collected.  All transfer functions, in EDI and EMTF XML, and an image of each transfer function are included in a ZIP file.
This data product contains magnetotelluric (MT) transfer functions (TFs), which define the frequency dependent linear relations between components of electromagnetic (EM) field variations measured at a single site for horizontal electric and magnetic channels. No vertical magnetic fields were collected in these surveys due to cultural noise and difficulty installing a vertical magnetic sensor. TFs are estimated from the raw EM time series by (a) Fourier transforming data in a series of short overlapping time windows, and (b) applying robust regression methods to calculate TFs from the resulting spectral field estimates. For remote reference processing, data from one or more synchronously recording sites are used for more effective signal/noise separation. The supplied TFs are calculated for periods ranging from 0.001 to 3000 seconds including variance estimations.
 
To interact with a MTH5 file, an open-source Python package is available `mth5`, which can be installed through PyPi or Conda. Visit https://mth5.readthedocs.io/en/latest/index.html for more details.  Alternatively, any computer language that can read HDF5 can read an MTH5.
  
To progrommatically download files from Science Base, a user can use an open-source Python package `sciencebasepy`. For more details visit https://github.com/DOI-USGS/sciencebasepy.
 
References relevant to this release include: 
 

Wight, D., 1991, MT/EMAP Data Interchange Standard, The Society of Exploration Geophysicists, accessed January 19, 2021, at https://library.seg.org/pb-assets/technical-standards/seg_mt_emap_1987-…;
Kelbert, A. (2019), EMTF XML: New data interchange format and conversion tools for electromagnetic transfer functions, Geophysics, https://doi.org/10.1190/geo2018-0679.1.
Peacock, J. R., Kappler, K., Ronan, T., Heagy, L., Kelbert, A., Frassetto, A. (2022) MTH5: An archive and exchangeable data format for magnetotelluric time series data, Computers & Geoscience, 162, https://doi.org/10.1016/j.cageo.2022.105102.

 
This data release contains MT data for all sites in this collection.  The contents of the shapefiles are included in the "Entity and Attribute Information" section of the metadata.