DOI: 10.5066/p14kaq3m
title: Magnetotelluric data from the Clear Lake Region, Northern California

description: The Clear Lake Volcanic Field (CLVF) in northern California is a young (2 Ma-0.008 Ma) volcanic system that could be hazardous upon eruption. Magnetotellurics (MT) is a passive electromagnetic geophysical method that measures subsurface electrical resistivity by recording the Earth's electrical response to natural time varying magnetic fields. To further understand the magmatic system of the CLVF MT data were collected with the end goal of producing a 3D subsurface electrical resistivity model. MT data were collected in multiple field campaigns between 2021 and 2022 on both public and private lands. Included in this data release is an HDF5 file per MT station that includes time-series data for each run, all runs combined and downsampled to 1 sample per second, and the estimated transfer function. These are formatted as MTH5 files (Peacock et al., 2022, https://doi.org/10.1016/j.cageo.2022.105102). All MT transfer functions are included in a ZIP file in EDI format, and an image of each transfer function is included in a ZIP file.
 
This data product contains MT transfer functions (TFs), which define the frequency dependent linear relations between components of electromagnetic (EM) field variations measured at a single site. Two types of TFs are provided for most sites, impedance data relating horizontal electric and magnetic fields, and tipper data, relating vertical and horizontal magnetic fields. All of the TFs provide input data for subsequent MT inversion and interpretation. They 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 a range of periods including variances.  
 
To interact with a MTH5 file, an open-source Python package is available `mth5`.  It can be installed through PyPi or Conda. Visit https://mth5.readthedocs.io/en/latest/index.html for more details. Any programming language that can read an HDF5 will also work.
  
To programmatically 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.

Transfer functions can also be found at on the Earthscope EMTF Spud repository (https://ds.iris.edu/spud/emtf) under the DOI: 10.17611/DP/EMTF/GMEG/Clearlake.  Time series can also be found on the Earthscope Data Management Services under the network code 1Z.
 
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://www.seg.org/Portals/0/SEG/News%20and%20Resources/Technical%20Standards/seg_mt_emap_1987.pdf. 
 
      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.