USArray magnetotelluric transfer functions (MT TFs) calculated at USArray MT sites installed by Oregon State University, as well as other community magnetotelluric transfer functions, are available from the SPUD EMTF repository in both XML and EDI formats. The international magnetotelluric community is invited to use the EMTF repository to archive their MT TFs. Please contact Anna Kelbert for support; all data formats are accepted and a data citation is created upon submission of data to the searchable repository.
The XML format for electromagnetic transfer functions and related conversion software was developed at Oregon State University under an NSF award, and later improved and updated by the USGS Geomagnetism Program, with continued support from IRIS. A living software repository is available in the SeisCode EMTF-FCU project File Conversion Utilities project. Detailed documentation and usage examples are provided by Kelbert (2009).
The EMTF data product contains magnetotelluric (MT) transfer functions (TFs), which define the frequency dependent linear relationship between components of electromagnetic (EM) field variations measured at a single site. Two sorts of TF are provided for most sites: 2×2 impedance tensors, and vertical field TFs, which respectively relate horizontal electric and vertical magnetic components (the outputs in the TF relationship) to the horizontal magnetic field vectors (the corresponding inputs).
Note that for some sites only impedances are provided. All of the TFs, which can be justified if external source fields are quasi-uniform (varying over length scales that are long compared to the skin depth), 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 the resulting sequence of Fourier coefficients. For remote reference estimates data from a second site are used for noise cancellation, and to reduce bias due to noise in the predicting magnetic channels. Apparent resistivities and phases, which can be computed from the off-diagonal components of the impedance tensors, are frequently used in 2D interpretations. The EMTF data product includes estimates of the TFs for a range of periods (which may vary, depending on instrumentation and deployment time), as well as estimation error covariances.
XML Format and Conversion Tools
The XML format used for EMTF products in SPUD is designed as a modern replacement for EDI. The format and software that converts between the EMTF XML and EDI were initially developed at Oregon State University under an NSF award, and later improved and updated by the USGS Geomagnetism Program, with continued support from IRIS. A living software repository is available in the SeisCode EMTF File Conversion Utilities project. Detailed documentation and usage examples are provided by Kelbert (2009).
To cite the IRIS DMC Data Products effort:
- Trabant, C., A. R. Hutko, M. Bahavar, R. Karstens, T. Ahern, and R. Aster (2012), Data Products at the IRIS DMC: Stepping Stones for Research and Other Applications, Seismological Research Letters, 83(5), 846–854, https://doi.org/10.1785/0220120032.
To cite the IRIS Magnetotelluric Transfer Functions data product or reference use of its repository:
- Kelbert, A., G.D. Egbert and A. Schultz (2011), IRIS DMC Data Services Products: EMTF, The Magnetotelluric Transfer Functions, https://doi.org/10.17611/DP/EMTF.1.
To cite a particular EMTF survey, or a specific transfer function:
- please see the Citation Info panel of the EMTF of interest. For example, to cite the USArray MT transportable array, use:
Schultz, A., G. D. Egbert, A. Kelbert, T. Peery, V. Clote, B. Fry, S. Erofeeva and staff of the National Geoelectromagnetic Facility and their contractors (2006-2018). “USArray TA Magnetotelluric Transfer Functions”. https://doi.org/10.17611/DP/EMTF/USARRAY/TA. Retrieved from the IRIS database on Oct 30, 2019
- Kelbert, A., S. Erofeeva, C. Trabant, R. Karstens, and M. Van Fossen (2018), Taking magnetotelluric data out of the drawer, Eos, 99, https://doi.org/10.1029/2018EO112859. Published on 27 December 2018.
- 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.
- Kelbert, A. (2019), Electromagnetic Transfer Functions File Conversion Utilities (EMTF FCU), USGS Software Release, https://doi.org/10.5066/P98PPLDV.
- Anna Kelbert, USGS
- Gary Egbert, OSU
- Adam Schultz, OSU
- Mick Van Fossen IRIS DMC
- MT Transfer Functions released : MT Transfer Functions released
- MT XML Transfer Functions : MT XML Transfer Functions
- MT Transfer Functions citation : Survey DOI attribution and citation preferences enabled
Geomagnetism Program, U.S. Geological Survey
College of Earth, Ocean and Atmospheric Sciences, Oregon State University