USArray magnetotelluric transfer functions calculated from USArray MT stations by Oregon State University are available from the SPUD EMTF repository in both XML and EDI format and also directly from the DMC’s website in the EDI format.
An XML format for electromagnetic transfer functions and related conversion software was developed by OSU with funding from IRIS. The software and documentation is available in the SeisCode EMTF-FCU project.
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 developed at Oregon State University with support by IRIS. The software and related documentation are available in the SeisCode EMTF File Conversion Utilities project.
Oregon State University
Anna Kelbert, OSU
Gary Egbert, OSU
Adam Schultz, OSU