Estimation of magnetotelluric transfer functions from radio transmitters

A new method to estimate magnetotelluric (MT) transfer functions using radio transmitters as the source has been developed using an extended version of Weidelts C function expansion. The expansion coefficients are found from solving a mixed determined linear inverse problem by a truncated singular-value decomposition technique with a truncation level determined automatically from the data. Synthetic tests with both noise-free and noisy data show the new method is accurate and robust with respect to lack of transmitters in certain bands. A test profile with 42 stations from a dump-site investigation in the Netherlands shows that the estimated transfer functions are remarkably stable and smooth as a function of frequency and distance along the profile. Compared with the standard band-averaging technique, even the formal error bars are generally smaller and more realistic with the new method because the bias introduced by assuming that transfer functions are constant in a small subband is removed.