《Single station magnetotelluric impedance(egbert》.pdf

GEOPHYSICS, VOL. 61, NO. 4 (JULY-AUGUST 1996), P. 964-970,5 FIGS. Short Note Single station magnetotelluric impedance estimation: Coherence weighting and the regression M-estimate Gary D. Egbert* and Dean W. Livelybrook‡ INTRODUCTION the EM fields at the local site, was proposed and developed by Gamble et al. [1979a, b] and Goubau et al. [1978, 1984]. The The first step in the interpretation of magnetotelluric (MT) 1 2 RR MT impedance estimates are often substantially improved data involves estimating 10 -10 frequency-domain imped- (less biased and more stable) over those obtained using the ances, Z(w), from the raw electric and magnetic field time 6 single station LS approach. series e(t), h(t) (approx. l0 real numbers/site) [e.g., Swift, Second, the simple LS estimator implicitly assumes a Gaussian 1967, Sims et al., 1971]. Superficially, this initial data reduction distribution for the errors in equation (1). This assumption step is almost trivial. Making the usual MT assumptions that often fails for MT data because of the nonstationarity of both the external sources are spatially uniform, and allowing for