Some comments on magnetotelluric response function estimation

A new set of computational procedures are proposed for estimating the magnetotelluric response functions from time series of natural source electromagnetic field variations. These combine the remote reference method, which is effective at minimizing bias errors in the response, with robust processing, which is useful for removing contamination by outliers and other departures from Gauss-Markov optimality on regression estimates. In addition, a nonparametric jackknife estimator for the confidence limits on the response functions is introduced. The jackknife offers many advantages over conventional approaches, including robustness to heterogeneity of residual variance, relative insensitivity to correlations induced by the spectral analysis of finite data sequences, and computational simplicity. These techniques are illustrated using long-period magnetotelluric data from the EMSLAB Lincoln line. The paper concludes with a cautionary note about leverage effects by high power events in the dependent variables that are not necessarily removable by any robust method based on regression residuals.