MAGNETOTELLURIC STATIC SHIFT: ESTIMATION AND REMOVAL USING THE COKRIGING METHOD

Abstract

We present and test a new method to correct for the static shift affecting magnetotelluric MT apparent resistivity sounding curves. We use geostatistical analysis of apparent resistivity and phase data for selected periods. For each peri- od, we first estimate and model the experimental variograms and cross variogram between phase and apparent resistivity. We then use the geostatistical model to estimate, by cokrig- ing, the corrected apparent resistivities using the measured phases and apparent resistivities.The static shift factor is ob- tained as the difference between the logarithm of the correct- edandmeasuredapparentresistivities.Weretainasfinalstat- ic shift estimates the ones for the period displaying the best correlation with the estimates at all periods.We present a 3D synthetic case study showing that the static shift is retrieved quitepreciselywhenthestaticshiftfactorsareuniformlydis- tributed around zero. If the static shift distribution has a non- zero mean, we obtained best results when an apparent resis- tivity data subset can be identified a priori as unaffected by static shift and cokriging is done using only this subset. The method has been successfully tested on the synthetic CO- PROD-2S22DMTdatasetandona3D-surveydatasetfrom Las Cañadas Caldera Tenerife, Canary Islands severely af- fectedbystaticshift.