An Investigation of Static Shifts in MT Data from Parnaiba Basin Using Central-Loop TEM Data

Abstract

An attempt has been made to develop a simple framework for identifying and classifying magnetotelluric (MT) static shift in a sedimentary basin. environment. The concept of regional resistivity behavioural patterns is proposed and used to identify distorted soundings and the pattern zonations effectively map geological targets. The nature ofthe causative bodies is well defined and their depth. locations can be sought using the constraints provided by transient electromagnetic (TEM) data. In general, most of the static shift problems found in the Parnaiba basin can be explained in the context of intrusive volcanism and small outcrops or patches of conductive materials. Unlike conventional practice, we use a TEM transmitter loop of the same size as the MT electric dipoles thus yield. ing a better representation of the subsurface resistivity distribution in the vicinity of the MT observational station. The TEM and MT data at stations where the ground is almost homogeneous are in excellent agree ment suggesting that any of the MT polarizations can be used to determine the true subsurface resistivity structure. For areas of complex geology in the study region, we suggest that more accurate structural information can be gleaned from the combined central-loop and coincident- loop TEM data as against the popular approach that uses just the central-loop data in the joint analysis of MT and TEM data. For such areas, it was found that of the two sets of unrotated MT apparent resistivity curves measured at each location, the higher one at the short periods overlapping with the TEM data is in accord with the TEM data and this provides a criterion for selecting the appropriate MT data set for 1-D interpretations.