A study of static shift removal from magnetotelluric data

Abstract

Near-surface inhomogeneities can distort magnetotelluric (MT) data, thereby limiting interpretational accuracy. In the specific case of near-surface galvanic distortion the resulting frequency independent shift in apparent resistivity sounding curves can be most readily identified in data which are predominantly one-dimensional (1 D). We here consider data from a detailed grid survey which are thoroughly contaminated by parallel offsets. The data were collected over a limited portion of the basalt-covered Paraná basin in Brazil. The data appear largely 1 D over a three decade bandwidth down to a low frequency limit of about 0.1 Hz. A realistic conceptual model consists of a variable thin-sheet over a basin in which the layer parameters vary sufficiently slowly (laterally) that a 1 D interpretation of the static corrected data is valid down to some maximum depth (in this case basement). We consider the performance of three methods which attempt to remove static offsets and provide assessments of the configuration of the geoelectric units of the basin. The three methods consist of curve shifting, statistical/spatial averaging, and the application of parameter constraints. The unambiguous removal of static offsets undoubtedly requires one or more independent constraints at each measurement location. In the absence of such control the three methods are necessarily statistical and each must supply a constraint that has least conflict with the data characteristics. Simple spatial averaging of our data proves effective but the degree of success is generally not known. Parametric constraints, applied as uniform layer resistivities, allow phase-only (non-distorted) data inversion. Inverse models obtained from this approach are found to provide accurate estimates of the depth to the main resistivity interface, the base of the basalt cover, in the vicinity of a control well.