Modelling the effects of ocean and sediments on electromagnetic fields, example from the Gawler Craton, South Australia

Abstract

Magnetotelluric (MT) data have been frequently collected over the past few years in form of 2D and 3D surveys across much of the Gawler Craton. The increasing coverage of sites allows a regional analysis of the underlying resistivity distribution with the help of 2D and 3D inversion routines and helps constrain the delineation and nature of geological boundaries. Increasingly, the resistivity models have been combined with existing geological and geochronological knowledge to analyse the tectonic evolution of the Gawler Craton. The pitfall of large-scale regional analyses of MT data is the conductive influence of the ocean and thick sedimentary sequences on the MT responses. In the case of the Gawler Craton, the sediments, with resistivities of around 10 Ohm m and thicknesses up to a few kilometers, contribute significantly to the inductive effect and can cause artefacts in the resistivity modeling. It is therefore essential to differentiate the inductive effect of the lithosphere and that of sediments and sea water. We present a way of quantifying the sediment and ocean effects by means of 3D forward modeling of the electromagnetic fields associated with them. We show that such an analysis is highly beneficial to further modeling of lithospheric structures.