A magnetotelluric traverse across the Adelaide Geosyncline

Abstract

This paper reports on the results of the first detailed magnetotelluric experiments across the Adelaide Geosyncline. The eastern end of the traverse is located on the Stuart Shelf, from where it then traverses the Adelaide Geosyncline and ends on the western side of the Murray Basin. The five components of electromagnetic time variation data at 15 sites were collected by the EMI MT-1 system. Regional strike direction (south?north) was first determined and found to be consistent with the strike of geological trends. The dimensionality indices indicated that in this specific coordinate system the MT data could be described as two-dimensional. We analyse, model, and interpret the data using several modern processing and inversion techniques (i.e. Occam2-D inversion, 2-D rapid relaxation inversion and 2-D finite element modelling). Thus an electrical conductivity structure model across the Adelaide Geosyncline has been determined. The model shows: (1) that a conductive sequence of 10?200 ohm-m which has depths from a few hundred metres to about 20 km in the Adelaide Geosyncline may represent the sediments, (2) an resistive body with a resistivity of >1000 ohm-m found in the Stuart Shelf at depths from 100 meters to 10 km may be interpreted as volcanic rock, (3) the old basement underlying the sediments was modelled to have a resistivity of 600 ohm-m, (4) There are two extremely conductive structures (1 ohm-m) within the interpreted sediments. One is located near the Torrens Hinge Zone with a maximum depth of 5 km. This vertical structure appears to intersect the easterly dipping sediments, and to be along a direct continuation of the Spencer Gulf graben structure. The second one is located within the thick pile of sediments of the Adelaide Geosyncline. This extremely conductive zone appears to extend to the lower crust.