Exploration Geophysics - Volume 26, Issue 4, 1995

A high-resolution aeromagnetic survey was undertaken by the Australian Geological Survey Organisation (AGSO) across the Bendigo 1:250 000 map sheet area during February to April 1994. For part of this period, high-resolution ground magnetometers were also deployed to test for the way that short-period variations of the geomagnetic field (including micropulsations) affect data recorded by the aircraft.

These ground instruments consisted of the newly developed Helium base-station for AGSO aeromagnetic surveys, and two ring-core fluxgate magnetometers developed by the School of Earth Sciences at The Flinders University of South Australia. The Helium instrument was deployed at the Bendigo airport, in the south-west corner of the area, and the ring-core instruments were buried in the north-west and north-east corners of the area.

Preliminary results indicate that while the phases of continuous Pc3 micropulsations of about 20 s period change little across the map sheet area, the amplitudes can vary significantly. For longer period geomagnetic variations (60 s to 600 s), there are significant differences in the amplitudes and phases of the total-field values measured at different sites. Induction vectors indicate a shallow zone of higher conductivity between the east and west measurement sites.

Data from a single base-station magnetometer subtracted from airborne survey data will not adequately remove the short-period geomagnetic variations of the total field recorded during this survey period — they are an additional noise source in the survey data.

The noise budget of an aeromagnetic survey is inherently high, having contributions from several components, for example, instrumentation, the platform, aircraft motion, navigation errors and effects of the temporal variations of the geomagnetic field. Variations in the geomagnetic time series are studied in terms of their contribution to the noise budget of high-sensitivity, high-spatial resolution aeromagnetic surveys. We quantify the electromagnetic and magnetic induction effect and show, from first order approximations, that these are insignificant. An experiment was undertaken to test the spatial coherence of the time varying component of the geomagnetic field. Over a baseline of 17km, and during quiet magnetic conditions, the temporal variations are incoherent within the micropulsation band. This latter result suggests that the limiting factor in resolving the temporal constituent of the noise budget of an aeromagnetic survey is the spatial coherence of micropulsations.

This paper reports on the results of the first detailed magnetotelluric experiments across the Adelaide Geo–syncline. 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.