Airborne geophysics for natural resource management, Angas Bremer Plains, SA

Abstract

Airborne electromagnetics (AEM), magnetics and radiometrics flown over the Angas-Bremer alluvial plain southeast of Adelaide, SA, provide information for natural resource management in an area of intense viticulture with a shallow saline water table, and a deeper fresh-water Tertiary aquifer at risk of salinisation. A specific aim of the surveys was to delineate shallow palaeochannels that could be pumped to lower shallow saline water tables. These are not apparent in the data, either due to absence, or inability to image such features due to lack of contrast in physical properties. The surveys give a new view of the landscape, geology and regolith framework of the area. There is no evidence for a fault (postulated from resistivity soundings 30 years ago) that has been modelled as a recharge conduit to the deep aquifer. However, several previously unrecognized faults have been interpreted from the data and drill information. These offset the Cenozoic aquifers by up to 80 m. AEM and drill data have helped to determine a new set of structure contours for the base of the aquifer system which will assist more robust groundwater modelling. AEM response in the aquifers correlates loosely with measured water salinity, but the relationship is not tight enough to allow imaging of high quality groundwater from the AEM. The DEMs constructed from the surveys give better accuracy to landscape modelling in the area, and the radiometrics have the potential to form the basis for upgraded soil maps. Ten broad land management units across the area have been defined by reference to geophysical properties and drill hole information.