A geophysical investigation of dryland salinity at Cressy, Tasmania

Abstract

Geophysical methods have been used at Panshanger Estate near Cressy, Tasmania, to map the distribution of salt-affected areas, and the underlying catchment structure. Frequency-domain electromagnetic (FEM) techniques were used to detect near-surface salt-affected zones based on their high conductivity. Areas visibly affected by salt were found to coincide with areas displaying the highest conductivities. Other areas displaying moderate to high conductivity did not appear to be salt-affected but may represent areas of future salt risk. Magnetic, gravity and time-domain electromagnetic (TEM) techniques, in conjunction with drill logs from holes near the field area, indicate that the western half of the field area is underlain by shallow Jurassic dolerite, while to the east and northeast, the dolerite is covered by a thicker layer of sediment. In the western part of the survey area, high conductivities occur close to the edges of bedrock highs, indicating that bedrock topography may be controlling groundwater movement. These bedrock highs generally correlate with surface topographic highs, and the elevated conductivities, indicative of high salt storage, are measured over depressions and creeks. A sinuous conductivity anomaly in the northeast of the field area occurs on slightly elevated ground, in an area where Tertiary cover is thick. There was no visible evidence of salinity at the surface over this feature. It was interpreted as a shallow palaeochannel based on EM measurements. An investigation borehole intercepted an upwardly fining sandy clay sequence, representative of a meandering fluvial system. EC 1:5 analysis of the sediment core and water analysis indicates that the palaeochannel may act as a mechanism of salt transport and storage.