The RESOLVE frequency domain HEM system has been used to map the distribution of near-surface<br>clay-rich sediments in and around the Riverland irrigation districts of South Australia.<br>Approximately 12,000 line-km was surveyed at line spacings of 150 or 300 m. The data were<br>recalibrated with measurements from down-hole induction logs and then inverted using a 1-D layeredearth<br>model. In order to improve the sensitivity to the unknown aspects of the section, the inversion was<br>constrained with as much local geological and hydrologeological information as possible. These<br>constraints included information about the depth of the water table, the conductivity of the groundwater,<br>the variability of the conductivity and thickness of three sedimentary units, and the geomorphic history<br>of the area.<br>The resulting detailed map of the distribution of the Blanchetown Clay is being used to model the<br>recharge behavior of the area, which in turn can be used to help predict the future course of salinity<br>inflows to the River Murray. If more areas are to be released for irrigation, the map could be used to<br>select areas of thicker clay that are preferred locations for such developments.<br>The results of the inversion also allow us to reconstruct the strandline-dominated paleo-topography left<br>when the sea retreated from the Murray Basin in the Early Pliocene. The survey also revealed a hitherto<br>unsuspected, deeper variability in conductivity following the Pliocene strand line pattern. The cause of<br>this pattern is not clear. It could be due to variation in the porosity of Loxton Sands or to strandlinecorrelated<br>variability in the elevation of the contact between the Bookpurnong Beds and the Lower<br>Loxton sands.


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