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Open File Report 175

Constrained inversion of Resolve electromagnetic data - Riverland, South Australia

Brodie R, Green A, Munday T


This report describes a subset of activities undertaken CRCLEME as part of contribution to the Riverland and Tintinara projects of the South Australia Salinity Mapping and Monitoring Project conducted under the auspices of The National Action Plan for Salinity and Water Quality (NAP). Specifically, it concerns the development of constrained inversion methodologies to invert RESOLVE helicopter EM data in order to map the location and thickness of near-surface clay-rich materials. In the Riverland region, the focus of this report, these materials are principally associated with the Blanchetown Clay sedimentary unit. The Blanchetown clay, and units of like texture, act to delay the rate of groundwater recharge and thus locations where it is present are more favourable for agricultural development.

After a preliminary analysis to select the optimum system and survey characteristics, approximately 12,000 line-km were surveyed in the Riverland region at line spacings of 150 or 300 m. The data were recalibrated with measurements from down-hole induction logs and then inverted using a 1-D layered-earth model. In order to improve the sensitivity to the unknown aspects of the ground section, the inversion was constrained with as much local geological and hydrological information as possible. These constraints included information about the depth of the water table, the conductivity of the groundwater, the variability of the conductivity and thickness of two sand units. An understanding of the geological and geomorphic history of the area was also used to help define the probable geometry, distribution and disposition of relevant sedimentary units.

A map of the distribution and thickness of the Blanchetown Clay and like-units was generated. The results of the inversion also allowed us to reconstruct the beach strandline-dominated palaeo-topography left when the sea retreated from the Murray Basin in the early Pliocene. They also can be used to define the landscape left after the demise of Lake Bungunnia. This latter surface, defined by the top of the Blanchetown Clay, is a complicated result of fluvial and aeolian redistribution processes that have, in some locations, reworked clays to positions well above the maximum level of the lake.

The survey also revealed a hitherto unsuspected, deeper variability in conductivity following the Pliocene strand line pattern. The cause of this pattern is not, as yet, clear.

There are a number of potential uses for a detailed map of the distribution of the Blanchetown Clay. At present it is being used to model the recharge behaviour of the area as an input into a model predicting the future course of salinity inflows to the Murray River. If more areas are to be released for irrigation, the map could also be used to select areas of thicker clay that may be preferred locations for irrigation. Finally, areas of thicker clay are might also be the preferred location for disposal of saline water from salt interception schemes

Dr. Tim Munday
Project Leader
December 2003


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