Airborne TEM for the recovery of basin scale solute distribution; Perth Basin, Western Australia.

Summary

The distribution of groundwater salinity is a key input for management of water resources. Estimates of the three dimensional distribution of groundwater salinity below areas spanning thousands of square kilometres may be required. Airborne transient electromagnetic methods provide the possibility of recovering first pass large scale solute concentration distributions provided lithological influences on electrical conductivity distribution are not dominant. The Allanooka airborne TEM survey is located in the northern most portion of the Perth Basin in Western Australia. We use data from this Airborne TEM survey combined with data recovered from monitoring wells to highlight the steps used to construct a first pass large scale solute distribution model. We provide a method for converting airborne TEM datasets to estimates of solute concentration distribution for sandstone dominated sediments at a basin scale. For the Allanooka monitoring well network, base line empirical relationships are developed between laboratory derived total dissolved solids and formation conductivity derived from wire line logs. This relationship is then extended to include airborne TEM derived formation conductivities. Appropriate layer discretisation of input seed models for inversion of the airborne TEM data set are based on analysis of resistivities derived from wire-line logs. The interpretation of the inverted airborne TEM was assisted by geological constraints and high resolution seismic reflection transects. Selected inversion statistics were also mapped throughout the 3D volume to provide a quick method for assessing the “importance” of particular layers to the outcome of the inversion.

An approximate volume of low solute concentration sandstone dominated formation below the regional water table was extracted from the airborne TEM data. Our first pass basin-scale Airborne TEM derived 3D solute concentration provides a starting point for more detailed interpretation to commence.