IMPROVING THE RELIABILITY OF CONDUCTIVITY MODELS FROM TIME-DOMAIN AIRBORNE ELECTROMAGNETIC DATA FOR GROUNDWATER AND SALINITY MAPPING - LOWER BALONNE, QUEENSLAND, AUSTRALIA

TEMPEST airborne electromagnetic (AEM) data were acquired as part of an integrated study of a section of the Lower Balonne catchment in southern Queensland, Australia. Over 28000 line<br>kilometres of surveying was carried out at 250 and 400 metre line spacing covering an area of approximately 18000 km2. The purpose of the study was to evaluate the salinity risk in an area where there has been clearing of native vegetation and an increase in surface water diversion for cotton irrigation. Several generations of conductivity predictions derived from the AEM data were assessed against independent surface and borehole conductivity measurements. We were disappointed with the comparisons involving the initial set of conductivity predictions. We attributed the bulk of the disparity between the observed and predicted conductivity values to an assumption of a resistive basement at depth across the survey area when, in fact, borehole conductivity logs revealed the presence of a conductive unit at depth. A final set of conductivity predictions was generated with (1) a reference conductivity model that was conductive at depth, and (2) a more comprehensive parameterization of the variable AEM system geometry. These predictions were more highly correlated with the independent conductivity measurements. This provided the confidence to use the conductivity structures as a framework to aid interpolation of drillhole lithological information to produce a three dimensional sedimentary architecture of the Lower Balonne catchment. This architecture was a significant input into the groundwater study.