Constrained inversion of helicopter AEM data for mapping the Blanchetown Clay

Abstract

A RESOLVE frequency domain helicopter electromagnetic (HEM) survey has been flown in and around the Riverland irrigation districts of South Australia. The purpose was to map the distribution and thickness of near-surface clay-rich sediments which impact on irrigation, groundwater and salinity management strategies. The survey data were re-calibrated after their conventional processing by utilising independent ground geoelectric data. Data were inverted using a five-layer 1-D parameterisation of the Earth. Reduction of the ambiguity in the unknown aspects of the geological section was sought by constraining the inversion with as much local geological and hydrological information as was available. Groundwater depth information was incorporated as an extra datum to constrain the upper layer thicknesses. A combination of drillhole lithologic, groundwater and downhole conductivity data were used to construct a spatially variable reference model and impose constraints on the estimated parameters in the form of prior probability information. The resulting detailed map of the thickness of the Blanchetown Clay is more detailed than previous compilations based on drilling. The results also provide insight into other important hydrogeological features of the Riverland area.