An Examination of Frequency Domain and Time Domain HEM Systems for Defining Spatial Processes of Salinisation Across Ecologically Important Floodplain Areas: Lower Murray River, South Australia

Floodplains play an important role in catchment hydrology, representing a zone in which groundwater is shallow, and groundwater atmosphere interactions through evapotranspiration (ET) are more pronounced. Spatial patterns in evapotranspiration develop due to the variable distribution and type of floodplain sediments, patterns of vegetation type, floodplain elevation and geometry. Where groundwater is saline, as in the lower River Murray in South Australia, evapotranspiration concentrates salt and can lead to floodplain salinisation, vegetation dieback or health decline. Modelling these systems can assist our understanding of which areas of vegetation are at high risk from salinisation in order to target them for management. However, models of this scale require detailed data for parameter population and validation. Geophysical, particularly electrical, methods have the potential to provide detailed spatio-temporal information on the distribution of salinity in soils and groundwater, thereby assisting our understanding of floodplain processes that characterise the lower River Murray, in South Australia. In this paper we review this potential, and examine the relative merits of high resolution airborne electromagnetic technologies, and consider the relative performance of the RESOLVE frequency domain helicopter EM (FDHEM) and the SkyTEM time domain helicopter EM (TDHEM) systems for defining variations in near surface conductivity and sediment salt load across the Bookpurnong Floodplain. Results from coincident surveys are reviewed as are strategies for the inversion of these data. Data are examined against available borehole information including sediment chloride content and groundwater conductivity.