Integrating ground penetrating radar and ground-based high resolution EM to improve understanding of floodplain dynamics

Abstract

We report on results from a coordinated geophysical and drilling program on highly conductive floodplains near Mildura, Victoria. High resolution ground TEM (the Zonge NanoTEM system, configured as a towed rig) and low frequency GPR (the Mala Pro Ex system, combined with a towed 25 MHz antenna) surveys were run over three lines to evaluate the effectiveness of near surface geophysical methods for resolving important floodplain characteristics, including the depth to water table, location of perched water lenses, and extent and location of flushed zones near waterways. We were specifically concerned with whether the results from the two techniques could be used together to better inform the hydrogeology of these environments. The TEM data were processed using standard techniques, i.e. depth sections were prepared based on smooth-model inversion. The GPR data were initially processed using standard techniques to produce wiggle traces. Due to the conductive nature of this environment and the use of a relatively low frequency GPR system, the results from the standard processing were not satisfactory. These data were then reprocessed using conductivity information from the TEM section to improve velocity estimates at each GPR sounding. Results show improved resolution of water table elevation and delineation of river flush zones, while also providing information about lithological variations in the unsaturated zone. In summary, improving velocity estimation by incorporating information about the conductivity structure of the survey area has improved the interpretation of GPR data collected in this study, allowing us to interpret the TEM and GPR data sets together.