Improvements in mapping of floodplain dynamics by integrating drilling information with airborne EM, ground penetrating radar and ground based, high resolution EM

Abstract

Improved knowledge of alluvial architecture is becoming crucial to investigators to assist in understanding the dynamics of large river systems like the Murray in southern Australia. Historically, information has been gathered by analysis of drill-hole information, which provides information about fine-scale vertical structure. This is often interpolated laterally over large areas with little consideration to the influence of depositional process, consequently with often poor results. Increasingly, airborne electromagnetic surveys (AEM) have been used to help fill in information gaps. Even this information is coarser than needed to define fine structure. This leaves a niche for high resolution ground geophysical surveys, both to validate the AEM, but also to fill in where other methods are too coarse. For this study, we report the results from a coordinated geophysical and drilling program on highly salinised floodplains near Mildura, Victoria. Three accessible lines were chosen over recently acquired AEM surveys. A high-resolution electromagnetic survey and a low-frequency (25 MHz) ground penetrating radar survey (GPR) were run over each line. Where available, drillhole information was collected and compared with the geophysical data. Comparison of the three data sets show good correlation between small to medium scale vertical and lateral variations in the geophysical data with observable aspects of the alluvial architecture. Much of the response is strongly correlated with the location, depth and salinity of local groundwater. The higher resolution ground-based techniques also assist in informing interpretation of the broader scale and coarser resolution AEM data.