Interpretation of geophysical data for salt hazard identification and catchment management in Southwest Western Australia

Abstract

Dryland salinity is a major problem in dryland agricultural areas throughout the world. The problem results from the remobilisation of salt by rising groundwater through a variable regolith, and the adverse impact of the salty water on vegetation and soil structure. Salinity is predicted to claim some 30% of the agricultural area in the southwest of Western Australia. For effective design of remedial treatments we collected geophysical data that reflected differences in the character of surface and subsurface materials within the regolith. We believed there was information in these data that was an invaluable aid in the management of salt-affected land. In order for land managers to incorporate this information in the planning process we developed a semi-automated, integrated interpretation strategy that draws on all available data, including geophysical data, to generate "salt hazard" maps. Salt hazards were defined as sites in the landscape where salty groundwater is discharging, or is likely to discharge at some time in the near future. The "salt hazard" maps pinpointed existing and potential discharge points, and also provided information about the underlying cause. This information was then used by land managers to design effective remediation strategies. The interpretation strategy was tested initially at Broomehill in the southwest of Western Australia using a manual interpretation technique. The sites identified were visited in the field and 90 percent were found to show signs of surface salinity. This correlation between interpreted salt hazards and existing salinity gave us considerable confidence in the predictive power of our interpretation strategy. A more automated interpretation method was used to identify further hazard sites that might appear as salinity in the future. This paper discusses the concept of salt hazards, the application of salt hazard maps and shows results from studies at Broomehill and Toolibin Lake (part of the National Airborne Geophysics Project).