Salt mapping in the Lower Macquarie area, Australia, using airborne electromagnetic data

Abstract

An airborne electromagnetic survey was flown over 13 000 km2 in the Lower Macquarie Valley in central New South Wales, Australia. Whilst bulk conductivity values from the survey provide useful insights into the regional hydrogeology of the area, mapping salinity and salinity hazards requires the derivation of salt store values from the survey data. This paper describes the methodology of deriving salt stores from conductivity data and accompanying physical measurements. A post-flight sampling programme involving physical, chemical, electromagnetic and geological examination of cores from eight drill holes provides field-based measurements through which the bulk conductivity data can be related to salt stores. Linear models were generated to understand the relationship between salt stores, conductivity and other parameters, and to produce an optimum salt store map from the data. A strong relationship was evident between observed salt stores and drill hole-derived observations, which include parameters such as conductivity, lithology, stratigraphy, lithostratigraphy/weathering and depth. The main predictor for salt stores was observed conductivity with only a minor influence due to lithology. Significant salt stores are predicted at depth in the northern part of the survey. Superficial salt stores are generally low (<4 kg/m3) in the Lower Macquarie and should not pose a threat to native vegetation or agricultural activities.