Regolith mapping in hypersaline environments: a comparison of SAM with helicopter TEM

Abstract

Direct detection of bedrock structures using conventional electromagnetic and electrical surveys is very difficult in terrains covered by regolith saturated with hypersaline groundwater. The resistivity contrast between the brine-saturated regolith and the fresh, crystalline bedrock is usually much greater than any resistivity contrast within the bedrock itself. Mapping bedrock faults and shears by detecting their expression as changes in depth of weathering at the base of the regolith is considered to be the most effective application of electromagnetic and electrical methods to gold exploration in hypersaline environments. SAM is more effective than airborne electromagnetics (HoistEM) for mapping the regolith expression of structures, because the SAM transmitter dipole enhances current channelling into linear features, and SAM collects much higher resolution data. The two-dimensional current flow from the SAM dipole transmitter produces clearer images of linear, low-resistivity features than does the three-dimensional ?smoke ring? current flow induced by a time-domain electromagnetic loop transmitter. SAM surveys have imaged the base of regolith beneath 100 m of brine-saturated lake sediments, whereas airborne TEM transmitters have not resolved structure deeper than 50 m over the same ground.