A Comparison of Airborne and Ground Electromagnetic Techniques for Mapping Shallow Zone Resistivity Variations

Both airborne and ground TEM methods have been used for many years for detection of deep conductors, especially where the prospective zone is overlain by conductive overburden. Recent concern over environmental problems has caused a refocus of this method towards shallow zone resistivity variations, particularly in areas affected by dryland salinity and groundwater pollution. To be useful, the method must be economical and combine reasonably high lateral resolution with a quantitative interpretation of resistivity variation with depth in the shallow zone (top 30 m below surface).

The National Landcare Program has funded a survey program to test the airborne TEM technique in a range of geologial environments around Australia. The airborne TEM data was collected by World Geoscience Corporation’s QUESTEM system. In each survey area a variety of ground surveys have already been performed, using the techniques favoured by the responsible state government authority.

In two areas studied so far (Wanilla, South Australia and Cressy-Longford, Tasmania) a comparison of airborne and ground TEM measurements demonstrated good agreement, especially with the recently developed very early QUESTEM channels. Improvements in these very early channels between the flying of Wanilla and the flying of Cressy-Longford are also evident. The comparison showed that very early channels are sampling the shallow zone of interest, without being significantly affected by conductivity at greater depth, except in areas where thin resistive layers are present at the surface. The transparency of thin resistive layers applied to both the ground and airborne TEM systems.

The airborne TEM was effective for mapping areas of salt storage and provided a qualitative picture of resistivity variations with depth. Current developments in both airborne and ground TEM are aimed at improving the effectiveness of these techniques in shallow zone investigations. Gathering of data at very early times after very rapid turn-off times and effective data inversion schemes will lead to a better interpretation of resistivity variations with depth.