Airborne electromagnetic methods are practical tools for hydrogeophysical mappings on the watershed scale. Because of the rapid development in high quality electronic a number of new airborne systems have been developed in the last decay and the preferred platform has moved from fixed wing systems to helicopter system. When mapping more shallow geological structures in resistive terrain the frequency domain systems are often used. A key point when doing airborne geophysics for groundwater mapping is the accuracy of the applied system. Basically the processing and inversion of the data has limited value if the data quality is low, data are heavily filtered or they are biased. These data can be used for mineral prospecting where the conductivity contrast between the target and the background host differs by orders of magnitude but in groundwater surveys contracts between the target (freshwater bearing sediments) and the surroundings can be as low as a factor 1.5 demanding accurate measured responses and well know system parameters.
The processing and inversion tools need to fully justify the data and the survey specification. System parameters like transmitter waveform, low pass filters, gates etc. has to be modeled in the forward mapper of the inversion scheme rather than corrected for. Flight altitude and roll and pitch must be handled as inversion parameters with a priori value of what was measured by the instrumentation. Formulating the inversion algorithm so the individual dataset are not inverted independently but actually inverted using the a priori knowledge that the geology is three dimensional has led to the development of new inversion schemes like laterally constrained inversion (quasi 2D inversion) or spatial constrained inversion (quasi 3D).
In the talk an overview of the different techniques will be given and there will be some discussion on new processing and inversion techniques. The main focus will be on two surveys from different part of the world. In both surveys data were collected using the high resolution airborne system, SkyTEM. The first example is from an area around the Murray river in Australia where the boundary between fresh and salt ground water was mapped. The second example is from a very large survey in Denmark. The geology is complex and partly of glacial origin. The aim of the survey is to give a detailed input to a geological model being built.