Airborne Transient EM Methods and Their Applications for Coastal Groundwater Investigations

For more than half a century airborne electromagnetic (AEM) methods have been used worldwide for cost-effective resistivity mapping of areas larger than several hundred km². The technical developments and intensive use of these systems, principally in mining exploration during the first decades, led to the development of helicopter transient EM (HTEM) systems. Since the 2000s these systems provide the best lateral and vertical resolution for environmental exploration, and they still keep a good depth of investigation allowing the exploration from the first meters to depths of several hundred meters. This chapter focuses on helicopter borne transient electromagnetic (HTEM) systems, which are well suited for the detection of low resistive targets such as salt water intrusion in coastal zones. AEM methods are based on the diffusive induction phenomenon. It is a key tool for building realistic hydrogeological models; however it requires an understanding of its limits, and some insight into data processing modeling is necessary. They require careful processing, and removal of cultural EM noise, present in most survey areas, is mandatory in order to get high quality results. Accurate modeling of the data and of the system is also critical. The modeling is most often based on least-square optimization algorithms giving smooth or layered model descriptions of ground. In this chapter we describe the AEM method in detail and we discuss processing and inversion of data. To demonstrate the results from an investigation, we end the chapter with a case study of a SkyTEM survey made in the volcanic island of Mayotte where key geological structures and salt water intrusion were successfully mapped.