The Use Of Airborne Electromagnetic Systems For Hydrogeological Investigations

Traditionally, airborne frequency and time domain electromagnetic systems have been used for mineral prospecting, often in parts of the world with little environmental electromagnetic <br>disturbance. However, the increased focus on hydrogeological investigations in many parts of the world has led to a growing interest in the possibility of using airborne systems for such purposes, also in densely populated areas (Sengpiel and Siemon 1998). This raises many interesting questions as to the resolution capabilities of AEM systems and their sensitivity to disturbing influence from ambient electromagnetic noise, and the presence of man-made structures such as power lines, buried cables, and fences. The data quality of earlier AEM systems was such that they were mainly used as “bump detectors” capable of indicating the presence of good conductors. Quantitative interpretation of the data was often not warranted (Huang and Fraser 1999), but with the improvement of AEM systems in general new possibilities of quantitative interpretation have appeared (Macnae et al. 1991). Compared with ground systems, AEM systems have a larger footprint giving rise to reduced horizontal resolution, and a more narrow bandwidth resulting in inferior vertical resolution. Furthermore, it is often very difficult to estimate the system noise, especially that due to the motion of the aircraft, and not very much is known about the coupling effects to man-made structures in developed areas. On the positive side, the density of AEM measurements far exceeds the density of traditional ground surveys and the production rate is more than 1000 km per day. This paper presents analyses of the resolution capabilities of present-day transient electromagnetic (TEM) systems and makes comparisons between airborne and the corresponding ground systems for a number of hydrogeologically relevant models.