Analysis of helicopterborne electromagnetic data for hydrogeological investigations

Traditionally, frequency and time domain airborne electromagnetic (AEM) systems have been used for mineral prospecting, often in parts of the world with little environmental electromagnetic 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, even in densely populated areas (Sengpiel and Siemon 1998). This raises questions about 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). Traditionally, helicopterborne frequency-domain electromagnetic (HEM) data have been processed to produce iso-resistivity maps using lookup-tables, and transformations of the apparent resistivity and the centroid depth obtained from the table lookup have been used to produce pseudosection images. With the general improvement of HEM systems quantitative interpretation has become an option (Sengpiel and Siemon 1998). In general, the objectives of a hydrogeophysics survey are; detection of non-permeable boundaries of a potential aquifer, often coinciding with the clay-sand boundaries, discernment of internal structure in the aquifer and mapping of near-surface capping clays reducing the vulnerability of the aquifer. This paper presents quantitative analyses using one-dimensional (1D) models of the resolution capabilities of a modern HEM system with 5 frequencies. Because a frequency domain ground system equivalent to the HEM system does not exist we have chosen a profile oriented, multi-electrode DC geoelectrical system (CVES) with a comparable depth penetration and compared the resolution capabilities of the HEM system to that of the CVES system for a number of hydrogeologically relevant models.