Application of frequency-domain helicopter-borne electromagnetics for groundwater exploration in urban areas

Abstract

Airborne geophysical methods have been used successfully in groundwater exploration over the last decades. Particularly airborne electromagnetics is appropriate for large-scale and efficient groundwater surveying. Due to the dependency of the electrical conductivity on both the clay content of the host material and the mineralisation of the water, airborne electromagnetics is suitable for providing information on groundwater resources, water quality, aquifer conditions and protection levels. Frequency-domain helicopter-borne electromagnetic systems are used to investigate near-surface groundwater occurrences in detail even in rough terrain and populated areas. In order to reveal the subsurface conductivity distribution, the quantities measured, the secondary magnetic fields, are generally inverted into resistivity–depth models. Due to the skin-effect the penetration depths of the electromagnetic fields depend on the system characteristics used: high-frequency data describe the shallower parts of the conducting subsurface and the low-frequency data the deeper parts. Typical maximum investigation depths range from some ten metres (highly conductive saltwater saturated sediments) to several hundred metres (resistive hard rocks). In urban areas there are a number of man-made sources affecting the electromagnetic measurements. These effects on the secondary field values are discussed on the basis of synthetic data as well as uncorrected and corrected field data. The case histories of different hydrogeological setups in Indonesia, The Netherlands and Germany demonstrate that airborne electromagnetics can be applied to groundwater exploration purposes even in urban areas.