3D electromagnetic modelling in frequency domain - studies of underground measurements in a salt mine

Abstract

In recent years efforts have been made to find underground sites suitable for waste disposals. A potential geological host rock for such a disposal is rock salt, which is used as a natural barrier separating the waste from the environment. To manifest the suitability of the location the geologic structure of the surrounding rock has to be investigated in detail. As only non-destructive methods have to be used to investigate the barrier system, geophysical methods seem to be suitable. Examinations were performed to determine whether the electromagnetic induction method in the frequency domain is a useful tool for such investigations. The measurements were carried out in a salt mine gallery. Brine seeps into it along a few meters of the profile. This marks a big brine reservoir imposing an anomaly on to the data set. Apart from this qualitative interpretation, studies were done to analyze the effect of the gallery, gallery crossings, voluminous brine zones and homogeneous areas. To analyze 3D structures like the gallery or voluminous brine scenarios, a numerical finite-difference modeling code was used. It is shown that in the homogeneous area the data can be interpreted with a full-space model. The influence of gallery crossings can be neglected. The explanation for the data of the anomalous zone lies in the existence of a brine reservoir close to the surface.