f Field meauserements by using geoelectrical nullarrays on fissured limestone in the Swiss Jura

Until now no systematical field studies have been carried out by using geoelectrical arrays, having zero potential difference over a homogeneous halfspace. We call hereafter such configurations as null-arrays. In this paper we present field results obtained over near-surface fissures in limestone by using three different null-arrays. The null-arrays were selected according to three possible types of the current distribution around the current electrodes. In the first type of the current distribution the current is propagating radially. In the second one the current lines are more or less parallel with each other. In the last one the current flow is considered as it would be of a dipole origin. The first type of null-arrays was realized by a three-electrode geometry. The second type was realized by the so called Schlumberger null-array and in the last case both dipole equatorial and dipole axial configurations were transformed into null-array geometries. The connecting line of the potential electrodes in the null-arrays are always perpendicular to that in the classical arrays. This feature made it possible to obtain a very quick joint application of these two array types and also to compare the results between the traditionally used arrays and the null-arrays. All geoelectric profiles were evaluated in a close vicinity of a quarry, where the fissures were directly seen. As we found, the null-arrays located the fissures with a higher accuracy and provided a more significant anomaly than the classical configurations did, especially in case of a thicker overburden. Since the null-arrays and classical arrays give complementary information we recommend their combined use in such near-surface studies. Besides the theoretical aspects, these field test measurements have been found to be very useful in hydrogeological applications, since fractures and fissures are the most important geological features for determining groundwater vulnerability.