Electrical borehole measurements for the mapping of fracture zones in crystalline rock

Abstract

The mise-à-la-masse method has been used for delineation of fracture zones in crystalline rock. Measurements have been carried out at two different sites in Sweden, Finnsjö and Stripa. Measurements in Finnsjö show that the mise-à-la-masse method can be used to interpret the orientation and the dimensional shape of a fracture zone. The greatest distance over which a significantly large fracture zone could be detected from the current electrode was 150 m. In the Stripa mine mise-à-la-masse measurements have given valuable information on the geometry of a large fracture zone reached at the bottom of a vertical borehole even though the measurements were to be made in a mine where the possible locations for the potential electrode were limited to a few boreholes. In Stripa the distance between the current electrode and the potential electrode was at most about 500 m. An apparent resistivity has been defined and used for qualitative interpretation of the data. The qualitative interpretation was successful in the Finnsjö measurements where potential data were available over a large surface. Interpretation has also been made with a semi-quantitative model where the conductivity, width and orientation of a conductive sheet has been fitted to the data. In Stripa this model has given an indication of the orientation of a large fracture zone.