Resistivity logging as a tool for identifying initial weathering in crystalline rocks

In rock engineering construction projects it is common to make core drillings to quantify rock quality using different classification systems such as weathering and rock quality designation. The classifications are done on a metre scale by a geologist on site. However this is a subjective assessment where the human factor comes into play. Important observations might be overlooked or interpreted incorrectly. Therefore a more objective method is needed. The scope of this paper is to show how even low degrees of weathering of rocks lowers the resistivity, an effect that otherwise might be overlooked by the geologist because the core appears unaltered and is thus not reflected in the standard rock quality measures. This was done by means of thin‐section microscopy, point counting and resistivity logging on gneisses and amphibolites from two drill cores done in connection with the construction of the Hallandsås Tunnel, Southern Sweden. The study showed that the resistivity logs can detect even low grades of weathering of amphibolites, which can be important in determining the mechanical properties of the rock. The result suggests that focusing on the transitions between different lithologies is important because the rocks at the lithological contacts are more susceptible to fracturing, water flooding and weathering. This might not be obvious from the regu‐lar geological core description but this is indicated by the resistivity logs.