Difference in Resistivity Explained by Initial Stage of Weathering

Two core drilled boreholes in Southern Sweden were logged and the cores were inspected in order to correlate the results with the geological features. The resistivity log showed that the amphibolites in the two boreholes had very different resistivity. In one borehole the resistivity was high, whereas it was lower in the other. The visual inspection of the cores could not give an explanation as the amphibolites appeared similar. However, microscopy of the low resistivity amphibolites showed weathered pyroxenes, probably caused by introduction of water along the lithological contact between the amphibolites and the regional gneiss. The resistivity logs can thus detect even low grades of weathering of amphibolites which can be important for the mechanical properties of the rock.