Investigations over two major tectonic zones in the Hallandsås Horst, Southern Sweden were carried out in connection with a railway tunnelling project. A combination of resistivity imaging, core drilling and geophysical logging was found to provide a good overview of the variation in rock quality and detailed information on the engineering geological characteristics and genesis of the strata. Resistivity data, acquired along parallel lines over the zones using multi-electrode equipment, were processed by means of automatic inversion software to generate sections showing the variation of resistivity with depth. Wireline core drilling at selected points gave an almost complete core recovery, which revealed major sedimentary sequences and heavily weathered crystalline rocks that were previously unknown or only partially known despite conventional core drilling and percussion drilling. Geophysical borehole logging confirms the resistivity imaging results, and would be a particularly valuable complement to core logging in cases where significant core loss occurs.