Resistivity surveying with combined profiling and sounding, also called continuous vertical electrical sounding, has been carried out along large parts of the planned railway tunnel through Hallandsås using multi-electrode data acquisition equipment. The measured data was processed by means of inverse numerical modelling. (Dahlin 1996) The first surveying was carried out over the Southern Marginal Zone in 1995 as part of complementary investigations of the rock quality (Dahlin et al. 1999). Due to the good results achieved, the technique has since been applied along most of the 8.6 km long tunnel line. In the surveys carried out in 1998 larger electrode separations, with 800 metre cable layouts, were used for increased depth penetration. Expanded measurement protocols were also employed for enhanced resolution. The results (see figure 1) display a strong variation in modelled resistivities along the line, where in particular the Southern Marginal Zone and the Möllebäcken Zone stand out as low resistive zones. Apart from these, several low resistive zones of smaller extent are evident, which can be interpreted as fractured or weathered zones. Limited parts of the tunnel stretch exhibit the high resistivities that are normally expected for crystalline rock of low fracture density below the groundwater level. The resistivity models were used, along with other available geophysical data, as a basis for creating conceptual models of the structural geology and the variation in quality of the rock. These models were used as a basis for siting test drillings, to verify and refine the conceptual models. A comparison with core drilling data shows that the resistivity results give a good overview of the structural geology and variation in relative rock quality. It was, however, not possible to correlate e.g. Q-values with resistivity on a detail level. This is partly dependent on the differences in scale and resolution of the two types of data, but there may also be a lack of a simple connection between Q-value and resistivity. It is quite possible that a better correlation could be achieved with a different parameter as measure of the rock quality. Furthermore, the resistivity models as well as drilling data and documentation from the tunnelling (Dahlin and Sturk 1998) shows a strong inhomogeneity of the rock in Hallandsås, which means that the individual boreholes are probably only representative for a very small volume in their vicinity.


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