Comparison between Geoelectrical Imaging and Tunnel Documentation

The documentation from the use of a tunnel boring machine (TBM) is used for the evaluation of geoelctrical imaging. Rock type, RQD and weathering are visually compared with resistivity data extracted from different levels in the inversion result. The three types of resistivity, i.e. high, low and intermediate resistivity, can be correlated to certain types of rock mass conditions. The comparison shows that high resistivity corresponds well with good quality gneiss. The tendency is that low resistivity corresponds to a varying lithology with fractured contacts or merely rock with very poor quality (RQD < 25). The intermediate resistivity agrees often with areas with amphibolite having a dominating RQD of 50-75 (good quality). Even though there are exceptions from this tendency the evaluation has given valuable information about how to use the resistivity data in the future work in the tunnel. Previously there has been a large focus on the low resistivity zone in order to find poor rock conditions. But the comparison has shown that the high resistivity areas are interesting because they give a fair chance for good quality rock. This is just as important for the contractor to know as the location of rock with poor quality.