Resistivity and IP measurements at the Bolmen Tunnel and Ådalsbanan, Sweden

Abstract: In all tunnel construction projects geophysical methods are important as a foundation for engineering geological conceptual models. Detailed information about the condition and the physical properties of rocks at depth is desirable, and geophysical methods have the potential to exploit these properties. This work is based on geophysical investigations with focus on the geoelectrical methods. The methods are direct current (DC) resistivity and induced polarisation (IP). The use of the continuous vertical electrical sounding (CVES) gives continuous data in a time and cost efficient way. The methods have been tested at two tunnel investigation sites in Sweden, at the Bolmen tunnel and Ådalsbanan. Complementary magnetic and VLF (Very Low Frequency) measurements has also been carried out. Pre-investigations of tunnelling construction projects commonly include core drilling. The resistivity and IP methods provide significant geological information and by including these methods at an initial stage, the selection of drilling site can be optimized. By combining different geophysical methods with drilling a more reliable model can be compiled. The resistivity method is based on the fact that each rock has a specific physical character. The physical property of the rock can then be interpreted in terms of geological properties and in some cases even allow an assessment of the rock mass quality. The main purpose in this work was to estimate the applicability and the limitations of these methods used for pre-investigation in tunnel construction projects. This material includes geological interpretations and estimations of the quality of the rock based on geoelectrical methods. With the resistivity method several low resistive zones were detected and interpreted as weakness zones. Many of these were verified by data from earlier tunnel investigations. The IP method gave information about the character (e.g. water and clay content) of the fracture zones and served as a complement to the resistivity method. These methods provided general information, from which a geological model was estimated. The results show that the resistivity and IP are useful methods in pre-investigation for sub-surface tunnel constructions.