ERT-SIP versus Seismic Tomography - Geophysical Mapping of a Planned Highspeed Railway Track, a Case Study from Norway

Within an infrastructure offensive in Norway the authorities plan and will build many hundred kilometers of new roads and rail tracks. One of the focuses is towards a high speed railway (HSR) system as a sustainable future transport. Due to the complex geology of large parts of Norway drilling campaigns often lead to a big question mark. To fill the gap between these drillings, reduce costs and improve planning security there is a demand in high resolution geophysical methods. NGI had the possibility to test, among other methods, high resolution ERT-IP and seismic refraction tomography to determine shallow bedrock (1-20 m) below a glacial infill on two test sites. While we assumed seismic refraction tomography to be the most reliable tool our study shows that our high resolution ERT model results in a lower deviation from existing drilling informations and supplementary revealed two steep weakness zones which were completely hidden in the seismic model. Additionally we calculated the spectral information of the IP data using the Cole-Cole relaxation model. The model provides additional information about the sedimentary cover but reveals contradicting informations about the weakness zones which cannot be further explained due to currently non-existing petrophysical models, especially for hardrock.