| Abstract: | Refraction seismics with the shotpoints and the hydrophone cable on the sea-bottom, have become the standard geophysical method for investigating rock quality before constructing offshore tunnels in Norway. In connection with the construction of a sub-sea tunnel by the Norwegian Public Roads Administration, research work was carried out to compare two low-velocity zones, indicated by refraction seismics with other methods. A special resistivity cable for pole-dipole measurements on the sea-floor, with 10 m between the electrodes, was constructed. A 200 m long profile, crossing the two low-velocity zones, was measured with all combinations of electrode distances. The two zones were detected as low-resistivity zones. A special data processing technique to enhance the anomalies is described. Resistivity soundings in a seawater environment to detect fracture zones in the bedrock underneath the bottom sediments, are discussed. It is concluded that severely fractured zones, which may cause difficulties for the tunnel construction, can be detected both with sea-surface and sea-floor arrays using long electrode spacings. |
