Technical Report NTB 90-43

The borehole radar investigation program Stage III of the SCV-site has comprised single hole reflection measurements with centre frequencies of 22 and 60 MHz. Single hole reflection measurement with both omnidirectional and directional antennas have been performed in the boreholes C1, C2, C3 and the D-holes (D1 – D6). Crosshole tomographic measurements as well as crosshole reflection measurements have been made between the boreholes C1 – C2, W1 – C1 and W1 – C2. The range obtained in the single hole reflection measurements was approximately 100 m for the lower frequency (22 MHz) and about 60 – 70 m for the centre frequency 60 MHz. In the crosshole measurements transmitter-receiver separations from 20 to 120 m have been used.

The Stage III radar investigations have essentially confirmed the three dimensional description of the structures at the SCV-site. The conceptual model of the site which was produced based on the Stage I data included three major zones (GA, GB, and GH), two minor zones (GC and GI) and a circular feature (RQ). The major features are considered to be the most significant at the site and are all observed in the Stage III boreholes close to their predicted locations. The circular feature RQ has also been found in two of the additional tomograms at the predicted location. RQ is seen as a ring shaped feature in the attenuation tomograms and as a single spot anomaly in the slowness tomogram.

The results indicate that the zones are not homogeneous but rather that they are highly irregular containing parts of considerably increased fracturing and parts where their contrast to the background rock is quite small. The zones appear to be approximately planar at least at the scale of the site. At a smaller scale the zones can appear quite irregular.