Technical Report NTB 87-28

Crosshole Investigations –Results from Seismic Borehole Tomography

A system for seismic crosshole measurements has been designed, built and tested. The system can be used both for small-scale (i.e. 10 – 200 m) and large-scale (i.e. 200 – 1000 m) operations.

The design includes borehole receivers, amplifiers and recording system. The receivers can be used down to 700 m depth in slim boreholes.

Much work has gone into the development of analysis methods. Tomographic algorithms have been developed for the analysis of seismic data. The development includes basic theory as well as numerical methods.

Special care has been taken to minimize systematic errors. Many data quality checks have been made.

Field tests have been carried out at the large-scale test site at Gidea and at the small-scale test site at Stripa.

In the large-scale test, some zones of fractured rock were found. In addition, there appears to be a relatively large area of rock without any major anomalous features.

It appears that problems associated with large-scale crosshole seismics are still substantial. Further work is needed to solve the problems with ray-bending and anisotropy.

In the small-scale test the measurements could be carried out with high precision. Several zones with different properties are visible in the tomograms.

It is our opinion that the technique for small-scale crosshole seismics is now developed to a level where it can be utilized as a useful tool for rock-quality assessment.