Crosshole seismics including geotomography for investigation of foundations : By, T L Norw Geotech Inst PublN177, 1989, 12P

Abstract

Engineering geophysics represents a valuable supplement to other engineering geological investigations. In particular, the cross hole seismic method can rightly be claimed to be unique in providing detailed information of large rock volumes. Assessment of rock mass quality is of current interest in planning and completing rock caverns as well as for foundation purposes. Rock and soil site investigation require site geometries where cross sections can be formed between boreholes and/or exposed surfaces as tunnels or ground surface. To take full benefit of this in-situ site evaluation technique, computerised geophysical tomography should be applied. Interborehole scanning by a defined acoustic source/receiver configuration gives rise to a point by point reconstruction of the seismic velocity field or of the acoustic sound alternation properties in the actual section. Norwegian Geotechnical Institute has experienced the applicability of the cross hole method in dam site evaluation, pre-investigation of ground properties for tunnel projects and for general rock mass descriptions. Obviously, the most useful results are achieved where the client has a well defined problem to solve and where the cross seismic results can be verified, or can be given supplementary interpretations. This paper gives a brief description of the instrumentation. Further, two dam site foundation investigation projects and one main underground highway tunnel project, all taking benefit of cross hole seismic measurements, are presented.