ABSTRACT:

Surface seismic surveying methods are recommended during the early phase of a geotechnical site characterization programme, preferably prior to drilling and excavation activities, with the objective of detecting critical zones for more detailed invasive follow-up investigations, such as boreholes. By combining the evaluation results of reflection seismic profiling and refraction tomographical inversion in a joint representation, i.e. hybrid seismic surveying, many interpretational ambiguities are eliminated. Critical zones of rock weakness and reduced stiffness, attributable to weathering deconsolidation and tectonic faulting, are identified in a reliable manner. By using the appropriate recording equipment, the acquisition of the data of the two above mentioned methods can now be combined into one single field operation, resulting in a significant reduction in time and costs for the field crews. The significance of the key data acquisition parameters with regard to the imaging resolution and to the desired investigation depth is discussed.

1 INTRODUCTION

The direct relationships between the compression wave and the shear s-wave velocities on one hand and rock strength on the other are well documented in literature (N. Barton, 2006). Although these relationships are well known to the geomechanical practitioner, the systematic and timely application of engineering surface seismological surveys for the detection of weakness zones still is not an integral part in many geotechnical site characterization programmes. The seismic techniques commonly in use for geomechanical engineering are 1.1 Surface seismic methods A1 Conventional high resolution reflection seismic profiling. A2 The seismic refraction method of p-wave diving wave tomography. A3 The seismic refraction method of s-wave diving wave tomography. A4 MultichannelAnalysis of SurfaceWaves(MASW) for the determination of the shear wave velocity field (active, using => controlled seismic sources, and passive methods). A5 Spectral Analysis of Surface Waves (SASW); dual channel recording of the ground unrest).

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