ANISOTROPY DETECTED IN SHALLOW CLAYS USING SHEAR‐WAVE SPLITTING IN A VSP SURVEY1

Abstract

AbstractShear waves can provide valuable information about seismic anisotropy. On entering an anisotropic medium, a shear wave generally splits (shear‐wave splitting) into a fast and a slow quasi‐shear wave with polarizations fixed by the elastic properties of the medium and direction of travel. If the medium contains planar discontinuities with common normals, the fast shear wave will be suitably propagated if its polarization lies in the plane of the discontinuities. Measuring this polarization, using a VSP geometry with oriented three‐component geophones in the borehole, offers the possibility of monitoring the orientation and density of the discontinuities as a function of depth.Such a shear‐wave VSP was carried out in an uncased 0.3 m diameter borehole drilled to a depth of 120 m in the north of The Netherlands. The upper 80 m of the sequence, consisting of a glacial till and sands and clays of Pleistocene age, was studied. The clays in this sequence have been subjected to glacial deformation and as a result are overconsolidated and locally fissured.In our shallow VSP experiment, shear‐wave splitting and therefore anisotropy was identified at various geophone depths for one source offset. Hodograms showed a consistent polarization of the fast shear‐wave component over a large depth interval. Under the assumption that the anisotropy was caused by planar discontinuities with common normals, this polarization direction gives the strike of the fissures in this interval. The polarization direction of the fast S‐wave did not correspond exactly with the strike which was obtained from geological information on the fissures. The geological information was from undisturbed oriented 70 mm core samples taken at 3 m intervals in the borehole. The discrepancy, however, could be explained in terms of dipping fissures, and such a dip was confirmed by the geological and geotechnical information.The orientation of fissures is an important factor in the directional deformation and strength characteristics of clays as far as geotechnical behaviour is concerned. This study thus illustrates a practical application of shear‐wave splitting observed in shallow shear‐wave VSP for geology and geotechnical engineering.