PARIS BASIN VSPS - CASE-HISTORY ESTABLISHING COMBINATIONS OF FINE-LAYER (OR LITHOLOGIC) ANISOTROPY AND CRACK ANISOTROPY FROM MODELING SHEAR WAVE-FIELDS NEAR POINT SINGULARITIES

This paper examines shear-wave splitting in multi-offset VSPs at a borehole site in the Paris Basin, and demonstrates that the sedimentary structure must have, at least, orthorhombic anisotropic symmetry. It is suggested that the most likely cause for such an orthorhombic symmetry is the combination of fine-layer (or lithologic) anisotropy and crack anisotropy. Transition zones (close to shear-wave singularities), where there are rapid changes of shear-wave polarizations and delays, can account for the anomalous observed waveforms. Observations of large variations in shear-wave polarizations in propagation directions close to point singularities, are probably the first positive identification of the effects of singularities in sedimentary basins.