Paris Basin VSPs: case history establishing combinations of fine-layer (or lithologic) anisotropy and crack anisotropy from modelling shear wavefields near point singularities

Abstract

S U M M A R Y 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.

A review of a quarter century of International Workshops on Seismic Anisotropy in the crust (0IWSA–12IWSA)

“…Note that both radial and azimuthal integration were used and the full-wave seismograms are exact without approximations. Bush and Crampin (1991) confirmed the effects of point singularities in observations of a multi-offset VSP in the Paris Basin.…”

Modelling Orthorhombic Anisotropic Effects for Reservoir Fracture Characterization of a Naturally Fractured Tight Carbonate Reservoir, Onshore Texas, USA

“…S 1 is polarized parallel to the fracture and travels at a velocity similar to what would be observed in an isotropic medium (Lynn and Thomsen 1990) and thus faster than S 2 which is polarized perpendicular to the fracture set orientation and delayed by fractures. Wild and Crampin (1991) defined crack-induced anisotropy as extensive dilatancy anisotropy (EDA), when combined with the layer-induced or lithologic anisotropy that yields an orthorhombic anisotropic symmetry with three mutually perpendicular planes of symmetry (Bush and Crampin 1991). The degree of P-wave anisotropy due to different crack densities (C d = 0.001, 0.01, 0.1 and 0.2) in the modelled carbonate reservoir as observed in Fig.…”

Anisotropy and attenuation of crosshole channel waves from the Antrim Shale gas play, Michigan Basin

“…1995;Slater, 1997 or direct bedding-induced anisotropy in the Antrim Shale, and such anisotropy is believed to be a common feature in Ž . sedimentary rocks Bush and Crampin, 1991 . The strong oscillations in sonic log and gamma Ž .…”

“…Bush and Crampin, 1991 . Finally, we conclude our studies by modeling the observations using synthetic seismograms, which provide further support to our interpretation.…”