Estimation of Azimuthal anisotropy from VSP data using multicomponent velocity analysis

Abstract

Observation of azimuthal shear wave anisotropy can be useful for characterisation of fractures or stress field. Shear wave anisotropy is often estimated by measuring splitting of individual shear-wave events on VSP data; however this method may become unreliable for zero-offset (marine) VSP where the seismogram often contains no strong individual shear events but many low-amplitude PS conversions. In this paper we introduce a new approach to estimation of fast and slow shear wave velocities and orientation of polarization planes based on the multi-component velocity analysis. This technique is applicable to zero-offset VSP data and should take advantage of the presence of a large number of shear wave events with the same velocity. The main idea is to estimate the velocity for a given polarization direction by measuring the coherency of the seismic signal of a large number of events as a function of the apparent velocity. The algorithm was tested on marine 3C VSP acquired in the North-West Shelf of Australia. These tests show good agreement between anisotropy parameters (magnitude and orientation) derived from the VSP and cross-dipole sonic log data.