Estimation of Azimuthal anisotropy from VSP data using multicomponent velocity analysis
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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.
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