VSP analysis for azimuthal anisotropy: AVAZ, VVAZ and S-wave splitting in Altamont-Bluebell field

Khaled Al Dulaijan, Gary F. Margrave, Joe Wong

Within the Altamont-Bluebell survey, multiple VSP datasets were acquired. The first dataset was a conventional zero-offset VSP. The second dataset was six shots of offset VSPs. The objective of those shots was to estimate VTI Thomsen parameters to aid with 3D processing of seismic data, and also to create a HTI model for fracture characterization of the reservoirs. However, these offset VSPs were limited in terms of depth, offset, and azimuthal coverage, and walkaway VSPs would have been a better choice for such an objective, but certainly more expensive. The third dataset was a 4-component VSP. Its objective is S-wave splitting analysis for fracture characterization of the reservoirs.

In this paper, we began with the raw field data, applied processing, including some twists in order to use surface seismic methods of AVAZ and VVAZ on VSP data, which resulted in final products of azimuthal anisotropy intensity and orientation parameters. Offset VSPs were processed through the VSP-CDP transform, then AVAZ analysis was applied. A VVAZ workflow is developed here for offset, walkaround, or walkaway VSPs using a method for surface seismic, and interval anisotropy properties are calculated for each receiver. For AVAZ and VVAZ, deeper levels including the deeper target of Wasatch180 are more reliable because of better coverage. S-wave analysis is carried out using Alford (1986) 4-C rotation to separate fast and slow modes. This method assumes that the symmetry axis is vertically invariant. To overcome this assumption, a layer stripping technique was applied using Winterstien and Meadows (1991).