VTI analysis on crosswell seismic data

Many studies of anisotropy have been based on the detection and analysis of the signal from shear wave birefringence (or splitting) in two component or four component data. In general, this type of signal is weak and difficult to detect. The special layout in a crosswell seismic survey makes it possible to acquire the direct SH and SV wave using a 3C borehole receiver system. The possibility of anisotropy analysis using such a dataset was investigated with data acquired by the China National Petroleum Corporation in 1997. The stratified data we studied are in the cretaceous, and include the Yao-jia and Qing-shankou formations in this area. Drilling results indicate that the Yao-jia Formation is generally sand rich with delta sedimentary environments in the depth interval, 150-182 m. The Qing-shankou formation can be divided into (Q2+3) and Ql members and is composed of limestone and alternating layers of mudstone and shale, with deep lake subfacies and turbidity current environments in the depth interval 182-302 m. In the data acquisition procedure, we used two downhole sources, which were based on different working principles: (a) weight drop and (b) downhole vibrator. The data were recorded using a three-level array receiver system with 3C geo-phones. The data acquired with two different sources at the same observation interval showed good consistency. Velocity tomography, reflection imaging and Q-factor tomography was performed on 40 CSG (Common Source Gather) data of P wave. However, a single 3C CSG data set that included direct shear wave with a high ratio of S/N was used for analysis of anisotropy. By performing 3C reorientation on these data, the direct SH and SV wave were separated. Because of the high S/N in the CSG, we were able to pick the first breaks of SH and SV wave to perform interval velocity inversion. The inversion procedure was stable and convergent. We found that the difference between Vsh and Vsv was not a constant. This raised the possibility that it might be caused by anisotropy of the formation. We show that the Vsh and Vsv ratio calculated from direct shear recorded on 3C geophones is a reliable indicator for the analysis of anisotropy.