My 3DVSP Image does not Look Like My 3D Seismic?

Abstract

3D surface seismic data and 3DVSP data both provide velocity information and 3D images of the subsurface. In many cases however, 3DVSP and 3D seismic images are different. In this paper I explain the reason why one should not always expect subsurface images to be the same. With a 3DVSP survey, for each image point around the well, there is generally only one source-receiver azimuth contributing to this image point, while for 3D surface seismic, the image point is generally constructed from source-receiver pairs in multiple azimuths. Therefore, in an azimuthally anisotropic subsurface, 3DVSP and 3D surface seismic image amplitudes will be different. When recorded in environments that exhibit a strong AVO response, the absence of near zero-offset raypaths in 3DVSP data also leads to different image amplitudes compared to 3D surface seismic data which normally has close to zero-offset rays. It is shown that 3DVSP imaging is more sensitive to depth velocity model errors than is 3D surface seismic. 3DVSP data provides the most reliable interval orthorhombic parameters which should be used for both VSP and surface seismic processing and imaging. Reflected waves should be used when inverting traveltimes to improve the structural velocity model derived from 3DVSP data.