Highly irregular and nearly vertical salt flanks are difficult to image with surface seismic methods. Borehole seismic salt proximity surveys (SPS) have been widely used to delineate and map salt flanks adjacent to a borehole. The accuracy and reliability of the estimated salt flank from SPS data depends on the sedimentary velocity model around the borehole and above the salt, as well as the 3D ray-tracing method used to calculate seismic ray paths and travel times from seismic source to borehole receivers. Presently most of salt-exit–point calculations from SPS data are based on 3D isotropic ray-tracing algorithms. It has been recognized however that many sedimentary rocks exhibit vertical transverse isotropy (or VTI anisotropy) to seismic waves. Neglecting anisotropy could significantly affect the accuracy of the computed salt exit positions determined from SPS data. This paper presents a 3D anisotropic ray-tracing methodology to determine salt face exit points from VSP salt proximity survey data. We derive an analytic 3D vector ray equation across any curved anisotropic interface in sedimentary model. Through numerical tests imitating typical SPS case examples we demonstrate the effectiveness of our methodology and significant impact of anisotropy in determining 3D salt face exit points from SPS data.


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