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Abstract

PS velocity analysis consists of three operations: 1) true CCP (Common conversion point) determination, called here X-focusing, an analogue of DMO, 2) short spread hyperbolic move-out focusing, called here H-focusing, an analogue of NMO, and 3) co-depthing of PP and PS images. Each of these operations depends essentially on a parameter expressed as a simple function of P and S rms velocities and vertical traveltime: Thomsen’s γeff for 1), paraxial focusing term Fc for 2) and γ0 for 3). Time domain velocity analysis provides an initial estimate of these parameters, but full resolution of the PP-PS velocity problem is obtained in the depth domain. Our technique, the CCP-Scan technique, involves Pre Stack Depth Migration and scanning of P and S velocities. It permits, through refined measurement of γeff and Fc, the resolution of PS short spread move-out in a true CCP. It permits the (short spread) focusing and depthing of PP and PS images to a common depth, in a layer stripping sequence. Isotropic tools, in the last layer, suffice to yield separately all short spread parameters. However, simultaneously accounting for all of them, (i.e. ensuring PP and PS images focusing mad co-depthing), is not possible with an isotropic description and in fact reveals polar anisotropy. Further improvement in medium or long spread move-out involves scanning of anisotropy parameters, if, necessary.

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