Zusammenfassung
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 geff for 1), paraxial focusing term Fc for 2) and g0 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 geff 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 and 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.
Nutzer