Published:January 01, 1999
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.
Figures & Tables
Shear Waves from Acquisition to Interpretation
“This book, produced for use with the third SEG/EAGE Distinguished Instructor Short Course, addresses the practical aspects of multicomponent data acquisition, processing, and interpretation. The first part of the book is devoted to overcoming the difficulties associated with shear-wave acquisition. Converted-mode operation is covered in detail using real-life examples. The particularities of sea-bottom receivers also are examined. The second part reviews the processing and the main challenges of the shear-converted modes: static corrections, gathering, velocity analysis, and compensation for shear-wave splitting in axial anisotropy. The book gives a detailed description of processing sequences, and 2D and 3D results, yielding natural axis orientation of layers, are compared in shear and PS converted modes. The third part is devoted to case histories in which new attributes, such as VP/VS ratio, crack density, or fracture orientation, are illustrated in a reservoircharacterization context. These case histories can guide the geophysicist to decide if a particular geologic situation can be handled best using shear waves.”