GeoScienceWorld
Volume

3C Seismic and VSP:

Converted waves and vector wavefield applications

By James Gaiser

Abstract

3C seismic applications provide enhanced rock property characterization of the reservoir that can complement P-wave methods. Continued interest in converted P- to S-waves (PS-waves) and vertical seismic profiles (VSPs) has resulted in the steady development of advanced vector wavefield techniques. PS-wave images along with VSP data can be used to help P-wave interpretation of structure in gas obscured zones, of elastic and fluid properties for lithology discrimination from S-wave impedance and density inversion in unconventional reservoirs, and of fracture characterization and stress monitoring from S-wave birefringence (splitting) analysis. The book, which accompanies the 2016 SEG Distinguished Instructor Short Course, presents an overview of 3C seismic theory and practical application: from fundamentals of PS-waves and VSPs, through to acquisition and processing including interpretation techniques. The emphasis is on unique aspects of vector wavefields, anisotropy, and the important relationships that unify S-waves and P-waves. Various applications and case studies demonstrate image benefits from PS-waves, elastic properties and fluid discrimination from joint inversion of amplitude variations with offset/angle (AVO/A), and VSP methods for anisotropic velocity model building and improved reservoir imaging. The book will be of interest to geophysicists, geologists, and engineers, especially those involved with or considering the use of AVO/A inversion, fracture/stress characterization analyses, or interpretation in gas-obscured reservoirs.

  1. Page 1
    Abstract

    In Chapter 1, a number of important definitions related to the key concepts for 3C seismic and VSP applications important for their acquisition, processing, and interpretation are provided. Included are the basic concepts of elastic waves (P- and S-waves), anisotropy, and the rock physics behind using these tools together in seismic exploration. Also, the applications that have motivated us to acquire S-waves are discussed and a brief overview of the challenges faced in learning how to use PS-waves and VSPs in a cost-effective manner are described.

  2. Page 29
    Abstract

    In Chapter 2, an historical overview of the development of S-waves and VSP in the 20th century is presented. Early efforts focused on building strong S-wave sources; but after advancements in P-wave AVO and recognition of S-wave splitting, PS-waves began to dominate industry research in the 1990s. Although converted waves were well understood, VSP technology focused primarily on P-wave applications. As experience increased, the scope of applications broadened to include S-wave splitting and imaging through gas in addition to lithology discrimination.

  3. Page 99
    Abstract

    In Chapter 3, the fundamental concepts of PS-wave seismology are discussed. This includes propagation velocities in anisotropic media, the elastodynamics involved in the generation of converted waves, reflection and transmission coefficients from elastic impedance contrasts, and the up- and downgoing wavefields recorded in VSPs. Various modeling techniques are discussed to highlight 3C seismic interpretation and responses of the full wavefield at the surface as well as VSP responses of downgoing and upgoing wavefields in the earth.

  4. Page 155
    Abstract

    In Chapter 4, the basic source radiation patterns and equipment used to collect 3C and VSP data are described, along with free-surface and seabed responses. Acquisition geometries are reviewed in terms of the illumination properties of P- and PS-waves, with an emphasis on common-offset-vector (COV) data and the properties of minimal data sets (MDSs). Application of these concepts to VSP geometries is discussed because they follow the same principles and guidelines as for surface seismic acquisition.

  5. Page 201
    Abstract

    In Chapter 5, processing and analysis methods are provided in detail for PS-waves in terms of key processing stages. Unique for 3C seismic data are rotation of coordinate axes, statics computation, S-wave splitting analysis, polarization filters, and vector fidelity compensation. In terms of transformation to zero offset, vertical VP/VS (? 0) for registration, common-conversion-point (CCP) gathering, and converted-wave NMO are emphasized. Other important basic processing steps are also reviewed: deconvolution, dual-sensor summation, and elastic wavefield decomposition. In terms of VSP data, conventional processing and wavefield separation are reviewed, and advanced anisotropy analyses are highlighted.

  6. Page 273
    Abstract

    In Chapter 6, prestack migration algorithms and velocity model building are discussed, including OBN and VSP examples. Migration applications are reviewed for PS-waves dealing with anisotropy, S-wave splitting, velocity model building, joint tomography with P-waves, and imaging through gas-obscured zones. Also, VSP imaging of the full wavefield (primaries and free-surface multiples), tomography, interferometry and salt proximity are included.

  7. Page 341
    Abstract

    In Chapter 7, PS-wave inversion applications are separated into two groups for fracture and stress properties, and for lithology and fluid discrimination. Principles of S-wave inversion for long wavelength birefringence properties of splitting orientation and magnitude are demonstrated with case studies. Inversion for short wavelength elastic properties of P- and S-wave impedance and density are illustrated for joint AVO/A analysis with P-waves. Also, VSP examples are included along with unconventional reservoir, and time-lapse monitoring applications.

  8. Page 419
    Abstract

    In Chapter 8, business model considerations are discussed that relate to identifying realistic business drivers for shear-waves, and research directions for 3C seismic and VSP. PS- and P-waves are treated as a unified vector wavefield coupled by anisotropy, rather than as competing wavefields. Opportunities of 3C seismic and VSP for monitoring and evaluating unconventional reservoirs are also discussed, along with advancements for PS-wave processing and interpretation technology.

  9. Page 439
    Abstract

    This appendix shows the basic wave equations for the acoustic case and for a linearly elastic medium. Stress and strain are the most important wavefield properties for elastic media, so they are emphasized to help visualize the properties of these 3 × 3 tensors. Also shown are solutions to the wave equation and their dispersion relationships.

  10. Page 459
    Abstract

    This appendix illustrates various source radiation patterns related to VP/VS ratios of elastic media. The force term from the wave equation is defined in terms of point forces and a moment tensor. Radiation patterns of wavefield amplitude are illustrated in polar diagrams for point forces on the free surface. These are based on the theory of Miller and Pursey (1954) for computing the displacement field in a half-space. They derive asymptotic expressions for the far-field strength at infinity in the solid. Synthetic seismograms from a single explosive charge are compared with the wavefield from vertical arrays of distributed charges. These are illustrated in the space domain computed from acoustic finite-difference models. The last example shows the P-wave radiation patterns from shear couples.

  11. Page 471
    Abstract

    This appendix shows the simplified Voigt notation for the matrix of stiffness coefficients and various symmetry properties of geological models that reduce the number of coefficients to manageable levels. Also, solutions of the Christoffel equation illustrate wavefield properties of slowness, polarization, phase, and group properties in VTI/VPA and orthorhombic media. The last part of this appendix illustrates the stiffness tensor in terms of anisotropic components of Lamé parameters incompressibility and rigidity to demonstrate the essential properties of conventional SV-wave anisotropy.

  12. Page 497
    Abstract

    In this appendix, basic elastodynamic properties are reviewed for vertically heterogeneous anisotropic media. Also, PS-wave reflection and transmission coefficients are discussed in relationship to the interface scattering matrix and for special cases at the free surface and seabed.

  13. Page 505
    Abstract

    This paper has been reformatted and is reprinted from The Leading Edge, 1999, 18, 1306–1311.

  14. Page 513
    Abstract

    This paper is reprinted from The Leading Edge, 2001, 20, 1042–1047.

  15. Page 519
    Abstract

    This paper is reprinted from The Leading Edge, 2003, 22, 106–112.

  16. Page 525
    Abstract

    This paper has been reformatted and is reprinted from The Leading Edge, 2003, 22, 300–309.

  17. Page 533
    Abstract

    This paper is reprinted from SEG, Expanded Abstracts, 2005, 2329–2332.

  18. Page 537
    Abstract

    This paper is reprinted from The Leading Edge, 2008, 27, 1242–1250.

  19. Page 545
    Abstract
    Corresponding author, E-mail: richard.bale@cggveritas.com

    This paper is reprinted from First Break, 2009, 27, 73–83.

  20. Page 557
    Abstract

    This paper is reprinted from The Leading Edge, 2013, 32, 86–92.

  21. Page 563
    Abstract

    These are the references in the book.

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