Foundations of Anisotropy for Exploration Seismics
- K. Helbig, Rijksuniversity of Utrecht and Free University of Amsterdam, The Netherlands
Over the last few years, anisotropy has become a "hot topic" in seismic exploration and seismology. It is now recognised that geological media deviate more or less from isotropy. This has consequences for acquisition, processing and interpretation of seismic data and also helps determine the cause of anisotropy and adds to our knowledge concerning the structure of the medium at scales beyond the resolution of the seismic method.
This volume addresses the theoretical foundations of wave propagation in anisotropic media at an easily accessible level. The treatment is not restricted to exploration seismology. The book commences with fundamental material and covers the description of wave propagation in anisotropic conditions by means of slowness and wave surfaces. It continues to explore the theory of elasticity, the interaction of elasticity and material symmetry and conditions imposed by the stability of the medium. Wave propagation in general anisotropic solids are discussed referring in particular to singular and longitudinal directions. Slowness and wave surfaces in transversely isotropic media and in the planes of symmetry of orthorhombic media is presented and then moves on to wave propagation in orthorhombic media by means of "squared slowness surfaces". The latter part of the book deals with layer-induced anisotropy showing how a particular internal structure of a medium leads to anisotropy and how much of this structure can be recovered by "inversion" of the modelling algorithm. A few fundamental aspects of exploration seismology are also discussed.
The final chapter discusses how concepts which were developed by Kelvin, but only recently understood, can be utilised to determine the symmetry class and orientation of an elastic medium.
For researchers in exploration geophysics, seismologists and those working in material sciences and solid acoustics.
- Published: January 1994
- Imprint: PERGAMON
- ISBN: 978-0-08-037224-2
...an excellent monograph with a sound philosophical basis and a unique message.
P Hubral, Geophysics Journal Internation, 1994
...it is a delightful book, and I have learnt many things from it.
Stuart Crampin, American Scientist, Volume 83, 1994
...any serious researcher interested in anisotropy will find very valuable information in this excellent monograph
P. Hubral, Journal International of Geophysics, 1995
This is not a single overview of the subject but an in-depth examination of its fundamentals...is an important explanatory text on anistropy in exploration geophysics.
B.C. Scheffers, The Leading Edge, December 1996
Table of Contents
Chapter headings and selected contents: Fundamentals. What is homogeneity? What is anisotropy? What is dispersion? What causes anisotropy of wave propagation. Appendix 1A: Analytical derivation of the relation between anisotropy and dispersion. Tools for the Description of Wave Propagation under Piecewise Homogeneous Anisotropic Conditions. Ray velocity and normal velocity. The ray-slowness surface; slowness and wave surface as polar reciprocals. Snell's Law. Appendix 2A: Formal description of the transformations used in this chapter. Analytic expression for inversion (reflection in a circle). Analytic derivation of the tangent curve from the footpoint curve. Analytic description of polar reciprocity. Elasticity. Tensors and vectors. Infinitesimal strain. Basic symmetries of the elastic tensor and the contracted notation. The elastic constants and material symmetry. Appendix 3A: The relation between elastic constants and rotational symmetry. Reduction of an arbitrary rotation to a sequence of rotations about one axis each. Tensors of rank two under rotation of the coordinate system. Appendix 3B: Invariants of the elastic tensor. Contraction of the elastic tensor on itself. Appendix 3C: FORTRAN subroutines for operations on elastic tensors in four- and two- subscript notation. Elastic Waves - The Dispersion Relation and some Generalities about Slowness and Wave Surfaces. The wave equation. Elements of inflection of the slowness surface. Slowness, polarization and symmetry. Singular directions. Appendix 4A: Orthogonality of polarization vectors. Appendix 4B: Explicit versions of the characteristic equation. Appendix 4C: Subroutines for the Kelvin-Christoffel matrix. Stability Constraints. The general stability condition. Stability conditions for isotropic, hexagonal, cubic, strong tetragonal and orthorhombic media. One-Parameter Expressions for the Slowness Surfaces of Transversely Isotropic Media and the Slowness Curves in the Planes of Symmetry of Orthorhombic Media. Decoupling of the across-plane polarization and a parameter expression for the "coupled" slowness curves. The "basic curves", the separating ellipse and the general shape of the "coupled" slowness curves. The associate parameter, the representing point, and a geometric construction for the polarization. A nomogram for the in-plane polarization. Appendix 6A: Closed explicit expressions for the coupled slowness curves in the symmetry planes of orthorhombic media. One-Parameter Expressions for the Wave Curves in the Symmetry Planes of Orthorhombic Media. Expressions for the wave surface in Cartesian and polar coordinates. Cusps. A measure of anisotropy. Squared Slowness Surfaces and Squared Slowness Curves. Phenomenology of slowness surfaces in the squared domain. The "framework" in the planes of symmetry. Deviations from the "framework" in the planes of symmetry. Appendix 8A: Determination of the type of the squared slowness curve. Appendix 8B: Properties of the square transformation. Coordinate grids. Straight lines. Symmetrically centered conics with aligned axes. General centered conics. Appendix 8C: Geometric tools for the conversion of a squared slowness curve to the ordinary domain. Geometric determination of curvature. Causes of Anisotropy: Periodic Fine Layering. Simple quasi-static strain modelling. Constraints on layer-induced anisotropy. Inversion of the compound stiffnesses to constituent stiffnesses. A nomogram for the determination of layer parameters. Appendix 9A: Generalized averages. Anisotropy and Seismic Exploration. Elliptical anisotropy. An equivalence theorem for surface-to-surface seismics. Some aspects of reflection seismics. Eigentensors of the Elastic Tensor and their Relationship with Material Symmetry. Rudimentary definition of a tensor space. Strain tensors and wave propagation. Coordinate-free representation of "wave-compatibility". Eigentensors and symmetry. Eigensystems of specific symmetries. Determination of the symmetry class. Appendix 11A: Construction of media with particular eigentensors. Reconstruction of an elastic tensor from its eigensystem. Construction of an eigensystem with particular eigenvectors. References. Index.
220 line drawings, 195 lit. refs.