It is proposed to investigate the behaviour of waves upon the plane free surface of an infinite homogeneous isotropic solid, their character being such that the disturbance is confined to a superficial region, of thickness comparable with the wave-length.— Lord Rayleigh (1842–1919)
It is proposed to investigate the behaviour of waves upon the plane free surface of an infinite homogeneous isotropic solid, their character being such that the disturbance is confined to a superficial region, of thickness comparable with the wave-length.
We often are taught in college that dispersion is not an issue in seismic data processing, although sizable differences can occur when we try to match velocities estimated from core-plug, sonic-log, and surface-seismic measurements because the data span orders of magnitude in frequency. In the 10- to 100-Hz band of typical surface-seismic data, surely velocity is independent of frequency.
However, that is an oversimplified view of real seismic data. It is pretty nearly true for seismic body waves (P, S, and mode-converted) moving around the deep subsurface, but in the near surface, observed velocities often show strong dispersion because of layering and associated large velocity contrasts. Deeper in the earth, there are occasional interfaces across which large velocity jumps occur, including contacts between sedimentary rock and salt or igneous basement. In the shallow subsurface, however, dramatic velocity changes are commonplace at the base of weathering, the water table, or the seafloor. Because the layering is so shallow, waves emitted by a source can become trapped in surface layers
Figures & Tables
Elements of Seismic Dispersion: A Somewhat Practical Guide to Frequency-dependent Phenomena (SEG Distinguished Instructor Series No. 15) covers selected effects encountered in the acquisition, processing, and interpretation of reflectionseismic data. The material, based on the 2012 SEG Distinguished Instructor Short Course, shows how those phenomena arise, how they can be characterized, and the important information they contain. The text shows how spectral decomposition and time-frequency methods have led to improved understanding and use of nonlinear harmonics, near-surface guided waves, layer-induced anisotropy, velocity dispersion and attenuation, interference, and Biot reflection. Accessible discussion is augmented by examples, figures, and references to primary literature for further study. This book will interest technical managers and those who work in acquisition, processing, and interpretation of seismic data. (DISC on DVD, 761A, is also available.)