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Slip-slidin' away—some practical implications of seismic velocity anisotropy on depth imaging

University of Calgary

Robert W. Vestrum and Jennifer M. Leslie

Kelman Seismic Processing, Calgary, Alberta, Canada

Corresponding author: D. Lawton, donl@geo.ucalgary.ca

The first 20% of the full text of this article appears below.

Recent advances in imaging mean that we no longer need to ignore seismic velocity anisotropy. Laboratory and field studies provide compelling evidence that shales exhibit intrinsic transverse isotropy (TI). In this symmetry class, the seismic velocity parallel to the laminations is greater than that perpendicular to the layering, and the difference can be as high as 30%. In flat-layered clastic sequences, such as in the Western Canada sedimentary basin, the TI axis of symmetry is essentially vertical, and isotropic depth migration of reflection seismic data tends to overestimate the true depths of reflectors. This occurs because the imaging velocity, based on moveout analysis, is generally greater than the true vertical velocity.

Figure 1 is a photograph of flat-lying clastic strata in the Western Canada sedimentary basin near Calgary, Alberta. These rocks are composed of alternating sandstones and shales of Cretaceous age, with individual strata being significantly less in thickness than a typical seismic wavelength ( = 100 m). Although individual strata may be isotropic, it has been shown that the entire sequence can be considered as being equivalent to a single TI layer. The bedding-normal velocity is the slow-velocity axis and is defined as V₀. Conversely, the fast-velocity direction is parallel to bedding and is defined as V₉₀.

View larger version (147K): [in this window] [in a new window]   Figure 1. Thinly layered clastic sediments from the Western Canada sedimentary basin near Calgary. These rocks are considered transversely isotropic (TI) with the fast velocity (V90) parallel to bedding and the slow velocity (V0) perpendicular to bedding. Layering is at a much finer scale than a seismic wavelength (). (Photograph courtesy of D. Spratt.)