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The resolution of narrow low-velocity zones with the generalized reciprocal methodNormal access

Author: Derecke Palmer
Journal name: Geophysical Prospecting
Issue: Vol 39, No 8, November 1991 pp. 1031 - 1060
DOI: 10.1111/j.1365-2478.1991.tb00358.x
Organisations: Wiley
Language: English
Info: Article, PDF ( 1.48Mb )

Summary:
The depth to the surface of a refractor and the seismic velocity within the refractor are very often intimately related. In the shallow environment, increased thicknesses of weathering occur in areas of jointing, shearing or lithological variations, and these zones of deeper weathering can have lower subweathering refractor velocities. This association is important in geotechnical investigations and in the measurement of weathering thicknesses and sub-weathering velocities for statics corrections for reflection seismic surveys.

Algorithms, which employ forward and reverse traveltime data and which explicitly accommodate the offset distance through the process known as refraction migration, are necessary if detailed structure on a refractor and rapid lateral variations of the seismic velocity within it are to be resolved. These requirements are satisfied with wavefront construction techniques, Hales’ method and the generalized reciprocal method (GRM).

However, these methods employ refraction migration in fundamentally different manners. Most methods compute an offset distance with an often imprecise knowledge of the seismic velocities of the overlying layers. In contrast, the GRM uses a range of offset distances from less than to greater than the optimum value, with the optimum value being selected with a minimum-variance criterion.

The approach of the GRM is essential where there are undetected layers and where there are rapid variations in the depth to a refractor and the seismic velocity within it. In the latter situations the offset distance necessary to define the seismic velocities can differ considerably from the value required to define depths.

The efficacy of the GRM in resolving structure and seismic velocity is demonstrated with three model studies and two field examples.


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