Ray-path concepts for converted-wave seismic refraction
Steve Hearn 1 2 Alan Meulenbroek 11 Velseis Pty Ltd and School of Earth Sciences, University of Queensland, PO Box 118, Sumner Park, QLD 4074, Australia.
2 Corresponding author. Email: steveh@velseis.com
Exploration Geophysics 42(2) 139-146 https://doi.org/10.1071/EG10030
Submitted: 27 October 2010 Accepted: 26 May 2011 Published: 28 June 2011
Abstract
P-wave reflection-statics solutions typically incorporate P-wave refraction data, derived from the first breaks of the production data. Similarly, converted-wave refractions, taken from inline-component recordings, can be exploited to yield S-wave receiver statics, required in the processing of converted-wave reflection data. This methodology requires extensions to well known P-wave refraction analysis methods. This paper outlines extensions of the slope-intercept method and the reciprocal method, required to analyse converted-wave refractions. We discuss the computation of S-wave time-depths and describe how the observed ratio of S-wave to P-wave time-depths can provide a useful estimate of the near-surface V P/V S ratio, which is of interest in the analysis of engineering rock strengths.
We also include discussion of several related practical issues, with particular reference to dynamite sources. When the source is buried in the refractor, the required reciprocal times cannot be directly measured from the raw travel-time data. They can, however, be easily derived via correction using measured intercept times. Often converted-wave refractions are of poorer quality than conventional P-wave refractions, such that reversed refractions may not be available over some parts of the spread. In this situation, the preferred time-depth quantity cannot be computed. However, delay-times derived from single-ended data can be substituted, particularly if lateral variations in refractor velocity are allowed for.
The concepts outlined here are used in a companion paper to correct S-wave receiver statics in a coal-scale dataset from the Bowen Basin in central Queensland.
Key words: converted-wave, delay-time, reciprocal method, seismic refraction, slope-intercept.
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