Abstract

Evidence that shear (S) waves are much more important in seismic surveys than currently believed was found in each of two deep well tests conducted some time ago. Wave tests were recorded along vertical lines, following procedures which are now designated 'vertical seismic profiling.' The results may be divided into (1) evidence that shear (S) waves are produced by in-hole dynamite charges and by the resulting compressional (P) waves, and (2) evidence that the S-waves subsequently produce P-waves. The proof of S-wave production is quite conclusive. Even if we say that only P-waves are set up in the immediate vicinity of the shot, some S-waves are then generated within a radius of 10 to 100 ft to form what we will call a direct or 'source S wave.' Other S-waves are set up by conversion of P-wave energy to S-wave energy at interfaces hundreds and thousands of feet from the dynamite charge. In contrast to the P to S conversion, the evidence for S to P conversion is less conclusive. The source S-wave generated near the shot was found to have a long-period character, with many cycles which are believed to be controlled by the layering near the shot. The PS converted waves, which appear later, resemble the P-waves that produce them. The interference to primary reflections by multiple reflections and/or converted waves produces complex signals at points deep in the well which require directional discrimination to separate up-traveling waves from down-traveling waves.

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