Chapter 4: EXPLORATION APPLICATIONS
Raymond L. Eastwood, John P. Castagna, M. G. Justice, M. D. McCormack, S. S. Lee, K. Suyama, T. Imai, H. Ohtomo, K. Ohta, T. Takahashi, James E. Fix, James D. Robertson, William C. Pritchett, 1986. "EXPLORATION APPLICATIONS", Shear-Wave Exploration, S. H. Danbom, S. N. Domenico
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Results from theoretical models, laboratory experiments, and full-waveform sonic log data converge toward a common interpretational model for Vp/Vs ratios in sedimentary rocks. Moreover, we show that Vp/Vs ratios obtained from sonic logs may be significantly different than those obtained from seismic reflection velocity analysis due to dispersion.
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Application of shear (S)-waves in seismic petroleum exploration is in a critical stage of development. Propagation of these waves and of the historically applied compressional (P)-waves in a sedimentary section are affected differently by rock physical properties. Principally, propagation velocity and, in turn, reflection amplitude of P-waves is affected by both rock incompressibility and rigidity, whereas, that of S-waves is affected by rock rigidity only. Because of this difference it is possible, for example, to verify P-wave reflection amplitude variation due to pore fluid change (e.g., brine to a gas-brine mixture), that affects rock compressibility and not rigidity, by the absence of a variation in amplitude of the corresponding S -wave reflection. Additionally, this difference makes it possible to distinguish elastic from calcareous portions of the sedimentary section by comparison of P- and S-wave interval velocities derived from corresponding P- and S-wave reflections bracketing the interval.
First to utilize S-waves were earthquake seismologists who deduced composition of the earth from P- and S-wave propagation paths. Application of S-waves in petroleum exploration was delayed by disappointing theoretical and model studies due, principally, to S-wave velocity anisotropy in layered media. Also contributing to this delay was lack of an effective S-wave source of sufficient energy. Viable land S-wave sources now include (1) explosive charges, pioneered by Russian geophysicists, (2) weight-drop devices, and (3) horizontal vibrators, a modification of vertical vibrators used in the Vibroseis © method. Marine S-wave sources presently are not available; nonetheless, reflections of S-waves converted at the ocean bottom from and to pressure waves at the source and receiver end, respectively, provide the possibility of marine S-wave exploration.