Figures & Tables
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.