Chapter 3: FIELD EQUIPMENT AND ACQUISITION PROCEDURES
A new shear (S)-wave source, the M3 Marthor has been developed from a prototype, the M1 Marthor. It is a weight-drop source, producing reversed polarity blows on a baseplate which is adaptable to off- or on-road operations. With internal swings of the hammer and internal strikes on the baseplate, it is a compact, safe, and maneuverable S-wave source.
Seismic surveys were carried out to compare effectiveness of the S-wave vibrator and the M3 Marthor in generation of S-waves. In each case recordings were obtained under identical conditions for both sources. Frequency analysis shows that the M3 Marthor source signal spectrum is broader than that of the S-wave vibrator because of high-amplitude, low-frequency components. Onset of the first seismic event with the M3 Marthor is more abrupt than that obtained with the S-wave vibrator, greatly facilitating computation of static corrections. This is an important advantage in view of the usual highly variable S-wave reflection static time differences. S-wave events are enhanced by the M3 Marthor's ability to produce reversed polarity signals to be subtracted, thereby enhancing SH waves and attenuating other wave types. In the case of VSP surveys, the ability to change polarity of the source signal provides identification of events that are difficult or impossible to identify on single-polarity recordings. The distribution of energy along the x,y, and z axes produced by an M3 Marthor blow has been calculated. This study demonstrates the efficiency of the M3 Marthor in restricting source-signal energy to the y component parallel to the desired SH-wave particle motion.
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.