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.