7. Marine Examples and Applications

Abstract

Introduction Because there is no viable S-wave source that can be deployed on the seafloor, multicomponent seismic data acquired in marine environments are constrained to data recorded by seafloor sensors (typically four-component [4C] sensors) and generated by air-gun arrays towed at the sea surface. Thus, SH-SH data are not available for marine applications. An SV-SV mode can be used in rare instances in which the seafloor is sufficiently hard for a downgoing P-wave to generate a robust P-to-SV mode conversion directly at the water-seafloor interface (Tatham and Goolsbee, 1984). The seafloor then becomes a secondary source from which a downgoing SV mode illuminates subseafloor strata. For those reasons, only two wave modes are emphasized in marine multicomponent seismic data — the P-P mode and the P-SV mode. The applications that are illustrated in this chapter apply equally well to onshore prospects. Those examples are collected into this chapter only because the data were acquired in a marine environment, not because there is some uniqueness to marine geology or to marine seismic data. One exception to this generalization is the use of 4C data to image near-seafloor strata in deep water. This application is unique to the marine environment because there is a large elevation difference between the surface source and the seafloor receiver that allows P-P and P-SV data to be processed like walkaway vertical-seismic-profile (VSP) data. This extension of VSP data-processing principles to marine 4C data allows near-seafloor strata immediately below a receiver station to be imaged with high resolution. No equivalent source-receiver geometry in which there is a large difference between the elevations of source and receiver can be used onshore except a walkaway VSP, which requires access to a deep well.