Panel Discussion on Potential for Seismic Shear-Wave Applications in the Marine Environment (1987)

ABSTRACT With renewed interest in seismic shearwave applications in geophysical exploration, attention is being further focused on marine applications of shear-wave (S-wave) data. Since shear-wave energy does not propagate through fluids--including sea water-we are dependent upon either bottom S-wave sources and receivers on the sea floor, or mode conversion to effect P-wave transmission in the water and S-wave propagation in at least a portion of the subsurface, or a combination of the two. A panel of experts from both major oil companies and seismic contractors has been assembled to discuss the state of the art and, perhaps more importantly, the future direction in seismic S-w8ve applications offshore. Each panel member will briefly review a portion of the problem and status of solutions, followed by an open discussion. The discussions will focus on three general areas:A brief description of seismic shearwave propagation in elastic media;Successes in acquisition and processing of mode-converted shear-wave data in the terrestrial environment; andThe status of acquisition and processing of mode-converted shear-wave data in the marine environment. DESCRIPTION OF SEISMIC S-WAVE PROPAGATION Seismic body waves travel through an elastic solid with two modes of propagation, Compressional, or P-waves and Distortional, or S-waves. The two types of energy propagation involve different particle motions, each with different propagation velocities responding to different elastic properties of the solid. Conventional P-wave propagation responds to the incompressibility of the material through which the waves are propagating. Since both fluids and solids have finite incompressibility, they will both support P-wave propagation. S-wave propagation, on the other hand, responds to the rigidity of the material through which the wave is propagating. Since fluids have no rigidity, shear waves cannot propagate through them. That is, seismic shear waves cannot travel through sea water. To gather seismic S-wave information in an offshore setting, we must resort to either locating the sources and receivers on the sea floor, or relying upon mode conversion at some interface at or below the top nf the consolidated (i.e. rigid) sediments. The panel will consider the latter mechanism more fully in their discussion. Mode conversion occurs whenever propagating energy impinges upon an interface, such as the sea floor or at bedding surfaces between sediment layers. In general, if a P-wave ray intersects an interface at a non-normal angle of incidence, four different rays emanate from the interface: reflected P and S waves, and transmitted (refracted) P and S waves. The partition of energy between these four wave types is dependent upon the angle of incidence and the contrasts in elastic constants (and density) across the interface. Further, if the medium on one side of the interface is a fluid, such as sea water, then no shear-wave is reflected (or refracted) into the fluid medium. Geographically, there are areas with seafloor sediments with properties that are favorable for mode conversion to a transmitted shear wave. In these geographic areas, shear-wave data can be gathered and interpreted. OBSERVATIONS OF MODE-CONVERTED SHEAR UAVES ON LAND Sources of conventional P-wave energy do generate S-wave reflections through mode-conversion during the reflection process.