Effects and Consequences of Transverse Isotropy in the Seafloor

Abstract

Transverse isotropy, a form of anisotropy with a single vertical axis of symmetry, is extremely widespread in marine sediments, but its effects on seismic interpretations have been inadequately quantified. This anisotropy increases with depth, just as propagation velocities themselves increase. Indeed, it is impossible to separate the effects of velocity gradients and anisotropy. Using elastic parameters appropriate for a carbonate sequence, we have used ray-tracing and wavefield modeling codes to investigate the consequences of ignoring anisotropy. If isotropy is erroneously assumed, sound-speed gradients will be underestimated, the thickness of the sedimentary sequence overestimated (often seriously) and the shear velocity overestimated (hence estimates of Poisson’s ratio will also be incorrect). Transverse isotropy may also affect the bottom reflection loss, but from our preliminary studies such effects appear to be minimal. From compressional-wave traveltime data alone the identification of transverse isotropy appears almost impossible. Shear-wave information is vital if the phenomenon is to be adequately characterized.