Sea-bottom shear-wave velocities and mode conversions

Abstract

Elastic parameters for shallow marine sediments were obtained from the literature (Hamilton (1976, 1979); Hovem et al. (1991); Esteves (1996)) and previously unpublished geotechnical data from offshore Brazil. The Brazilian data showed reasonable agreement with Hamilton's results except in the very shallow (above 10 m) sedimentary section. A second order equation to calculate VS as a function of depth in marine sediments is derived empirically down to a depth of 140 m. Analyses of transmission and reflection coefficients for compressional- and shear-wave energy mode conversion using Zoeppritz equations were performed for both the sea bottom and a typical hydrocarbon reservoir top of Tertiary age. It is concluded that most S-wave reflection data recorded on the ocean floor by OBC is related to upcoming energy converted at an interface at depth and not from a downgoing shear conversion at the ocean floor. It was also concluded that, using elastic assumptions, mode conversion (both P- to S- and S- to P-) of the up going energy is negligible in the shallow (above 160 m) sediments. © 2005 Sociedade Brasileira de Geofísica.