A technique for measuring in-situ compressional wave speed dispersion in marine sediments

Compressional speed dispersion exists in all marine sediments. If the dispersion is great enough it may play a significant role in acoustic interaction with the seabed. On the other hand if dispersion is weak, seabed models and databases can be substantially simpler. The ocean acoustics community is divided on this issue, in part because of the lack of observations. One of the experimental challenges has been to measure speed over several decades of frequency using a single technique so that observed speed changes cannot be due to different biases in the techniques. A simple in-situ experimental approach was developed that measures the critical angle as a function of frequency and thus infers the speed dispersion. Measurements on the mid to outer continental shelf (Malta Plateau and the New Jersey Shelf STRATAFORM area) show a weak dispersion over the band from /spl sim/10/sup 2/-10/sup 4/ Hz. This implies (via the Kramers-Kronig relations) that the compressional wave attenuation for these sediments is small and/or approximately linear over this band.