In situ measurement technique for compressional wave velocity dispersion

Abstract

Marine sediments can be considered a two-phase medium composed of solid sediment particles and fluid-filled interstices. Many theories of propagation through such an assemblage predict a dispersive compressional wave velocity. Granular sediments (i.e., sands and gravels) have been predicted and reported to exhibit velocity dispersion and/or a nonlinear frequency-dependent attenuation in the frequency range 102−104 Hz (Kramers–Kronig relations require velocity dispersion if a nonlinear frequency-dependent attenuation is observed). One of the experimental challenges has been to measure velocity over several decades of frequency using a single technique so that observed velocity changes cannot be due to different biases in the measurement techniques. A simple in situ measurement technique is described that measures the critical angle as a function of frequency from ∼102−104 Hz and thus infers the velocity dispersion over that band. Measurements on the mid to outer continental shelf (Malta Plateau and the New Jersey shelf STRATAFORM area) show little or no velocity dispersion. This implies that the compressional wave attenuation for these sediments is linear or nearly linear over this band. [Work supported by the ONR and NATO Undersea Research Centre.]