Sound speed dispersion measurements and their interpretation in the presence of a shallow buried layer.

Abstract

Sharing the same experimental apparatus, two complementary techniques were developed to measure the frequency‐dependent speed of sound in marine sediments during the SAX04 sea‐trial. The first technique enabled direct time‐of‐flight measurements of acoustic wave speed along all three Cartesian axes. The second technique determined the acoustic wave speed based on the arrival angle of pulses generated in the water column and refracted upon entry into the seabed. None of the results could be modeled or explained when the seabed was parametrized as a sand half‐space. However, both techniques suggested the presence of a thin muddy layer, 0.05–0.2 m thick within the top 1 m of the sediment. For the time‐of‐flight technique, in the vertical direction, the interference from the layer created considerable apparent variability in sound speed measurements. In the horizontal direction, the layer caused the measured sound speeds to be lower than what a simplified poro‐elastic model would predict, unless one accounts ...