A 1-km 2 area located 2 km off the Florida Panhandle (30 22 6 N; 86 38 7 W) was selected as the site to conduct high-frequency acoustic seafloor penetration, sediment propagation, and bottom scattering experiments [1]. Side scan, multibeam, and normal incidence chirp acoustic surveys as well Manuscript
Unlike the application of the Biot model for fused glass beads, which was conclusively demonstrated by Berryman ͓Appl. Phys. Lett. 37͑4͒, 382-384 ͑1980͔͒ using the experimental measurements by Plona ͓Appl. Phys. Lett. 36, 259-261 ͑1980͔͒, the model for unconsolidated water-saturated sand has been more elusive. The difficulty is in the grain to grain contact physics. Unlike the fused glass beads, the connection between the unconsolidated sand grains is not easily modeled. Measurements over a broad range of frequencies show that the sound speed dispersion is significantly greater than that predicted by the Biot-Stoll model with constant coefficients, and the observed sound attenuation does not seem to follow a consistent power law. The sound speed dispersion may be explainable in terms of the Biot plus squirt flow ͑BISQ͒ model of Dvorkin and Nur ͓Geophysics 58͑4͒, 524 -533 ͑1993͔͒. By using a similar approach that includes grain contact squirt flow and viscous drag ͑BICSQS͒, the observed diverse behavior of the attenuation was successfully modeled.
Elastic theory of wave propagation and the measured speed of sound in sandy ocean sediments indicate that such sediments are impenetrable to high-frequency sound at shallow grazing angles. The speed of sound in water-saturated, unconsolidated sand is in the region of 1700 m/s which, under the elastic theory of wave propagation, gives it a critical grazing angle in the region of 28°. At shallower grazing angles, refraction is not permitted, and total internal reflection is predicted. Recent experimental measurements contradict this view. Biot’s theory of acoustic propagation in porous sediments is the most likely explanation. Biot’s theory of acoustic propagation, as it applies to water-saturated sand, is reviewed. The speed of the slow wave is found to be higher than previously predicted. New input parameter values are deduced.
A high-frequency acoustic experiment was performed at a site 2 km from shore on the Florida Panhandle near Fort Walton Beach in water of 18-19 m depth. The goal of the experiment was, for high-frequency acoustic fields (mostly in the 10-300-kHz range), to quantify backscattering from the seafloor sediment, penetration into the sediment, and propagation within the sediment. In addition, spheres and other objects were used to gather data on acoustic detection of buried objects. The high-frequency acoustic interaction with the medium sand sediment was investigated at grazing angles both above and below the critical angle of about 30 . Detailed characterizations of the upper seafloor physical properties were made to aid in quantifying the acoustic interaction with the seafloor. Biological processes within the seabed and the water column were also investigated with the goal of understanding their impact on acoustic properties. This paper summarizes the topics that motivated the experiment, outlines the scope of the measurements done, and presents preliminary acoustics results. A preliminary summary of the meteorological, oceanographic, and seafloor conditions found during the experiment is given by Richardson et al. [1].
The discrepancy between acoustic measurements and the theoretical predictions was investigated in the case of water-saturated sand. Two theoretical models were considered: visco-elastic and poro-elastic solid models. The visco-elastic solid model could not be reconciled with reflection loss measurements and was rejected. The poro-elastic solid model using Biot's theory [J. Acoust. Soc. Am. 103, 2723-2729 (1998)] as formulated by Stoll [J. Acoust. Soc. Am. 70, 149-156 (1981)] was an improvement. It was investigated using an inversion process. Operative values of grain bulk modulus and the frame bulk and shear moduli of water-saturated sand were inverted from simple measurements--reflection loss, compressional and shear wave speeds and attenuations. Although the inversion process is nonlinear, in practice, it is well behaved and converges quite rapidly to a unique solution. The issue of imprecisely known parameter values was handled in a probabilistic manner. The inversion results, using published laboratory and in situ measurements, showed that further improvement was needed. In an attempt to find a solution, two possible hypotheses are put forward. (1) Composite materials: The possibility that the frame may contain fluid and that the pore fluid may contain loose grains. (2) Independent coefficient of fluid content: The possibility that porosity may change with pore fluid pressure. Inversion results were encouraging for both hypotheses. It is difficult to say which of the two hypotheses is superior, and the two hypotheses are not mutually exclusive. The new hypotheses represent a significant advance because they have the potential to resolve the remaining discrepancies. At this stage, alternative interpretations of the data are possible.
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