Sediment Acoustics: LF Sound Speed, HF Scattering and Bubble Effects

Abstract

LONG-TERM GOALSPhysically sound models of acoustic interaction with the ocean floor including penetration, reflection and scattering in support of MCM and ASW needs. OBJECTIVESThe objectives are to fill important gaps in our knowledge and understanding of ocean sediment acoustics, including (1) a new model to account for the recently measured low-frequency sound speed anomaly, (2) new scattering mechanisms to augment current Navy models of high-frequency bottom scattering, and (3) the study of time dependent propagation and scattering effects due to shallow water gas bubbles in the sediment. APPROACHNew propagation model development: Previous accomplishments in sediment acoustics include (a) the detection of sub-critical angle penetration (Chotiros 1989(Chotiros , 1995a, (b) the realization that the effective density is less than the physical density (Chotiros 1995b andWilliams 2001), (c) the composite medium extension of the Biot model (Chotiros 2002) which gave a better understanding of the boundary between frame and fluid, and (d) the relaxation mechanisms, also known as squirt flow, at the grain-grain contacts (Chotiros, Isakson 2004). There is a recently discovered anomaly that needs to be addressed: the low-frequency sound speed anomaly. Measurements at SAX04 (Hines, Osler, Scrutton, and Lyons, 2005), SAX99 (Williams et al. 2002) and a previous set of measurements by Turgut (Turgut, Yamamoto 1990) show sounds speeds below 2 kHz that are significantly lower than the lower-bound predicted by the Wood equation. One possible cause is the presence of minute gas bubbles in the sediment, which current instrumentation is unable to resolve. Another, and more likely cause, may lie within the dynamics of the skeletal structure. New bottom scattering model development: Initial measurements in 1960s (McKinney and Anderson 1964) and model studies, indicated that backscattering strength increased with frequency, and this was partially responsible for the frequency selection of the AN/SQQ-32 sonar. But later measurements did not uphold this trend. Measurements in the 1980s resulted in the APL-UW HFEVA bottom backscattering model. While this model was well-tuned to the 1980s data, it fit neither the earlier data nor the later data. These observations show that no one measurement, or program of measurements, has been able to capture the diversity of the seabed. A database of all the published measurements is 1