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Proceedings of Meetings on Acoustics

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POMA - 158th MeetingĀ Acoustical Society of America
Conference Location: San Antonio, Texas Conference Date: 26 - 30 October 2009
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Low-frequency seismo-acoustic propagation near thin and low-shear speed ocean sediment layers

Jon M. Collis

POMA Volume 8, pp. 070001 (December 2009); (10 pages)

Online Publication Date: December 09, 2009

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Accurate and efficient prediction of propagation over realistic models of elastic ocean sediments has been achieved recently using parabolic equations. A concern has been low-shear wave speed sediments that can become singular as the wave speed tends toward zero. A historic approach for treating a sediment of this type has been to assume it is a fluid and that effects due to elasticity are negligible. This approach does not account for second order effects such as energy loss due to frequency-dependent attenuation. In addition, thin sediment layers typical of those found at the ocean bottom interface have been difficult to treat numerically. At low frequencies, layers of this type can be treated as a 'massive' interface between the water and higher-shear speed sediment basement layers. To satisfy interface conditions across the layer, Rayleigh jump conditions are imposed [F. Gilbert, Ann. Geofisica XL, 1211 (1997)]. A consequence of this approximation is that interface and other wave types become dispersive where they were not previously. In this presentation, the massive elastic interface is benchmarked with an elastic parabolic equation and the effects of resultant errors are quantified.
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43.30.Dr Hybrid and asymptotic propagation theories, related experiments
43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics
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Evolution of navy diver exposure standards for deterministic underwater sound in the 100-500 hertz band

Frederick M. Pestorius, Edward A. Cudahy, and David M. Fothergill

POMA Volume 8, pp. 070002 (December 2009); (8 pages)

Online Publication Date: December 09, 2009

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US Navy development of Low Frequency Active (LFA) sonar at the end of the Cold War resulted in the adverse exposure of Navy divers to high levels of LFA sound. An initial 30 month study into the effects on Navy divers of deterministic waveforms in the frequency band 160-320 HZ was conducted. Sound pressure levels up to 160 dB re 1Ī¼Pa were utilized in a series of experimental observations. Auditory, animal, and human diver studies were completed. Additionally, modeling studies were conducted in the areas of cavitation, hyperthermia, tissue shearing, chest wall/lung displacement and rectified diffusion. Ultimately, 453 exposures using 87 different test subjects were conducted. These tests yielded a baseline set of guidelines for the exposure of Navy divers to deterministic sonar transmissions in the subject test band. The Navy has continued to refine and expand these guidelines to the present day utilizing both modeling and field exposure studies. A summary of the initial and subsequent testing and a tracing of the evolution of the current guidelines will be presented.
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43.80.Nd Effects of noise on animals and associated behavior, protective mechanisms
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The applicability of a small towed array system to the ocean bottom sediment classification problem.

William M. Carey and James F. Lynch

POMA Volume 8, pp. 070003 (May 2010); (11 pages)

Online Publication Date: May 19, 2010

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An autonomous array system is uniquely suited to perform near bottom ~1m ) measurements of horizontal wave number spectra that estimate water and interface wave numbers. Synthetic coherent processing provides the required gain to observe the interface wave peak ~40-50 dB below the peaks of water modes. Comparable measurements were performed with an array of seismometers (Muir et al, 1991). These spectra contain the information necessary to perform inversions that account for compressional and shear wave speed gradients. Spatial transforming the computed pressure field produces spectra comparable with measurements. Variations of the geoacoustic profiles show that the present of gradient produces additional identifiable spectral structure corresponding to shear waves trapped with in the sediments. In the experiments discussed the sound source was on the bottom and the array was towed by a vehicle. However it is feasible to either use a source of sound on the vehicle or on a companion vehicle to do surveys. This method is a fast, efficient and accurate ocean acoustic measurement tool useful in rapid waveguide characterization with the appropriate inversion technology.
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43.20.Jr Velocity and attenuation of elastic and poroelastic waves
43.30.Bp Normal mode propagation of sound in water
43.60.Gk Space-time signal processing, other than matched field processing
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Measurement of sound transmission through mud at Dodge Pond, Connecticut

William M. Carey and Allan D. Pierce

POMA Volume 8, pp. 070004 (May 2010); (8 pages)

Online Publication Date: May 19, 2010

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Depositional muddy sediments are slow bottoms and pose a problem for the sonic detection of buried ordnance. This paper addresses the question: can the frequency dependent dispersion be predicted and verified by measurements in areas where buried object detection is required? Wood and Weston (Acustica, V14, 1964) have indicated that muddy sediments in the kHz range have a compressional speed 3% less than water with a frequency dependent attenuation (less than that of sand). A theoretical treatment of "muddy sediments", the Card House Theory (Pierce and Carey, POMA (5), 7001, 2009), estimates the slow sound speed and frequency dispersion proportional to mud porosity. Preliminary Dodge Pond results obtained with a buried array (1 to 10 kHz) are presented and illustrate the importance of micro-bubbles on the dispersion characteristic. The initial measurements on the disturbed sediment were found strongly influenced by scattering from larger bubbles whereas the results after a period of 10 months showed the effect of a smaller size distribution of bubbles. Estimates of the dispersion characteristic of mud and the effect of micro-bubbles are discussed. Finally the application of an impedance tube to the characterization of mud is discussed.
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43.20.Hq Velocity and attenuation of acoustic waves
43.20.Jr Velocity and attenuation of elastic and poroelastic waves
43.30.Ft Volume scattering
43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics
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