William M. Carey

Nonlinear frequency-dependent attenuation in sandy sediments

This paper summarizes evidence of a nonlinear frequency dependence of attenuation for compressional waves in shallow-water waveguides with sandy sediment bottoms. Sediment attenuation is found consistent with alpha(f) = alpha(f(o)) x (f/f(o))n, n approximately 1.8 +/- 0.2 at frequencies less than 1 kHz in agreement with the theoretical expectation, (n = 2), of Biot [J. Acoust. Soc. Am. 28(2), 168-178, 1956]. For frequencies less than 10 kHz, the sediment layers, within meters of the water-sediment interface, appear to play a role in the attenuation that strongly depends on the power law. The accurate calculation of sound transmission in a shallow-water waveguide requires the depth-dependent sound speed, density, and frequency-dependent attenuation.

Sound sources and levels in the ocean

The standard definitions found in the American National Standards on Acoustics are applied to common sound sources used in both underwater acoustics research and naval sonar system operation. Recommended metrics are quantified for both continuous and transient sources of sound. Standard definitions are reviewed with theoretical sound source models. Requisite metrics are derived and applied to examples of energy sources of sound, such as transients from a small omnidirectional explosive, an air gun, a light bulb, and a dolphin click. A generic quantitative model of surface ship sonar system emissions is developed. Active sonar transmissions are analyzed with the requisite quantitative metrics required to characterize these emissions. These results should be useful in environmental assessments, biological experiments, and the sonar system design

Measurement of sound transmission and signal gain in the complex Strait of Korea

The experiment, The Acoustic Characterization Test III, was conducted in the oceanographically complex Strait of Korea to accurately measure the sound transmission under known environmental conditions. Geoacoustic profiles derived from geophysical measurements, measured bathymetry, and sound-speed profiles were the basis for range dependent parabolic equation (PE) calculations. Agreement between measured and calculated transmission loss was obtained with an attenuation profile in the near water-sediment interface layer with a dependence on frequency to the 1.8 power consistent with measurements in other sand-silt areas. Since the environment was oceanographically complex and the shipping noise levels were high, the coherency of the sound transmission was estimated using relative signal gain (RSG). RSG was taken as the difference between the gain calculated with PE and measured with the array and at longer ranges and higher frequencies was found to be approximately -2 dB with a signal gain coefficient of variation of 5%. This RSG degradation, attributed to the random signal phase fluctuations resulting from scattering from the surfaces and volume of the waveguide, yielded using a Gaussian coherence function a spatial coherence length of 30/spl lambda/ @ 400 Hz-40 km. In addition, high resolution imaging of five targets with two bottom mounted arrays illustrate the achievable performance of low-to-mid frequency active sonar in this environment.

Investigation of ocean acoustics using autonomous instrumentation to quantify the water-sediment boundary properties

Sound propagation in shallow water is characterized by interaction with the oceans surface, volume, and bottom. In many coastal margin regions, including the Eastern U.S. continental shelf and the coastal seas of China, the bottom is composed of a depositional sandy-silty top layer. Previous measurements of narrow and broadband sound transmission at frequencies from 100 Hz to 1 kHz in these regions are consistent with waveguide calculations based on depth and frequency dependent sound speed, attenuation and density profiles. Theoretical predictions for the frequency dependence of attenuation vary from quadratic for the porous media model of M.A. Biot to linear for various competing models. Results from experiments performed under known conditions with sandy bottoms, however, have agreed with attenuation proportional to f 1.84, which is slightly less than the theoretical value of f2 [Zhou and Zhang, J. Acoust. Soc. Am. 117, 2494]. This dissertation presents a reexamination of the fundamental considerations in the Biot derivation and leads to a simplification of the theory that can be coupled with site-specific, depth dependent attenuation and sound speed profiles to explain the observed frequency dependence. Long-range sound transmission measurements in a known waveguide can be used to estimate the site-specific sediment attenuation properties, but the costs and time associated with such at-sea experiments using traditional measurement techniques can be prohibitive. Here a new measurement tool consisting of an autonomous underwater vehicle and a small, low noise, towed hydrophone array was developed and used to obtain accurate long-range sound transmission measurements efficiently and cost effectively. To demonstrate this capability and to determine the modal and intrinsic attenuation characteristics, experiments were conducted in a carefully surveyed area in Nantucket Sound. A best-fit comparison between measured results and calculated results, while varying attenuation parameters, revealed the estimated power law exponent to be 1.87 between 220.5 and 1228 Hz. These results demonstrate the utility of this new cost effective and accurate measurement system. The sound transmission results, when compared with calculations based on the modified Biot theory, are shown to explain the observed frequency dependence.

Results from the Nantucket Sound autonomous underwater vehicle towed hydrophone array experiment

Results from several experiments with an autonomous underwater vehicle towed hydrophone array system demonstrate the ability of such a system to provide rapid, cost effective and accurate characterization of the shallow water waveguide. A synthetic aperture Hankel transform of the complex pressure as a function of range was used to estimate the eigenvalues and functions. Results showed that the vehicle was capable of towing the array in a steady and stable configuration sufficient for coherent processing and formation of a synthetic aperture Hankel transform. Transmission loss calculated with propagation codes that use depth and frequency dependent geo-acoustic profiles (compressional wave speed, density and attenuation) compared well with the measured transmission loss. The ability to perform coherent processing of the data obtained by this prototype system demonstrates that the system is an efficient and accurate alternative to traditional measurement techniques. Since the vehicle has the ability to change depth and course while measuring non-acoustic data such as currents, bathymetry, and sound speed; the time required for wide area characterization is significantly reduced

A Physical Explanation For Less Than Quadratic Recorded Attenuation Values

A review of experimental evidence shows that the low frequency attenuation of compressional acoustic waves in sandy marine sediments obeys a simple power law consistent with a frequency dependent attenuation proportional to fn , n =1.0. This observation is in general agreement with Biots model (1956) for frequencies less than 1 kHz. Recent theoretical work [A. D. Pierce etal., Proc. Oceans 2005, Brest, FR. (2005)] shows that in this lower frequency range, a quadratic dependence (n=2) is to be expected and is decreased by predictable modal propagation characteristics in shallow water. Physical properties and depth-dependent characteristics of the sediment are used to explain the apparent n=18 power law dependence of attenuation. Calculations performed using parabolic equation codes with simplified Biot geoacoustic profiles are compared to experimental results.

The Measurement of Oceanic Ambient Noise

Ambient noise investigations constitute one of the largest sections of the Journal of the Acoustical Society of America. Urick (1984) has summarized a good many of these experimental papers and his report is valuable as it updates and extends the work of Wenz (1972. In this chapter the discussion focuses on the key aspects of ambient noise by interpreting the experimental observations in light of the fundamental production mechanisms of ambient noise. The theoretical treatments of these source mechanisms can be found in Chapter 3, with the appendices containing detailed derivations. The source mechanisms are used in this chapter as part of our overview of the characteristics of measured ambient noise. Since the measurements span some 60 years, the following question arises: What was measured and how do these measurements compare with those currently performed?

Sensitivity of modal attenuation coefficients to environmental parameters

In waveguides that are nearly independent of range, modal attenuation coefficients (MACs) convey the influence of intrinsic sediment attenuation to propagating modes in the water. The intrinsic attenuation in the upper sediment layer in sandy-silty areas has been recognized as having a nonlinear dependence on frequency f. The frequency dependence of the MACs is particularly interesting. The MACs in a Pekeris waveguide decrease like f-1, even though the upper layer intrinsic attenuation increases like f-2. These results are consistent with calculations in a classic paper by Ingenito, For adiabatic propagation a perturbation formula is useful for calculation of the MACs. The formula demonstrates that downward refracting sound speed profiles (SSPs) dramatically change the behavior compared with a Pekeris waveguide. For example, depending on the strength of the downward gradient, the high-frequency power law of the MACs varies from about zero to one. Sensitivity of the MACs to various environmental parameters and profiles will be discussed, as well as their consequences for the average rate of decay with range of transmission loss. A recent experiment off Nantucket Island provides the opportunity for comparisons of experimentally determined MACs with those calculated from the perturbation approach

Detection and classification of buried targets and sub-bottom geoacoustic inversion with an AUV carried low frequency acoustic source and a towed array

An innovative bottom acoustic survey technique involving an Autonomous Underwater Vehicle (AUV) is presented. In this technique, the AUV carries a low frequency broadband acoustic source and a towed acoustic array and operates close to the seabed. The proposed technique has great potential to improve the capability of detecting and classifying buried or half buried targets, and providing a more reliable sub-bottom geoacoustic inversion method over a large area for geological and geophysical surveys. Specifically, since the source and the receiving array are very close to the seafloor, a variety of surface and reflected waves with good SNR is expected to be received by the hydrophone array. All of these waves have their own unique acoustic characteristics, which can be used for seafloor and sub-bottom geoacoustic inversion. Results from numerical simulations based on synthetic data will be presented to demonstrate how the geometric and physical properties of the seafloor, sub-bottom, and buried targets influence the received echoes in either the time domain, the frequency domain, or both. The simulation results will be discussed

Spatial coherence in shallow water

For applications such as beamforming and matched field processing, it is important to understand spatial characteristics of shallow water acoustic fields. Oceanic and geoacoustic variabilities introduce random fluctuations into the fields, for which an important characteristic is the spatial coherence. The spatial coherence of the acoustic field is described by the correlation function of the field. Results from a careful investigation are described of the sensitivity of the correlation function to variations in parameters of shallow water propagation. Parameters of particular interest are range and frequency. In order to demonstrate key sensitivities, stratified environmental models are used. The models include one or more sediment layers. Variability in the environment is assumed to arise from small random fluctuations in sound speed throughout the layer and at layer interfaces. Formulas based on a perturbation treatment of the Helmholtz equation are used.