Inference of Sound Attenuation in Marine Sediments from Modal Dispersion in Shallow Water

Experiments carried out by Bill Carey in the Hudson Canyon off the New Jersey coast provided a great wealth of data for the study of low frequency sound propagation in shallow water. His analysis of the transmission loss data, reported in a series of papers dating from the mid 1990s, indicated a non linear frequency dependence of sound attenuation in the sediment material. This work provided a large base of experimental evidence for the non linear dispersion predicted by the Biot theory of sound propagation in porous media, and it stimulated new studies on sound propagation in marine sediments by many researchers, including Bill Carey himself. This paper reviews the results obtained by Carey from the Hudson Canyon experiments and places them in the context of new results from more recent experiments using different techniques and observables. Analysis of results from the new work indicates that Carey's observation of non linear frequency dependence in sediment material in the Hudson Canyon applies to attenuation of sound in different types of marine sediments.

The study on the sound speed dispersion and the frequency dependence of sound attenuation in marine sediments is of great importance in understanding the physics of sound propagation in the sea bottom. Modal dispersion curves (MDC) have only been utilized to estimate the low-frequency sound speed. At higher frequencies (i.e., above a few hundred hertz), arrival time difference for different modes generally become smaller and MDC is difficult to extract because of the cross-mode interference. This paper applies time-frequency representations (TFR) with high time-frequency resolution (e.g., optimal-kernel and center-affine-filtered TFR) to broadband acoustic signals (e.g., 31g explosive and combustive source signals) detonated along the 15-km Northwest-Southeast tracks during the Seabed Characterization Experiment in the New England Mud Patch, which has a surface fine-grained sediment layer overlying a sandy bottom. The resulting high-resolution broadband (150–1500 Hz) MDC, in conjunction with TL data, are fed to a previously developed two-step dimension-reduced geoacoustic inversion algorithm including modal dispersion-based inversion for sound speed and energy-based inversion for attenuation [IEEE J. Ocean. Eng., 44, in print, (2019)]. The estimated sound speed and attenuation are compared with the theoretical results from seabed geoacoustic models for fine-grained sediments. [Work supported by ONR Ocean Acoustics (322OA).]The study on the sound speed dispersion and the frequency dependence of sound attenuation in marine sediments is of great importance in understanding the physics of sound propagation in the sea bottom. Modal dispersion curves (MDC) have only been utilized to estimate the low-frequency sound speed. At higher frequencies (i.e., above a few hundred hertz), arrival time difference for different modes generally become smaller and MDC is difficult to extract because of the cross-mode interference. This paper applies time-frequency representations (TFR) with high time-frequency resolution (e.g., optimal-kernel and center-affine-filtered TFR) to broadband acoustic signals (e.g., 31g explosive and combustive source signals) detonated along the 15-km Northwest-Southeast tracks during the Seabed Characterization Experiment in the New England Mud Patch, which has a surface fine-grained sediment layer overlying a sandy bottom. The resulting high-resolution broadband (150–1500 Hz) MDC, in conjunction with TL data, are ...

During the last decade the traditional view that acoustic attenuation in marine sediments is linear in frequency from seismic to ultrasonic frequencies has been re-examined [A. C. Kibblewhite, J. Acoust. Soc. Am. 86, 716–738 (1989)]. In particular, there is a growing body of evidence from the ocean acoustic and marine seismology communities that the characteristics of attenuation in marine sediments are compatible with the Biot–Stoll theory for porous media. The Biot–Stoll formulation predicts a range of frequency dependencies, according to which of a number of attenuation mechanisms, including viscous dissipation, is dominant. The question of how different attenuation mechanisms can be accounted for in a fluid description of sediments will be examined. The convention of complex sound speed does not account for all the effects of absorption due to viscous dissipation when quantities involving both the pressure and velocity field are considered. An example of such a problem is the Pekeris waveguide with a lossy basement. The effect of viscous dissipation can be accounted for by adopting a convention that amounts to introducing a complex quiescent density.

Acoustic attenuation in marine sediments may be measured either in situ, from the difference in signal level at two embedded probes, or from an inversion of transmission loss in the water column. Most in situ measurements are performed above 10 kHz and return an attenuation that scales as the first power of frequency, whereas transmission‐loss inversions are performed at lower frequencies, returning a near‐square‐law variation with frequency. The difference between the in situ and transmission‐loss estimates could be due to a combination of factors, including bottom roughness, which may lead to an enhanced attenuation from the transmission‐loss inversions. The nonoverlapping frequency bands of the two techniques could also account for the differences in the estimated scalings of attenuation with frequency. Thus, the attenuation could scale roughly as the square of the frequency below a few kilohertz, transitioning to a linear dependence above 10 kHz. Such behavior would be consistent with the recently dev...

This paper presents a method for estimating low frequency sound attenuation from information contained in normal modes of a broadband signal. Propagating modes are resolved using the time-warping technique applied to signals from light bulb sound sources deployed at relatively short ranges of 5 and 7 km in the Shallow Water ’06 experiment. A sequential inversion approach is designed that uses specific features of the acoustic data that are highly sensitive to specific geoacoustic model parameters. The first feature is the modal group velocity which is inverted for sediment sound speed and sediment layer thickness. The second feature is the modal amplitude function which is inverted for water depth and receiver depths. The third feature is related to the modal amplitude spectrum and is inverted for source depth and sound attenuation. In each subsequent stage, estimates from the previous stage(s) are used as known values. The sequential inversion is stable and generates geoacoustic model parameter estimates that agree very well with results from other experiments carried out in the same region. Notably, the inversion obtains an estimated value of 0.08 dB/λ in the band 120–180 Hz for the de-watered marine sediment characteristic of the continental shelf at the site.

The velocity dispersion and frequency dependency of attenuation in marine sediments are two important measures that provide the ability to test the validity of geoacoustic models as well as to estimate physical properties from the acoustic data. A widely‐used Biot–Stoll theory for elastic‐wave propagation in porous marine sediments seems to predict a narrow band of dispersion that was measured only in well‐sorted granular marine sediments [A. Turgut and T. Yamamoto, J. Acoust. Soc. Am. 87, 2376–2382 (1990)]. Recent measurements in silty‐sand sediments show almost linear frequency dependency of attenuation and mild velocity dispersion within the 3–80 kHz frequency band. The newly measured attenuation and velocity dispersion curves are in good agreement with those predicted by an extended Biot theory for sediments with a distribution of pore sizes [T. Yamamoto and A. Turgut, J. Acoust. Soc. Am. 83, 1744–1751 (1988)]. Simultaneous measurements of in situ acoustic‐probe data and chirp‐sonar reflection data are also used to estimate the model parameters of the extended Biot model. [Work supported by the Office of Naval Research.]

A new in situ method of measuring the attenuation constant of marine sediments is developed and described. This technique relies on the use of a submerged impedance tube that is pushed into the surface of the seabed. The acoustic impedance of the section of the tube filled with sediment is then measured in the frequency range 1–3.5 kHz using conventional impedance tube techniques and values of attenuation constant and sound speed determined. The system has been deployed on several sand sediments in shallow water. The attenuation constant of these sediments was found to be 0.01±0.002 Np/m, a value in the range expected for sand sediment at these frequencies. The sound speed determined by the method is of low precision but in agreement with the sound speed measured by the conventional pulse transit method. The technique offers the opportunity to make measurements of acoustic attenuation in an important frequency range between conventional high-frequency (50 kHz) techniques and remote low-frequency (below 500 Hz) methods.

The evaluation of many high-frequency bottom-interaction models requires as inputs sound speed and attenuation in marine sediment. A bottom probe system mounted on the underside of a cable-controlled underwater vehicle was used to make measurements at 15, 30, and 60 kHz, and at probe depth positions of 17, 42, and 67 cm. Sound-speed ratios are determined in-situ to 0.1% and attenuation to 0.2 dB/m. Measurements made in a 1 km by 1 km area of Puget Sound indicated small but significant spatial distributions of acoustic properties in a nominally uniform area. Attenuation coefficients were determined using the empirical relation K times frequency to the nth power. Exponent n was approximately 0.70. Past data in a Santa Barbara area indicated an n value of 1.19. These results indicate that for different sediments, exponent n is distributed about one with significant deviations. [Work supported by Naval Sea Systems Command.]

Existence of acoustic velocity dispersion and nonlinear frequency dependency of attenuation in marine sediments is investigated. A new wide-band acoustic probe system has been used during Boundary04 experiment at Malta Plateau to measure compressional wave speed and attenuation within 20-150 kHz frequency band. Observation of velocity dispersion in granular marine sediments has been previously reported by Turgut and Yamamoto (J. Acoust. Soc. Am., 87, 2376-2383, 1990), Maguer et al., (J. Acoust. Soc. Am., 108, 987-996, 2000), and Stoll (J. Acoust. Soc. Am., III, 785-793, 2002). However, most of the previous results were obtained by using different measurement techniques at low and high frequencies. Wide-band acoustic probe measurements show evidence of dispersion of compressional waves within 20-100 kHz frequency band in muddy silt. The observed dispersion is effectively modeled by an extended Biot theory (Yamamoto and Turgut, J. Acoust. Soc. Am., 83, 1988, pp. 1744-1751). In the extended Biot model viscous losses due to relative motion between the pore fluid and skeletal frame are calculated for marine sediment with nonuniform pore-size distribution. The extended Biot model that predicts comparable dispersion and attenuation to those measured in silty sediments briefly described. Finally, recent improvements on the wide-band acoustic probe system are discussed for future compressional wave velocity and attenuation measurements in granular marine sediments.

Marine sediment attenuation at low frequencies (under 5 kHz) is generally difficult to be directly measured by in situ probes embedded in the sediment, partly due to the very short propagation distances. An alternate experimental technique is to use single bottom bounce signals received by a vertical line array. The frequency dependence of the sediment attenuation is first obtained by comparing the amplitude differences of the sea floor reflection and the sub bottom layer reflection at different frequencies. The absolute attenuation is then obtained by using the previously estimated sound speed and layer thickness. Inherently there is uncertainty introduced in each stage of the attenuation estimation procedure. To evaluate the uncertainty of the attenuation estimate, the standard deviation of the signal fluctuation is mapped to the intermediate result first, and then Bayesian inversion results of the sound speed and the layer thickness are included in the final attenuation estimates. This uncertainty analysis is demonstrated by the estimation of the sediment attenuation from the low frequency chirp data collected in a variable water column environment in the Shallow Water 06 experiment. [Work supported by ONR Ocean Acoustics.]

Propagation and attenuation of acoustic waves in fluid-saturated sediments have been studied theoretically and experimentally. In situ acoustic transmission tests in saturated beach sand show that compressional waves are dispersive within a certain frequency band where the intrinsic attenuation is maximum. This indicates that low-frequency wave velocities in marine sediments are at least 5% to 10% less than the velocities obtained from high-frequency measurements, and viscous damping, due to the relative motion between solid skeleton and fluid, is the main damping mechanism in the frequency range of 1–30 kHz. The agreement between the experimental results and Biot’s theory enables the remote determination of porosity and permeability of marine sediments by using measured compressional and shear wave characteristics. Approximate relations are used to determine the porosity and permeability of the marine sediments using the measured acoustic wave velocities and attenuation.

This paper reports measurements of sound attenuation in marine sediments from two locations on the outer New Jersey continental shelf. At one site the sediment in the top 20 m is primarily sandy clay, while the other site includes a thin (3–5 m), over-lying layer of sand at the sea floor. The attenuation was inverted from close range, broadband data from light bulb implosions deployed at stations at the sites. The inversion method made use of the time-frequency dispersion information in signals received at single hydrophones. The signals were first processed by time warping to resolve the propagating modes at relatively close ranges (50–80 water depths). The inversion is carried out in two stages. The first stage inverted the sound speed and density by modeling the modal group velocities, and these estimates were used in the second stage to invert the attenuation from the modal amplitude ratios. The results provide estimates of low-frequency sound attenuation that can be compared to predictions from different models of sound propagation to assess the frequency dependence in the band from 100–500 Hz.

Simple approximate relations are proposed for the viscous attenuation per cycle of the fast compressional and shear waves in the low-to-intermediate frequency range. Corresponding closed-form formulas are derived for frequencies at which maximum viscous attenuation per cycle occurs according to the Biot-Stoll theory of elastic wave propagation in marine sediments. In the new formulas, Biot's approximation [M. A. Biot, J. Acoust. Soc. Am. 34, 1254-1264 (1962)] for the frequency-dependent viscosity correction factor F(f) and the assumption of relatively low specific loss (Q(-1)<(0.2) [J. Geertsma and D. C. Smith, Geophysics 26(2), 169-181 (1962)] are used to provide an accurate representation of the fast compressional and shear wave attenuation from low frequencies through a transition region extending to two or three times Biot's critical frequency f(c). The approximate viscodynamic behavior of marine sediments for the fast compressional and shear waves shows similarities to that of a "homogeneous relaxation" process for an anelastic linear element [A. M. Freudenthal and H. Geiringer, Encyclopedia of Physics (Springer-Verlag. 1958), Vol. 6].

Metals can be mobilized from contaminated sediments under variable environmental conditions. This paper discusses the effects of specific ions of the water column in conjunction with natural attenuation processes on the leaching of metals from marine sediments. In particular, the effect of the salinity and the presence of ions in the seawater, especially the chlorides of the water column, in leaching of metals was examined. Sediment samples were collected from sampling stations in the inner port of Piraeus, Greece. Due to the fact that natural attenuation is a slow procedure which consists of natural, chemical, and biological processes and is influenced by many factors, it was approached with experiments taking place under quite aggressive conditions. Sequential leaching tests in cycles of seven repetitions were performed. The results of these experiments showed that leaching of metals from contaminated sediments to the water column was influenced by the concentration of dissolved constituents. Initially leaching was significant with maximum concentration of leachable copper (Cu) 0.25mg/kg, lead (Pb) 0.0048mg/kg, and zinc (Zn) 0.28mg/kg, and then fell in the last repetitions. The leaching of Cu and Zn from contaminated sediments to the water column was positively correlated to the concentration of chlorides.

Data on low frequency attenuation in marine sediments is surveyed and evidence is cited for the attenuation coefficient at such frequencies being something other than directly proportional to frequency. The original model of Biot for propagation in porous media is examined and simplified versions are derived for the low frequency limit. Acoustic waves are governed by a wave equation of the standard form, but with an additional terms that is proportional to the third time derivative. The "slow wave" disturbances are governed by a simple diffusion equation, such as results from Darcy's law. In this limit, the model predicts for acoustic waves an attenuation coefficient that is proportional to the square of the frequency and inversely proportional to viscosity. It is shown that this model also implies that the attenuation for fixed frequency and porosity should be proportional to the square of the grain size. The commonly used hybrid model, commonly referred to as the Biot-Stoll model, is subsequently examined in the low frequency limit, and it is shown that this model always predicts an attenuation coefficient varying linearly with frequency in the low frequency limit, as the linear term will always dominate the quadratic term if the frequency is sufficiently low. An alternate to the Biot-Stoll model is proposed in which the nonviscous attenuation is regarded as caused by a continuous distribution of relaxation processes. The model is characterized by a function g(/spl tau/) that represents the relative contribution per unit range of relaxation time at a given value of relaxation time /spl tau/. It is demonstrated that there is a choice for this function that does lead to a prediction of a linear dependence on frequency. However, a more realistic prediction, in which g has vanishingly small values at lower relaxation times, yields a prediction in which the quadratic term dominates at low frequencies, but a linear dependence is evident for intermediate ranges of frequency.