Altan Turgut

Measurements of acoustic wave velocities and attenuation in marine sediments

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.

Acoustic wave propagation through porous media with arbitrary pore size distributions

In the Biot [J. Acoust. Soc. Am. 28, 168–178 (1956); 28, 179–191 (1956)] theory, the effect of frequency on the viscous forces within a porous medium is treated by replacing the kinematic viscosity ν by an oscillatory viscosity νF, in which F is a function of angular frequency ω, the kinematic viscosity ν, and the single pore size a. The mathematical expression of F for arbitrary distribution of pore sizes that can be used in the Biot theory without modification is presented. It is shown that porous media with a given permeability and porosity may be represented by an infinite number of pore size distributions. The velocities and attenuation of acoustic waves through such porous media are independent of the pore size distribution at the low‐frequency limit and at the high‐frequency limit, while they are strongly dependent on the pore size distribution in the intermediate frequency range. Comparisons between Hamilton’s [Geophysics 37, 620–646 (1972)] data of attenuation coefficient of compressional waves t...

Coherence of acoustic modes propagating through shallow water internal waves

The 1995 Shallow Water Acoustics in a Random Medium (SWARM) experiment [Apel et al., IEEE J. Ocean. Eng. 22, 445-464 (1997)] was conducted off the New Jersey coast. The experiment featured two well-populated vertical receiving arrays, which permitted the measured acoustic field to be decomposed into its normal modes. The decomposition was repeated for successive transmissions allowing the amplitude of each mode to be tracked. The modal amplitudes were observed to decorrelate with time scales on the order of 100 s [Headrick et al., J. Acoust. Soc. Am. 107(1), 201-220 (2000)]. In the present work, a theoretical model is proposed to explain the observed decorrelation. Packets of intense internal waves are modeled as coherent structures moving along the acoustic propagation path without changing shape. The packets cause mode coupling and their motion results in a changing acoustic interference pattern. The model is consistent with the rapid decorrelation observed in SWARM. The model also predicts the observed partial recorrelation of the field at longer time scales. The model is first tested in simple continuous-wave simulations using canonical representations for the internal waves. More detailed time-domain simulations are presented mimicking the situation in SWARM. Modeling results are compared to experimental data.

Acoustic field variability induced by time evolving internal wave fields

A space- and time-dependent internal wave model was developed for a shallow water area on the New Jersey continental shelf and combined with a propagation algorithm to perform numerical simulations of acoustic field variability. This data-constrained environmental model links the oceanographic field, dominated by internal waves, to the random sound speed distribution that drives acoustic field fluctuations in this region. Working with a suite of environmental measurements along a 42-km track, a parameter set was developed that characterized the influence of the internal wave field on sound speed perturbations in the water column. The acoustic propagation environment was reconstructed from this set in conjunction with bottom parameters extracted by use of acoustic inversion techniques. The resulting space- and time-varying sound speed field was synthesized from an internal wave field composed of both a spatially diffuse (linear) contribution and a spatially localized (nonlinear) component, the latter consisting of solitary waves propagating with the internal tide. Acoustic simulation results at 224 and 400 Hz were obtained from a solution to an elastic parabolic equation and are presented as examples of propagation through this evolving environment. Modal decomposition of the acoustic field received at a vertical line array was used to clarify the effects of both internal wave contributions to the complex structure of the received signals.

Inversion of bottom/subbottom statistical parameters from acoustic backscatter data

Inversion of statistical parameters of a bottom/subbottom scattering model is investigated by using genetic algorithms for both synthetic and real data. The bottom/subbottom scattering model used in the calculations is a modified version of Lyons, Anderson, and Dwan [J. Acoust. Soc. Am. 79, 1410–1422 (1986)] in which correlation between subbottom density, compressibility, and sound-speed fluctuations is established through Wood’s equation [A Textbook of Sound (Macmillan, New York, 1941)], and volume inhomogeneities are described by von Karman type autocorrelation functions [T. von Karman, J. Mar. Res. 7, 252–264 (1948)]. The inversion is posed as an optimization problem which is solved (by a controlled Monte Carlo search using genetic algorithms) to find the optimum set of statistical parameters that minimizes the quadratic deviation between measured and calculated backscatter data. A posteriori probabilities calculated at the end of the search are used for error estimation and indication of relative impo...

In situ measurements of velocity dispersion and attenuation in New Jersey Shelf sediments

The existence of acoustic velocity dispersion and frequency dependence of attenuation in marine sediments is investigated using in situ measurements from a wideband acoustic probe system during the Shallow Water 2006 experiment. Direct-path pulse propagation measurements show evidence of velocity dispersion within the 10-80 kHz frequency band at two silty-sand sites on the New Jersey Shelf. The measured attenuation in dB/m shows linear frequency dependency within the 10-80 kHz frequency band. The measured velocity dispersion and attenuation curves are in good agreement with those predicted by an extended Biot theory [Yamamoto and Turgut, J. Acoust. Soc. Am. 83, 1744-1751 (1988)] for sediments with a distribution of pore sizes.

Acoustic monitoring of the tide height and slope-water intrusion at the New Jersey Shelf in winter conditions

Waveguide invariant theory is used to describe the frequency shifts of constant acoustic intensity level curves in broadband signal spectrograms measured at the New Jersey Shelf during the winter of 2003. The broadband signals (270-330 Hz) were transmitted from a fixed source and received at three fixed receivers, located at 10, 20, and 30 km range along a cross-shelf propagation track. The constant acoustic intensity level curves of the received signals indicate regular frequency shifts that can be well predicted by the change in water depth observed through tens of tidal cycles. A second pattern of frequency shifts is observed at only 30 km range where significant variability of slope-water intrusion was measured. An excellent agreement between observed frequency shifts of the constant acoustic intensity levels and those predicted by the change in tide height and slope water elevations suggests the capability of long-term acoustic monitoring of tide and slope water intrusions in winter conditions.

Directional spectra observations of seafloor microseisms from an ocean‐bottom seismometer array

Observations of the directional spectra of seabed motion in shallow water were conducted off the New Jersey coast during the summer of 1987. Using a six‐point ocean‐bottom seismometer array, each instrument supporting a pressure transducer, and two horizontal and vertical accelerometers, measurements of gravity and seismic waves across the ULF/VLF band were collected in 12.5 m of water. Array dimensions were tuned particularly for directional spectra observations of short‐period seafloor microseisms. Directional spectra analysis indicates that in the short‐period microseismic band, 1.5–2.5 s, motion of the seafloor is primarily a result of slow seismic waves traveling at apparent velocities near 200 m/s. These propagation velocities for the microseismic band in shallow water are an order of magnitude less than microseismic velocities from similar studies on land. Contemporaneous measurements of the directional spectra of long‐period ocean gravity waves, 15–85 s, show an eastern direction of origin; short‐...

Chirp sonar sediment characterization at the northern Gulf of Mexico Littoral Acoustic Demonstration Center experimental site

Chirp sonar subbottom surveys have been conducted during a recent Littoral Acoustic Demonstration Center (LADC) experiment to invert bottom geoacoustic properties in the Northern Gulf of Mexico. Sediment properties such as density, porosity, and sound-speed profiles are inverted by using reflection amplitude and phase data obtained from a shallow-towed 2-12 kHz chirp sonar. High-quality subbottom images have been obtained with submeter resolution and up to 60 m penetration resolving several seafloor fault and diapir systems in the area. The attenuation coefficient is also estimated using the frequency shift method that seems to be relatively insensitive to reflection and transmission effects. The sound-speed and density structures of the LADC acoustic propagation tracks are efficiently mapped and made available for the numerical simulation studies of ambient noise and marine mammal acoustic propagation. The inversion results compare favorably with the previously reported sediment core data indicating that an accurate and rapid estimation of acoustical and physical properties of marine sediments is feasible.

Measurements of ambient seabed seismic levels below 1.0 Hz on the shallow eastern U.S. continental shelf

Measurements of ambient seismic noise levels in the range 0.03–1.0 Hz were made using ocean‐bottom seismometers (OBS) at four shallow‐water (<100 m) locations on the New Jersey Shelf and George’s Bank. Surface gravity‐wave‐induced seabed motion (single‐frequency microseism) was found to be dominant in the frequency range 0.03–0.3 Hz, with the high‐frequency cutoff strongly dependent on water depth. The peak seismic level in the water wave band was measured at 2.0×10−8 (m/s2)2/Hz in 12 m of water. This level was observed to decrease rapidly with greater water depth. Seismic interface waves (microseisms) of power level approximately 5×10−10 (m/s2)2/Hz were observed in the range 0.25–1.0 Hz. This microseism power level was found to be roughly constant in water depths from 12 to 70 m. A quiet ‘‘notch’’ between the two wave bands, in the range 0.15–0.3 Hz, was observed. The background seismic level in this notch was determined to be less than 5×10−12 (m/s2)2/Hz. Extrapolations of the observed pressures and sea...