Bottom Loss Data Research Articles

Measurements of mid-frequency bottom-interacting signals and geoacoustic inversion in Jinhae Bay, Southeast Korea.

Measurements of bottom-interacting signals over a frequency range of 4-12 kHz were made at two different sites in shallow water off the southeast coast of Korea. Although the distance between the sites was less than 20 km, the geological properties of surficial sediments were different. The surficial sediment at the site located on the north side was dominated by mud, and the measured bottom loss showed typical characteristics of sediment with a sound speed lower than the water sound speed. In addition, some interesting arrivals, which seemed to be reflected from a sub-sediment interface, were observed immediately following bottom arrivals in the frequency range of 4-8 kHz of some datasets. In contrast, sediment at the site located to the south was dominated by sandy mud, and the bottom loss showed typical characteristics of sediment with a higher sound speed. Geoacoustic parameters of surficial sediment were estimated using a simulated annealing method with an objective function comparing the measured bottom loss data with the Rayleigh reflection model. Surficial layer thickness was estimated based on arrival-time differences between bottom reflected and sub-bottom reflected signals. Inversion results are discussed in comparison with a bottom model constructed based on seismic data and core samples.

Toward a test of the existence of the Biot slow wave in sand using layer resonances

Geoacoustic models based on Biot's theory of poroelasticity have found some success in predicting the interaction of underwater sound with sandy sediments. One of the unique features of Biot theory is its prediction that poroelastic media can support two types of compressional waves—the conventional compressional wave, referred to as the “fast wave,” and an additional, slower compressional wave due to the out-of-phase motion of the saturating fluid with respect to the solid frame dubbed the “slow wave.” While Biot-based sand models have been shown to accurately fit measured compressional sound speed and attenuation data, shear wave sound speed and attenuation data, bottom loss data, and backscattering strength data, the so-called slow wave has not been directly measured in sand. Rather than attempt to directly measure the slow wave, it is the goal of this work to confirm or disprove the slow wave's existence from the resonances it causes in thin sand layers. It is expected that these resonances would be apparent in measurements of normal-incidence bottom loss. Finite element models are used to demonstrate the possible effectiveness of this approach. Experimental design and preliminary measurements are also considered. [Work supported by ONR, Ocean Acoustics.]

Sediment characterization from normal incidence bottom loss at the GLISTEN sea test

The GLider Sensors and payloads for Tactical characterization of the ENvironment (GLISTEN) sea test was conducted off the west coast of Italy in August of 2015. Led by the Centre for Maritime Research and Experimentation (CMRE), the effort included seven other international organizations including the Applied Research Laboratories from The University of Texas at Austin (ARL:UT). The mission of the sea test was to improve and understand long-range acoustic methods of environmental characterization in shallow waters. As part of the test, a team from ARL:UT deployed a remotely operated vehicle (ROV) which collected normal incidence bottom loss data from 6 to 20 kHz along with video and laser line profiling data. These data were taken along the propagation paths of the CMRE low-frequency transmission loss experiment. In this presentation, results will be shown from the bottom loss measurements for a variety of sediments that included both layering and entrained gas pockets. The acoustic data will be compared ...

서해 천해환경에서의 중주파수 해저면 반사손실 측정

ABSTRACT: KIOST-HYU joint acoustics experiment was performed on the western shallow water off the Taean peninsula in the Yellow Sea in May 2013. In this paper, mid-frequency (6~16 kHz) bottom loss data measured in a grazing angle range of 17 ~60° are presented and compared to the predictions obtained using a Rayleigh reflection model. The sediment structure of the experimental site was characterized by multi-layered sediment and the components of the surficial sediment consisted of various types of particles with a mean grain size of 5.9 ϕ. The model predictions obtained using the mean grain size were not in agreement with the measured bottom loss, and those obtained using the grain size of 4 ϕ, which was estimated by an inversion process, showed a best fit to the measurements. It would be because the standard deviation of the gain-size distribution of surficial sediment is 4.3 ϕ, which is much larger than those of other areas around the experimental site. Finally, the model predictions obtained using the geoacoustic parameters estimated from the inversion process for the surficial sediment layer and those corresponding to the mean grain size of 1.3 ϕ for lower layer are reasonably agreement with the measured bottom loss data.Keywords: Bottom loss, Rayleigh reflection coefficient, Geoacoustic parameters, Geoacoustic inversionPACS numbers: 43.30.Ma, 43.30.Pc†Corresponding author: Jee Woong Choi

A trans-dimensional polynomial-spline parameterization for gradient-based geoacoustic inversion.

This paper presents a polynomial spline-based parameterization for trans-dimensional geoacoustic inversion. The parameterization is demonstrated for both simulated and measured data and shown to be an effective method of representing sediment geoacoustic profiles dominated by gradients, as typically occur, for example, in muddy seabeds. Specifically, the spline parameterization is compared using the deviance information criterion (DIC) to the standard stack-of-homogeneous layers parameterization for the inversion of bottom-loss data measured at a muddy seabed experiment site on the Malta Plateau. The DIC is an information criterion that is well suited to trans-D Bayesian inversion and is introduced to geoacoustics in this paper. Inversion results for both parameterizations are in good agreement with measurements on a sediment core extracted at the site. However, the spline parameterization more accurately resolves the power-law like structure of the core density profile and provides smaller overall uncertainties in geoacoustic parameters. In addition, the spline parameterization is found to be more parsimonious, and hence preferred, according to the DIC. The trans-dimensional polynomial spline approach is general, and applicable to any inverse problem for gradient-based profiles. [Work supported by ONR.].

Trans-dimensional geoacoustic inversion of wind-driven ambient noise

This letter applies trans-dimensional Bayesian geoacoustic inversion to quantify the uncertainty due to model selection when inverting bottom-loss data derived from wind-driven ambient-noise measurements. A partition model is used to represent the seabed, in which the number of layers, their thicknesses, and acoustic parameters are unknowns to be determined from the data. Exploration of the parameter space is implemented using the Metropolis-Hastings algorithm with parallel tempering, whereas jumps between parameterizations are controlled by a reversible-jump Markov chain Monte Carlo algorithm. Sediment uncertainty profiles from inversion of simulated and experimental data are presented.

A true-depth passive fathometer

This paper applies a sequential trans-dimensional (trans-D) Monte Carlo algorithm for geoacoustic inversion to bottom-loss data estimated from wind-driven ambient noise at a drifting vertical array. The approach explored in this work provides range-dependent estimates of geoacoustic parameters and true-depth layering structure of the seabed, together with corresponding uncertainties. The Bayesian inversion is applied to incoherent estimates of seabed bottom loss, computed as the array drifts along a range-dependent track. The method adopts a layered representation of the seabed, where each layer is determined by sound speed, density, attenuation, and thickness. The number of layers is also included as an unknown parameter, which allows data-driven parametrization rather than an arbitrary choice of the parametrization for the seabed model. The trans-D Bayesian inversion method samples the joint posterior probability density of all model parameters to provide parameter estimates and uncertainties. A particle filter is used to update the estimated geoacoustic parameters from one array position to the next. The sequential inversion approach is demonstrated using data from the Boundary 2003 experiment, and compared to images of the seabed layering structure obtained by an active seismic system.

Geoacoustic Inversion of Mid-Frequency Bottom Loss Data in Shallow Water off the East Coast of Korea

Geoacoustic Inversion of Mid-Frequency Bottom Loss Data in Shallow Water off the East Coast of Korea

Geoacoustic Inversion of Mid-Frequency Bottom Loss Data in Shallow Water off the East Coast of Korea

Mid-frequency bottom loss measurements over a grazing angle range of 25 to 70° at frequencies of 6, 8, and 10 kHz were made off the east coast of Korea in October 2008. In this paper, geoacoustic parameters including the sediment sound speed, density, attenuation coefficient, and thickness of the surficial sediment layer are estimated via comparison between the measured bottom loss and a bottom reflection forward model based on the two-layered fluid sediment structure. A global search is performed over the search space of geoacoustic parameters by a genetic algorithm method, in which the weighted squared error between the data and the model predictions is used as an objective function to be minimized. The inversion result shows that a thin lower sound speed layer with a thickness of 0.4 m overlies the higher sound speed sediment. This result is consistent with marine geological observations conducted at the experimental site.

Bayesian geoacoustic inversion using wind-driven ambient noise

This paper applies Bayesian inversion to bottom-loss data derived from wind-driven ambient noise measurements from a vertical line array to quantify the information content constraining seabed geoacoustic parameters. The inversion utilizes a previously proposed ray-based representation of the ambient noise field as a forward model for fast computations of bottom loss data for a layered seabed. This model considers the effect of the array's finite aperture in the estimation of bottom loss and is extended to include the wind speed as the driving mechanism for the ambient noise field. The strength of this field relative to other unwanted noise mechanisms defines a signal-to-noise ratio, which is included in the inversion as a frequency-dependent parameter. The wind speed is found to have a strong impact on the resolution of seabed geoacoustic parameters as quantified by marginal probability distributions from Bayesian inversion of simulated data. The inversion method is also applied to experimental data collected at a moored vertical array during the MAPEX 2000 experiment, and the results are compared to those from previous active-source inversions and to core measurements at a nearby site.

Mid-frequency geoacoustic inversion using bottom loss data from the Shallow Water 2006 Experiment

Geoacoustic inversion work has typically been carried out at frequencies below 1 kHz, assuming flat, horizontally stratified bottom models. Despite the relevance to Navy sonar systems many of which operate at mid-frequencies (1-10 kHz), limited inversion work has been carried out in this frequency band. This paper is an effort to demonstrate the viability of geoacoustic inversion using bottom loss data between 2 and 5 kHz. The acoustic measurements were taken during the Shallow Water 2006 Experiment off the coast of New Jersey. A half-space bottom model, with three parameters density, compressional wave speed, and attenuation, was used for inversion by fitting the model to data in the least-square sense. Inverted sediment sound speed and attenuation were compared with direct measurements and with inversion results using different techniques carried out in SW06. Inverted results of the present work are consistent with other measurements, considering the known spatial variability in this area. The observations and modeling results demonstrate that forward scattering from topographical changes is important at mid-frequencies and should be taken into account in sound propagation predictions and geoacoustic inversion. To cope with fine-scale topographic variability, measurement technique such as averaging over tracks may be necessary.

Comparative analysis of mid-frequency bottom loss derived from two distinct measurement paradigms

This paper describes a comparison of acoustic mid-frequency bottom loss data derived from two distinctly different measurement methods, systems, and platforms. One data set was derived from measurements made by a passive drifting line array. The second data set was derived from measurements made from a Navy destroyer equipped with an AN/SQS-53C active sonar system that utilizes a hull-mounted volumetric cylindrical array. Data were collected off the west coast of the United States. Detailed comparisons and analysis of the derived co-located bottom loss data were made. The metric used for comparison was the difference of the estimated MGS curves at specific co-located sites. The set of MGS curves used as the metric for comparison between the two methods ranges from MGS-1, corresponding to a low-loss bottom, to MGS-9, corresponding to a high-loss bottom, in one tenth increments, i.e., MGS-1.1, MGS-1.2, to MGS-8.9, MGS-9.0. The results from the two methods were found to be in excellent agreement.

2Pb6-4 Geoacoustic inversion using mid-frequency bottom loss data in shallow water off the East Coast of Korea(Poster Session)

2Pb6-4 Geoacoustic inversion using mid-frequency bottom loss data in shallow water off the East Coast of Korea(Poster Session)

Sub-bottom scattering: A modeling approach

Bottom scatter strength data exhibit artifacts when the sub-bottom plays a role in the scattering process. The artifacts arise from the classical assumption in the data processing that the scattering process occurs at the water–sediment interface. Many of the current bottom scattering models, even those that explicitly treat sub-bottom scattering, do not address these artifacts. A new bottom scatter modeling approach is proposed. The essence of the approach is to (1) employ a geoacoustic model capable of predicting the sub-bottom insonified field, (2) avoid the usual plane-wave assumption, and (3) compute the received level in the time domain. The first condition suggests the analysis of bottom loss data concurrent with bottom scatter data and leads naturally to the idea of a self-consistent geoacoustic basis. The model is employed to analyze a data set that showed surprising discrepancies between low angle (<20°) and intermediate angle (28°–50°) bottom scattering strength. The proposed acoustic model predicts this discrepancy, which arises from the fact that the two angular regimes are controlled by entirely different scattering mechanisms. The difference in mechanisms suggests caution in the current and common practice of extrapolation from survey (intermediate angle) measurements to the low angles of interest for system performance prediction.

Interpreting and extracting sediment attenuation from measured bottom loss data in the frequency range 500–5000 Hz

The angle of intromission is the angle at which there is total transmission across an interface. This phenomena occurs between two ideal (lossless) half-spaces when the sound speed in the transmitting medium is lower and the density higher than in the incident medium. In low-energy environments (i.e., where clay and silty-clay sediment types dominate) the angle of intromission is often observed in measured bottom loss data. The loss at and around the angle of intromission is observed to be a strong function of attenuation in the host medium as well as the effects of gradients, random sedimentary layering, discrete sub-bottom reflecting horizons, roughness, and volume inhomogeneities. The physical mechanisms controlling reflection near the angle of intromission are explored in several data sets to demonstrate the potential for inverting the measured data for attenuation. [Work supported by the ONR/AEAS Program.]

Analysis of a bottom backscattering data set: Evidence of sub-bottom volume scattering

Low-frequency (under 1 kHz), low-grazing angle (under 30°) bottom backscatter data from the Messina Cone in the Ionian Sea are analyzed. Core data and bottom loss data both indicate that the sediment impedance at the water–sediment interface is too small to attribute the observed backscatter to rough surface scattering. It is hypothesized that the scattering arises from lateral discontinuities of turbidite layers in the sub-bottom volume. A self-consistent acoustic and geoacoustic model for treating such sub-bottom volume inhomogeneities is proposed and compared with the measured data. The model appears to successfully predict the grazing angle, frequency, and pulse length dependence of the measured data. The pulse length dependence of the measured data is hypothesized to arise from the lateral size distribution of the discontinuities. Thus the pulse length dependence of bottom scattering could be a useful measure of the lateral length scales of sub-bottom volume inhomogeneities. [Work supported by ONR/AEAS.]

Acoustic reflection from quasiperiodic sedimentary sequences

A useful theoretical measure of acoustic interaction with the seafloor is the plane wave reflection coefficient (R), which is commonly expressed as bottom loss (−20 log ‖R‖). Predictions indicate that when the seafloor is modeled as a refracting layer over a basement half-space, the bottom loss is proportional to frequency. Bottom loss measurements in the 50–1600-Hz band, however, frequently show a loss that is inversely proportional to frequency. For example, roughly one-third of a set of measured bottom loss data in the North Atlantic exhibit this anomalous frequency dependence. It is concluded that the anomalous frequency dependence is due to sedimentary fine-scale layering arising from turbidity currents. The evidence presented consists of (1) a high correlation between the anomalous bottom loss stations and the bounds of the abyssal plains and (2) favorable predictions between a model accounting for the layering and the measured data. The study area was the western North Atlantic including the Sohm, Hatteras, and Nares abyssal plains. A simple, approximate stochastic model of reflection from a quasiperiodic sedimentary sequence was developed that appears to account for the dominant physical mechanisms important in the reflection process in this type of physiographic province.

Acoustic model-based matched filter processing for fading time-dispersive ocean channels: theory and experiment

It is shown that the performance of a conventional matched filter can be improved if the reference (replica) channel compensates for the distortion by the ocean medium. A model-based matched filter is generated by correlating the received signal with a reference channel that consists of the transmitted signal convolved with the impulse response of the medium. The channel impulse responses are predicted with a broadband propagation model using in situ sound speed measured data and archival bottom loss data. The relative performance of conventional and model-based matched filter processing is compared for large time-bandwidth-product linear-frequency-modulated signals propagating in a dispersive waveguide. From ducted propagation measurements conducted in an area west of Sardinia, the model-based matched filter localizes the depths of both the source and receiving array and the range between them. The peak signal-to-noise ratio for the model-based matched filter is always larger than that of the conventional filter. >

The effect of an elastic basement in the interpretation of bottom loss data

A bottom loss simulator has been developed to model low‐frequency bottom loss measurements obtained at sites in the SW Pacific around New Zealand. The data for frequencies below 100 Hz have been grouped into two types according to the bottom environment. The first type is characteristic of continental margins where the bottom is composed of turbidite layers, and the second type is characteristic of deep‐water basins where the bottom is silt–clay material overlying the ocean basement. The bottom loss simulator consists of a simple geoacoustic profile, composed of a thin‐layer fluid sediment overlying an elastic basement, and a ray propagation model. Simulations were carried out with the model to study the effect of various types of solid substrates, including relatively low‐speed consolidated sediment material and high‐speed basalt. The model provides good agreement with both types of bottom loss data, using archival values from seismic and geological sources for the model parameters. The work demonstrates...

Acoustic reflection from ocean abyssal plains

Existing bottom loss and pulse response data, measured at a variety of grazing angles, frequencies, and locations (Sohm and Hatteras abyssal plains, Blake Plateau, Florida shelf, and Gulf of Mexico), will be reviewed. A comparison of data with existing theoretical models will be made.