Bottom Backscattering Research Articles

Bottom Multi-Parameter Bayesian Inversion Based on an Acoustic Backscattering Model

The geoacoustic and physical properties of the bottom, as well as spatial distribution, are crucial factors in analyzing the underwater acoustic field structure and establishing a geoacoustic model. Acoustic inversion has been widely used as an economical and effective method to obtain multi-parameters of the bottom. Compared with traditional inversion methods based on acoustic propagation models, acoustic backscattering models are more suitable for multi-parameter inversion, because they contain more bottom information. In this study, a Bayesian inversion method based on an acoustic backscattering model is proposed to obtain bottom multi-parameters, including geoacoustic parameters (the sound speed and loss parameter), partial physical parameters of the sediment, and statistical parameters of the seafloor roughness and sediment heterogeneity. The bottom was viewed as a kind of fluid medium. A high-frequency backscattering model based on fluid theory was adopted as the forward model to fit the scattering strength between the model prediction and the measured data. The Bayesian inversion method was used to obtain the posterior probability density (PPD) of the inversion parameters. Parameter estimation, uncertainty, and correlation were acquired by calculating the maximum a posterior (MAP), the mean values, the one-dimensional marginal distributions of the PPD, and the covariance matrix. Finally, the high-frequency bottom backscattering strength from the Quinault Range site was employed for inversion tests. The estimated values and uncertainties of various bottom parameters are presented and compared with the directly measured bottom parameters. The comparison results demonstrate that the method proposed herein can be used to estimate the sediment/water sound speed ratio, the sediment/water density ratio, and the spectral exponent of the roughness spectrum effectively and reliably.

Bottom and Suspended Sediment Backscatter Measurements in a Flume—Towards Quantitative Bed and Water Column Properties

For health and impact studies of water systems, monitoring underwater environments is essential, for which multi-frequency single- and multibeam echosounders are commonly used state-of-the-art technologies. However, the current scarcity of sediment reference datasets of both bottom backscatter angular response and water column scattering hampers empirical data interpretation. Comprehensive reference data derived from measurements in a controlled environment should optimize the use of empirical backscatter data. To prepare for such innovative experiments, we conducted a feasibility experiment in the Delta Flume (Deltares, The Netherlands). Several configurations of sonar data were recorded of the flume floor and suspended sediment plumes. The results revealed that flume reverberation was sufficiently low and that the differential settling of fine-sand plumes in the water column was clearly detected. Following this successful feasibility test, future comprehensive experiments will feature multi-frequency multi-angle measurements on a variety of sediment types, additional scatterers and sediment plumes, resulting in reference datasets for an improved interpretation of underwater backscatter measurements for scientific observation and sustainable management.

Application of a parametric array over a mid-frequency band (4–10 kHz) –measurements of bottom backscattering strength

Study on characteristics of bottom backscattering at midfrequency (usually referring to 1–10 kHz) is essential for underwater acoustic communication, subbottom structure detection, buried target detection, and antisubmarine sonar. Measurements of bottom backscattering strength in the grazing angle range of 70°–90° over a wide mid-frequency band of 4–10 kHz were taken using a parametric array transmitter system and an omnidirectional hydrophone. Details of the parametric array transmitter system are introduced. The experimental site locates in the South Yellow Sea of China, where the water depth is 37 m. Core data showed that the sediment was fine sand mixed with a small amount of shell fragments. The backscattering strength was extracted from monostatic backscattering data according to the sonar equation. A calibration experiment indicated that the distance from the parametric array to the seafloor should be larger than a conversion distance, where the difference-frequency source level and beamwidth of the parametric array tended to be stable, to efficiently generate low-frequency difference-frequency waves used to measure the bottom backscattering strength. The backscattering strength increases with the increase of the grazing angle, reaching a peak at the grazing angle of 90°. In the point of frequency, the frequency dependence of the backscattering strength is weak. The characteristics of bottom backscattering agree with a sandy site with similar sediment properties near the experimental site.

Toward monitoring and estimating the size of the HFO-contaminated seabed around a shipwreck using MBES backscatter data

Despite a progressive reduction of oil spills caused by the activity of maritime transportation, the latent sources of pollution still exist. Although the harmful impact of heavy fuel oil (HFO) on the marine environment is widely known, many shipwrecks cause contamination of the surrounding areas. In this paper, an approach to monitor the area of the HFO spill around a shipwreck is made using a bottom backscattering strength (BBS) obtained by a multibeam echosounder (MBES). As a case study, the s/s Stuttgart wreck located in the Gulf of Gdansk (Poland) is verified. Two different measurement campaigns have been carried out in shallow waters using low (190 kHz) and high (420 kHz) MBES frequency. The results indicate that the polluted area around s/s Stuttgart was estimated at 49.1 ha, which is around 18.3% more in comparison to the geological surveys made four years earlier.

Characterization of seafloor roughness from high-frequency acoustic backscattering measurements in shallow water off the west coast of India.

Measurements of the reverberation time series are made at frequencies of 8, 10, and 12 kHz, and the corresponding acoustic bottom backscattering strengths are estimated as functions of grazing angle. The experiment was conducted in the western continental shelf of India (off Kerala) in water depth of ∼61 m where hard sandy sediments of biogenic origin are predominant. The average values of two-dimensional (2D) spectral strength (w2) and exponent (γ2) of seafloor roughness are obtained by inverting bottom backscattering strength data with the help of a scattering model, utilizing the genetic algorithm method. Measurements of one-dimensional interface roughness height are also carried out using a single beam echosounder to analyze the variability of bottom roughness in terms of spatial frequency. The spectral parameters estimated from roughness height measurements are compared to that obtained from inversion results. The 2D spectral strength and exponent of seafloor roughness estimated from the two methods agree with each other and are consistent with the typical values associated with sandy sediments.

Intensity evaluation of bottom backscattering between high-frequency underwater acoustic instruments and its experimental validation.

Running several high-frequency underwater acoustic instruments simultaneously on board a surface or underwater platform can cause interference between the instruments. By combining ray theory and the high-frequency bistatic scattering model for water bottoms, this work presents a practical method for evaluating the relative intensities of such interference signals corresponding to different instrument-to-instrument distances. To examine this method's effectiveness, a series of lake experiments were conducted. The relative intensity of the interference signal was first evaluated by using the proposed method, and then it was measured for comparison. The experimental results showed good agreement between the evaluated and the measured intensities.

Direct-Path Backscatter Measurements Along the Main Reverberation Track of TREX13

The primary goal of the Target and Reverberation Experiment in spring 2013 (TREX13) was to identify the major physical mechanisms responsible for midfrequency reverberation. While both the sea surface and seafloor can contribute to reverberation, the seafloor is typically dominant in shallow water environments. To determine the level of this contribution at the TREX13 site, the bottom backscatter sonar (BBS) was deployed from a dive boat at multiple locations around the site. The BBS consists of a source and a receiver mounted on a short bracket that is suspended above the seafloor to measure direct-path bottom backscatter at 3 kHz. Data near normal incidence were interpreted as bottom reflectivity, which was used to quantitatively explain the range-dependence of the sediment composition at the experiment site. Two factors restricted the estimates of the bottom backscatter strength to the minimum grazing angle of 21 $^\circ$ : the currents at the experiment site made it difficult to position the system close to the seafloor, and the shallow water depth resulted in sea surface scatter contaminating small angle bottom backscatter. From the measured backscatter strength and by utilizing available environmental data, initial models of scattering strength indicate that at the shallow grazing angles of importance to reverberation, the scattering on the sand ridges is dominated by roughness scattering while in the muddy areas of the ridge swales, volume scattering dominates. The volume scattering from these mud areas is significantly stronger than the roughness scattering on the ridges by as much as 10 dB and may explain the substantial fluctuations observed in the reverberation as a function of range.

Acoustic backscattering measurement from sandy seafloor at 6–24 kHz in the South Yellow Sea

The acoustic bottom backscattering strength was measured at the frequency range of 6–24 kHz on a typical sandy bottom in the South Yellow Sea by using omnidirectional sources and omnidirectional receiving hydrophones. In the experiment, by avoiding disturbances due to scattering off the sea surface and satisfying the far-field condition, we obtained values of acoustic bottom backscattering strength ranging from −41.1 to −24.4 dB within a grazing angle range of 18°–80°. In the effective range of grazing angles, the acoustic scattering strength generally increases with an increase in the grazing angles, but trends of the variation were distinct in different ranges of frequency, which reflect different scattering mechanisms. The frequency dependence of bottom backscattering strength is generally characterized by a positive correlation in the entire frequency range of 6–24 kHz at the grazing angles of 20°, 40° and 60° with the linear regression slopes of 0.222 9 dB/kHz, 0.513 0 dB/kHz and 0.174 6 dB/kHz, respectively. At the largest grazing angle of 80°, the acoustic backscattering strength exhibits no evident frequency dependence.

A Method for Calculating Bottom Backscattering Strength Using Omnidirectional Projector and Omnidirectional Hydrophone

A method of obtaining bottom backscattering strength by employing an omnidirectional projector and omnidirectional hydrophone is proposed. The backscattering strength is extracted from monostatic backscattering data. The method was adopted in an experiment conducted in the South Yellow Sea of China. The seafloor surface was relatively smooth and covered by a small quantity of shell fragments, as observed through a digital camera system. Sampling data showed that the main component of the sediment at this experimental site was fine sand. In this paper, we detail the calculation method. Preliminary results of backscattering strength as a function of grazing angle (20°–70°) in the frequency range of 6–24 kHz are presented. The measured backscattering strength increased with the grazing angle and changed more rapidly at large grazing angles (60°–70°). A comparison of the data at different frequencies reveals that the measured backscattering strength substantially rises with the increase of acoustic frequency. A fitting curve of Lambert’s law against the measured data shows that the backscattering strength deviates from Lambert’s law at large grazing angles.

Simplification of roughness bottom backscattering model at small grazing angle in shallow-water

Bottom backscattering due to roughness seafloor is the main source of shallow water reverberation, especially in the waveguide with downward reflection profile or a calm sea-surface. Empirical backscattering models with a simple form has an important limitation to analyzing other characteristics of reverberation except for the intensity characteristics, which originates from optics and describes the relationship between the bottom backscattering strength and scattering grazing angle of plane-wave in half-infinite space. In the shallow water, such a plane-wave backscattering model cannot be used due to frequency dispersion. The model of bottom backscattering based on physical scattering principle is made to relieve such a limitation, but thereby bringing about another restraint by a geoacoustics model. The bottom backscattering model, which is formulated during modeling the full-wave reverberation theory at small grazing angle in range-independent shallow water waveguide, is simplified by combining with bottom reflection coefficient model which is independent of the geoacoustics model. The bottom reflection coefficient model as referred to the proposed phase parameter <i>P</i> in this paper is equivalent to velocity and density of sediment to describe sound field interacted with sea-bottom. Therefore simplification of bottom backscattering model can be handled by the phase parameter without any knowledge of bottom geoacoustic parameters. The angular dependency and intensity dependency of bottom backscattering due to roughness seafloor at small grazing angle are studied more in depth through such a simplified model. Marking 2/<i>P</i> as the cut-off point, the grazing angle is divided into two stages. Near the critical angle, as grazing angle is greater than 2/<i>P</i> and less than critical grazing angle, the angular dependency of bottom backscattering due to roughness seafloor is weighted by phase parameter of bottom reflection coefficient, while the intensity dependency is independent of phase parameter. At each small grazing angle, as grazing angle is less than 2/<i>P</i>, the angular dependency of bottom backscattering due to roughness seafloor is proportional to incident and scattering grazing angle squared and irrespective of phase parameter of bottom reflection coefficient which is like the empirical bottom backscattering model, while the intensity dependency is proportional to the fourth power of phase parameter. So the bottom has different influences on the angular dependency and intensity dependency of bottom backscattering in different stages of grazing angle.

Measurements of Midfrequency Acoustic Backscattering From a Sandy Bottom in the South Yellow Sea of China

Acoustic bottom scattering is the main source of reverberation background in target detection and recognition near the seafloor. Accurate estimation of bottom scattering strength is necessary for prediction of sonar performance in the presence of reverberation in shallow-water environments. A measurement of bottom backscattering strength at midfrequency in a shallow-water environment of the South Yellow Sea of China was carried out in April 2016, where the water depth was 39 m. The surface of the seafloor was relatively flat and covered by some shell fragments observed through a digital camera system. Core data showed that the main component of sediment at this experiment site was fine sand. The backscattering strength as a function of grazing angle (20°–70°) in the frequency range of 6–12 kHz was extracted from monostatic backscattering data, employing an omni-directional source and an omni-directional hydrophone. The measured backscattering strength increases with the increase of the grazing angle, and changes more rapidly at large grazing angles (60°–70°). Comparing the data at different frequencies, it is found that the measured backscattering strength slightly increases with the increase of the acoustic frequency. A fit of Lambert's law to the measured data shows that the backscattering strength deviates from the Lambert's law at large grazing angles. Finally, the fitted result of a scattering model to the measured data suggests that seafloor roughness scattering is larger than that from sediment volume heterogeneity in the measurement frequency band.

Measurements of mid-frequency bottom backscattering during KOREX-17

Measurements of mid-frequency bottom backscattering strength were made on May 30, 2017, as part of the Korea Reverberation Experiment (KOREX-17) in a shallow water region located at the south coast of Korea. The acoustic data were transmitted and received as functions of frequency (4, 6, and 8 kHz) and pulse length (2 and 4 ms) using semi-monostatic sonar system composed of omni-directional acoustic source and receiver, which was deployed from the R/V Mirae. The bottom backscattering strength were extracted from intensity-averaged reverberation level for 30 individual pings. The water depth measured by an echo sounder was about 35 m and bottom was approximately flat, having a slope less than 1° over the experimental area. The surficial sediment was estimated to be silty sediment, having a mean grain size of 6 phi from grain size analysis. In this talk, the measurement results of bottom backscattering strengths will be presented as a function of grazing angle and compared to the predictions obtained by Lambert’s law and APL-UW scattering model. [Work supported by Agency for Defense Development, Korea(UD160006DD).]

Measurement of bottom backscattering strength using single-beam echosounder

A quantified single beam echosounder was applied to detect and quantify the bottom echoes. Bottom backscattering strength (SB) is necessary to be quantified for accurate seabed mapping and classification. The SB can be measured by Lambert’s Law that enabled to interpret the bottom echo and to measure the SB values. The research was conducted in Tidung Island, Jakarta which had several seabed types, such as coral reef, seagrass, sand, silt, clay, mud, and mixed sediment. Acoustic data was acquired using SIMRAD EK15 with the frequency of 200 kHz and sediment grain size was analyzed using Wentworth grain size classification. The results show that the backscattering of each seabed type is different. Sediment with greater mean diameter had greater SB value, and seabed with rougher surface had greater SB value as well.

Measurement of mid-frequency acoustic backscattering from the sandy bottom at 6–24 kHz in the Yellow Sea

In a typical sandy bottom area of the South Yellow Sea, measurement of acoustic bottom backscattering strength within a frequency range of 6–24 kHz was conducted using omnidirectional sources and omnidirectional receiving hydrophones. In this experiment, with interference from scattering off the sea surface being avoided and the far-field condition being satisfied, we obtained acoustic bottom backscattering strength values ranging from −31 to −17 dB within a grazing angle range of 18°–80°. In the effective grazing angle range, the acoustic scattering strength generally increases with the increase in the grazing angle, but the variation trends were different in different frequency bands, which reflects different scattering mechanisms. The frequency dependence of the acoustic backscattering strength is characterized by a segmented correlation. In the frequency band of 6–11 kHz, the scattering strength is positively correlated with the frequency, and the average slope of the linear correlation is about 0.83 dB/octave; in the frequency band of 12–24 kHz, the scattering strength generally exhibits a negative correlation with the frequency, and the slope of the linear correlation is about −0.42 dB/octave.

A backscattering model for a stratified seafloor

In order to predict the bottom backscattering strength more accurately, the stratified structure of the seafloor is considered. The seafloor is viewed as an elastic half-space basement covered by a fluid sediment layer with finite thickness. On the basis of calculating acoustic field in the water, the sediment layer, and the basement, four kinds of scattering mechanisms are taken into account, including roughness scattering from the water-sediment interface, volume scattering from the sediment layer, roughness scattering from the sediment-basement interface, and volume scattering from the basement. Then a backscattering model for a stratified seafloor applying to low frequency (0.1–10 kHz) is established. The simulation results show that the roughness scattering from the sediment-basement interface and the volume scattering from the basement are more prominent at relative low frequency (below 1.0 kHz). While with the increase of the frequency, the contribution of them to total bottom scattering gradually becomes weak. And the results ultimately approach to the predictions of the high-frequency (10–100 kHz) bottom scattering model. When the sound speed and attenuation of the shear wave in the basement gradually decrease, the prediction of the model tends to that of the full fluid model, which validates the backscattering model for the stratified seafloor in another aspect.

An effective surface loss model for shallow water propagation and reverberation at mid-frequency that accounts for surface forward scattering

A transport theory approach has been developed for modeling shallow water propagation and reverberation at mid-frequencies with emphasis at 1-3 kHz. With this approach, sea surface forward scattering can be taken into account in a 2-D (range-depth) approximation. The effects of surface forward scattering on reverberation level for typical conditions can be significant (>10 dB), even though bottom backscatter typically dominates reverberation in shallow water. However, effects of surface forward scattering have not been treatable with traditional ray-based codes, since it is implicit in these codes that the surface interaction is a specular reflection combined with some loss, and accounting for the change in grazing angle due to forward scattering, crucial for reverberation modeling, does not naturally come into it. A method to account for forward scattering with ray-based codes will be described. Transport theory results are used to develop an effective surface loss model. It is referred to as TOTLOS, for the surface loss for the total field, reflected plus scattered. While it is referred to as an effective surface loss model, the model can yield either loss or gain, depending on the grazing angle and other parameters of the environment. [Work supported by PMW-120 and ONR Code 322.]

Analysis of Grazing Angle and Frequency Dependence on Acoustic Backscattering Pattern

This study aims to investigate the underwater acoustic backscattering pattern with respect to grazing angles and frequency. The narrowband impulses signal was generated from 8500 to 9500 Hz with 250 Hz increment, and the grazing angles were 10o, 20o, 30o, 40o, and 50o, respectively. The measurement was carried out in a 9m-width, 5m-wide and 0.5 m-water deep pond with muddy bottom with spatial-aliasing free hydrophone array placed at depth 15 cm and 35 cm. The experiment used ray tracing method for sensor placement. The measurement results showed that bottom backscattering pattern was observed at grazing angle 10o to 30o, and whilst from 40o to 50o the surface backscattering pattern was appeared on the loudness contour. The results also showed the intensity was higher at grazing angle 10o-30o at depth 35 cm, whilst from 40o to 50o was higher at depth 15 cm. It was confirmed from the measurements that the intensity has an exponential decay at grazing angle 10o, 30o, and 50o. Intensity as function of grazing angle was found to fit Lambert's rule at grazing angle 10o-40o at depth 15 cm and grazing angle 10o-30o at depth 35 cm. The results suggest that the acoustic backscattering pattern depends at grazing angle only. The intensity increased as the acoustic waves were backscattered because of reflection as well as interference with walls of the pond, rough bottom, and water surface.

A study on calibration for commercial split beam echosounder using the bottom backscattering strength from a fishing vessel near the South Shetland Islands, Antarctica

A study on calibration for commercial split beam echosounder using the bottom backscattering strength from a fishing vessel near the South Shetland Islands, Antarctica

Single-mode field excitation experiment in shallow water

Single-mode field has advantages in active detection, underwater communication, bottom backscattering measurement, etc. Some of papers have reported on the theory and simulation about the single-mode excitation by open- and closed-loop methods, respectively. The experiment in the tank has also been reported many years ago, but the experiment in the water has not been reported yet. In this paper, an effective closed-loop method for exciting the single-mode field is discussed, which can obtain the optimum estimation of the matrix of the Green’s function in the shortest feedback times, with high efficiency of emission of the single-mode signal. The experiments at the different sites in different seasons are reported. The vertical source array with the center frequency at 700 Hz was used as the source in the experiment. The experimental results show that when the source array spans almost the effective water depth the single-mode field with high quality can be excited. The first three single modes were excite...

Distributional factors of juvenile Walleye Pollock in the Musashi Bank area, northern Japan Sea

Most of juvenile walleye pollock (Gadus chalcogrammus) in the northern Japan Sea migrate to the bottom layer with 150-200 m depth of Musashi bank area in summer. Clarification of distribution factors of nursery area after the migration is important for resource management. We examined the factors of juvenile pollock after migration to bottom in summer 2014 and 2015. The distribution of density of juvenile pollock was obtained by quantitative echo sounder. The distribution of bottom sediment was discriminated by the side scan sonar. The bottom backscattering strength and sediment data were jointed to analyze bottom sediment of quantitative echo sounder survey transect line based on the Roxann method. In the result, bottom sediment was separated into clay, sand, and boulders. Distribution density of juvenile pollock was high in boulders area. Additionally, the density of juvenile pollock was higher at northward upslope than gentle landform and else way upslope in all boulders area. It is considered that northward ocean flow collided with the northward upslope of boulders make upward flowing and stagnate water, and the water forms the retention space of prey plankton that is favorable for juvenile pollock.