Surface Backscattering Strength Research Articles

Measurement and Analysis of Acoustic Surface Scattering for Oil Concentration

The life of maritime flora and wildlife are substantially impacted by oil's effects, and this has an immediate impact on livelihoods. The impact disrupts the lives of contaminated coastal communities. Because of the potential losses caused by oil pollution, the government enacts policies governing environmental conditions in order to maintain or improve the quality of the polluted sea. The purpose of this study was to calculate the acoustic backscattering value of oil using a single beam echosounder and compare the measurement results from a single beam echosounder with the results of gravimetric analysis. The data acquisition was carried out in March 2021 at the Marine Acoustic Laboratory, Department of Marine Science and Technology, Faculty of Fisheries and Marine Sciences, IPB University, using the SIMRAD EK-15 frequency of 200 kHz and oil used as a source of pollution, namely the dexlite type (density of 880 kg/m3). The results showed that the value of Surface Backscattering Strength (SS) decreased when contaminated with oil, namely -63.76 dB and -65.90 dB, but when oil was added back, the SS value increased to -50.81 dB. The relationship between oil concentration and surface backscattering strength shows a negative correlation between the variables, and the level of the relationship between variables is 0.85.

Measurement and Analysis of Acoustic Backscatter Value for Bottom Classification of waters Tidung Island

Sediment in a water has an important role for organisms, namely as a habitat, a place for foraging for food, and a place for spawning. These sediment can affect the composition of organisms in the water. The purpose of this study is to calculate the value of acoustic backscatter for the classification of the bottom of the water and to see the effect of sediment grain size on the backscatter value obtained from a single beam acoustic instrument. Data collection was carried out from 10 to 12 June 2021 in the water of Tidung Island, Seribu Islands, using the SIMRAD EK-15 single beam, single frequency 200 kHz instrument. Sediment sampling was carried out at 13 stations. The results showed that the waters of Tidung Island were dominated by muddy substrate which was classified based on the Surface Backscattering Strength (SS) value. Meanwhile, the grain size of the sediment affects the SVb value, where the large the grain size of the bottom sediment, the SVb value will be higher. The higher SVb value the SS value will be higher.
 Keywords: Bottom Classification, Acoustic Backscatter, Tidung Island

Detection of bottom substrate type using single-beam echo sounder backscatter: a case study in the east coastal of Banyuasin

This research was aimed to identify substrates type in the east part of the Banyuasin coastal waters using quantitative backscatter data from single-beam echo-sounding. The SIMRAD EK-15 was used to classify the seafloor substrate types. The ground truth was required for calibrating the acoustic result. Wet sieving methods and Shepard’s triangular diagram were used to analyze the ground truth samples. The acoustic data were filtered to extract the volume backscattering strength of bottom surface (SV) using Echoview 4.0. The data of bottom surface backscattering strength (SS) and SV were classified by using Hierarchical Cluster Method. Data of substrate type from the ground truth will be used as a guideline to classify the SS data to identify the substrate type based on the SS characteristic which associated with various types of the bottom substrate. The results showed the single beam capability in distinguishing the types of bottom substrate, namely clayed sand with the SS value ranges from -47.29 to -46.32 dB), silt + sand + clay with the SS value ranges from -51.00 to -48.54 dB), clayed silt with the SS value ranges from -53.47 to -52.24 dB), and silty clay with the SS value ranges from -56.89 to -55.94 dB.

Measurement and analysis of acoustic backscattering strength for characteristics of seafloor sediment in Indian Ocean WPP 572-573

Understanding and predicting the acoustic backscattering from seafloor substrates was essential to support the management and development of coastal area as well as offshore. The aims of this study were to quantify the acoustic backscattering strength from the seafloor substrates, classify the types of seafloor substrates, as well as correlation between the seafloor substrates and the value of surface backscattering strength (SS). The data collection was done in April to June 2016 in the Indian Ocean WPP 572-573 using hydroacoustic instrument SIMRAD EK60 with operating frequency of 38 kHz. Substrate sampling was used for data validation using Van Veen Grab. The results showed that the SS value for the type of sand substrate (-8.05 dB to -5.25 dB), silty sand (-8.15 dB to -7.03 dB), sandy silt (-11.30 dB to -11.35 dB), silt clayey (-14.71 dB to -12.17 dB) and clay substrate -23.55 dB. Between the SS value and the sand percentage had a very strong positive correlation, and a very strong negative correlation between the surface backscattering strength and the clay percentage. In contrast, a poor negative correlation was also found between the SS value and the silt percentage.

Seabed classification using surface backscattering strength versus acoustic frequency and incidence angle measured with vertical, split-beam echosounders

Abstract The multifrequency biplanar interferometric imaging technique (MBI) is applied to data from vertical, split-beam echosounders to produce sub-beam estimates of seabed surface-backscattering strength (Ss), incidence angle (θ), and roughness (${\rm {\cal R}}$). A simple model is used to quantify the variation of Ss versus θ = {2–20°} and acoustic frequency, f = {18, 38, 70, 120 and 200 kHz}. The coefficients of the angle- and frequency-dependent terms of the model indicate seabed material properties, principally small- and large-scale roughness and hardness. These indices are combined with the estimates of θ and ${\rm {\cal R}}$ to classify the seabed using unsupervised cluster analysis. This technique is applied to data from the Forty-Three-Fathom Bank, a seamount in the Southern California Bight. The resulting seabed classifications are consistent with the surficial lithology and the spatial distribution of known rockfish (Sebastes spp.) habitat. The method should be generally applicable to seabed classification.

Seabed acoustic classification in the Pelotas basin, Brazil

The present study sought to develop a seabed map of the region of the Pelotas Basin using acoustic methods. A total number of 1,507,823 seabed reflectivity data, collected during six oceanographic surveys, were processed to generate a seabed map. Data processing consisted of the classification of the acoustic parameter BSBS (Bottom Surface Backscattering Strength) obtained with the Scientific Echosounder EK 500 operating at a frequency of 38 kHz. BSBS is expressed in decibels (dB), and corresponds to a logarithm of the ratio between incident acoustic energy and the energy reflected by the seabed. Four BSBS value classes, associated with different sediment types, were established. High BSBS values are associated with coarse sediments, whereas low values indicate fine sediments. A seabed reflectivity map was generated using the Natural Neighbor method to interpolate the BSBS values organized according to the pre-established classes. Four features with high BSBS values were identified at 100-200 m depth. The largest one was found in the region of Santa Marta Cape and attributed mainly to consolidated seabed and/or the presence of biodetritic material, according to comparison with maps available in the literature. Above 500m depth, there was a predominance of acoustically low reflectivity sea floor, which was attributed to the presence of muddy sediment. Considering the lack of information on the seabed at great depths, the acoustic method was shown to be an alternative tool to obtain data on seabed characteristics in these regions.

Measurement of sea bottom surface backscattering strength by quantitative echo sounder

Measurement theory and method of the bottom surface backscattering strength (SS) using a quantitative echo sounder (QES) are discussed and applied in the ocean near Java, Indonesia. The frequencies of the QES were 38, 70, and 120 kHz. The measurements of bottom echoes and sampling of bottom material by a dredge were done simultaneously. Bottom characterization was based on analysis of particle size distribution for bottom samples taken during the survey. The SS value increases with the increase of the mean diameter of the bottom particles. The SS decreased with increasing frequency. The effectiveness of QES for measurement of the SS along with observation of the depth topography were demonstrated.

Quantifying Sea Bottom Surface Backscattering Strength and Identifying Bottom Fish Habitat by Quantitative Echo Sounder

A quantitative echo sounder (QES) was used to measure the bottom surface backscattering strength (SS) extensively off Southern Jawa Island, Indonesia. The average SS by the bottom echo integration method were plotted as a map with the depth contour to give a synoptic view of the topography. On the other hand, the results from the ring surface scattering (RSS) model gave us more detailed SS values in ping base. A simultaneous display of the fish volume backscattering strength and the bottom SS was useful to observe bottom fish habitats remotely. The bottom material estimated by the measured SS showed that the fish schools were abundant in the sand bottom in this area.

Measurements and modeling of low‐frequency air‐sea‐interface scattering

The acoustic scattering response of the undersea air‐sea interface can be quite sensitive to ocean surface conditions, scattering geometry, and acoustic frequency. This paper presents both measurements and modeling of low‐frequency (<1500 Hz) scattering strength. The modeling is based on a small slope approximation (SSA) that accounts for directional surface‐wave roughness spectra with low‐wave‐number cutoffs. To provide an operationally useful predictive tool, a set of surface‐wave spectral parameters (strength, exponent, and peak) are determined by nonlinear, multi‐parameter fits to broadband, vertically bistatic surface backscattering strength data, leading to a semi‐empirical bistatic model environmentally parameterized solely by the wind speed. The same SSA theory is also used to estimate the coherent‐reflection loss given the frequency, angle, and wind speed. These broadband models are then used to explore the dependence of both incoherent and coherent sea‐surface scattering on frequency, angle, and wind speed. [Work supported by ONR and PEO C4I & Space (PMW‐180).]

A new, semiempirical model for predicting bistatic surface backscattering strengths

A Lloyd-mirror model of bubble-cloud scattering, along with perturbation theory for the rough air–sea interface, have been used as a basis for generating a new algorithm for predicting the strength of acoustic backscatter from the ocean surface. Multiparameter curve fits of the model to acoustic data from the Critical Sea Test program (1988–1996) and other at-sea experiments have been used to derive a new, semiempirical description of the dependence of surface backscattering strength on the incident and scattered grazing angles, the acoustic frequency, and environmental descriptors. The model represents an advance on previous algorithms (Chapman–Harris, Ogden–Nicholas–Erskine) in several ways: (1) a much improved, physics-based description of the grazing-angle dependence, applicable to arbitrarily low grazing angles; (2) a much improved description of low-to-moderate sea-state bubble scattering; (3) can handle vertically bistatic geometries; and (4) is applicable to a broader range of frequencies (50 to 5000 Hz). [Work supported by ONR.]

The influence of bubble clouds on surface scattering strengths

One of the most comprehensive sets of surface scattering strength measurements was made during the CST experiments between August 1988 and May 1993, covering a frequency range from ∼70 to ∼1500 Hz. An extensive set of environmental measurements (including bubble cloud measurements) collected on several of these experiments make this a unique data set for examining the effect of sub-surface bubble clouds on surface backscattering strength. Analysis of these data have shown that wind speed, while the most important environmental descriptor, cannot by itself explain some important differences in the surface scattering strengths measured for the different CST experiments. These differences appear to be linked to the sub-surface bubble clouds which, in turn, are influenced by the dynamics of the upper layers of the ocean. Latest results from the continuing analysis of the surface scattering and bubble cloud data will be presented which illustrate the influence of the bubble clouds and indicate how the stability of the upper layers of the ocean play a role in distributing those bubble clouds. [Work supported by ONR.]

On the correlation of ambient noise levels and other parameters with backscattering strengths observed during the acoustic surface reverberation experiment (ASREX)

Preliminary results from a surface reverberation experiment performed in the North Atlantic during the winter of 1993–94 indicate that ambient noise levels may prove to be a convenient and accurate indicator of surface backscattering strengths. Backscattered energy from coherent sources ranging in frequency from 100 to 800 Hz were observed over a three month period. Both the backscattered returns and ambient noise were recorded by a 64-element vertical array while simultaneous measurements were made of a variety of environmental parameters, including: wind speed and direction, wave spectra, air and sea temperature and current fields, bubble plumes, etc., by investigators from SIO, WHOI, and IOS/BC, Canada. The nature of the data set lends itself to time series analysis techniques. Multivariate analysis of backscattering strengths as a function of ambient noise and various other environmental parameters will be presented and discussed. [Work supported by ONR.]

The impact of the background bubble layer on reverberation-derived scattering strengths in the low to moderate frequency range

The impact that a near-surface, range-independent background or persistent bubble layer may have on the derivation of sea surface backscattering strengths from reverberation measurements is examined. A simple ray model is proposed to account for the refractive and attenuating effects of the bubble layer and is used to calculate the modified insonification of the air–sea interface. This simple approach is validated against a highly accurate numerical solution. Scattering at the interface is handled via first order small perturbation theory. The combined propagation/scattering model is exercised in the low- to moderate-frequency range in order to examine bubble-induced modifications to sea surface backscatter calculations. Results indicate that the refractive effects due to the background bubble layer significantly enhance scattering levels as a function of wind speed. Furthermore, reasonable variations in the background bubble spectrum are shown to yield scattering levels that compare quite well with explosive source data from the 7th Critical Sea Test (CST-7) and the Chapman–Harris experiment. The model also predicts a weaker grazing angle and frequency dependence than evidenced in either data set.

Surface and volume scattering measurements using broadband explosive charges in the Critical Sea Test 7 experiment

Extensive measurements of low-frequency (70–1000 Hz) sea surface backscattering strengths have been made as part of the Critical Sea Test (CST) 7 experiment. These measurements were made for a variety of wind speeds from 3.0 to 17.5 m/s and for mean grazing angles from about 5 to 30 deg. The results from the CST-7 tests support the conclusions based on surface scattering strengths measured in previous CST SUS tests regarding the mechanisms controlling surface scattering and the frequencies and environmental conditions at which the mechanisms are important. As in previous tests, three scattering regimes were noted: a regime where results are generally consistent with scattering from a rough surface, a second regime where the frequency and grazing-angle dependences are much greater than expected from air–sea interface scatter but are consistent with scattering from subsurface bubble clouds, and a third regime that forms a transition zone between the other two regimes. The new measurements extend the range of environmental conditions over which these regimes are seen to substantially higher wind speeds than were obtained in the CST tests prior to CST-7. Volume scattering was observed to play a significant role in the measured results, with scattering from salmon the principal source of volume reverberation. Correlations of the measured scattering strengths were calculated for various environmental parameters and showed that the predictive ability of several wind speed-related parameters was improved by using a 2- to 4-h backaverage rather than the values taken during the runs.

Anomalous frequency behavior in low-frequency sea-surface backscatter

Surface backscattering strengths have been measured between 70 and 1500 Hz using broadband SUS charges as sources in the Critical Sea Test 7 at-sea test. While the results of the measurements for frequencies of 1 kHz and below have been presented previously, the extension of the results up to 1.5 kHz have shown some unexpected frequency behavior in the scattering strengths as a function of wind speed. For wind speeds of 15 m/s and higher, scattering strengths at a fixed grazing angle are found to peak in the 600- to 1000-Hz region and then decline with increasing frequency, while for lower wind speeds the scattering strengths increase roughly monotonically with increasing frequency. Possible explanations for this behavior will be discussed. [Work supported by ONR.]

Improved empirical descriptions of the dependence of surface backscatter on air-sea environmental descriptors

Between August 1988 and March 1992 six at-sea experiments were performed in which sea surface backscattering strengths were measured from ∼70 to ∼1000 Hz utilizing explosive (SUS) charges. (These were part of the Critical Sea Tests.) The latter of the six experiments (CST-7) was particularly interesting in that wind speeds as high as 17.5 m/s (at 10-m height) were encountered, which is 4 m/s higher than in any of the earlier experiments. Here, results obtained from CST-7 will be compared with those from the five earlier tests, and some important differences will be presented. Some new empirical formulas will be presented that describe the results from all six of the CST tests better than the Ogden–Erskine algorithm. (The Ogden–Erskine algorithm was formulated using data from the first five tests only and is now widely regarded as a much better description than the older Chapman–Harris formula.) Possibilities for improving still further the prediction of surface scattering strengths will be discussed. [Work supported by ONR.]

Surface scattering measurements using broadband explosive charges in the Critical Sea Test experiments

Extensive measurements of low-frequency (70–1000 Hz) sea surface backscattering strengths have been made as part of the Critical Sea Test (CST) experiments. These measurements were made during CST-1 through CST-5 for a variety of wind speeds from 1.5 to 13.5 m/s and for mean grazing angles from about 5 to 30 deg. These measurements have revealed several regimes in a frequency versus wind speed domain that probably correspond to at least two distinct scattering mechanisms. For relatively calm seas at high frequencies and for all wind speeds at lower frequencies, perturbation theory is found to give an adequate description of surface scattering. For rougher seas and higher frequencies, the Chapman–Harris empirical curves are adequate predictors of the levels of surface scattering. In between these two regimes is a transition region where the scattering strengths are more difficult to predict, as they depend on the details of the surface and wind characteristics. These observations lead to the idea that there are two mechanisms that dominate the scattering of sound from the surface. In the perturbation theory regime, air–water interface scattering is the dominant mechanism; in the Chapman–Harris regime, another mechanism such as scattering from subsurface bubble clouds must dominate the scattering process. The transition region is then the part of the frequency and wind speed domain where the two effects are competing.

Measurements of ocean surface and bottom backscattering strengths in the northwestern Atlantic Ocean

Experimental measurements of ocean surface and bottom backscattering strengths were carried out in the northwestern Atlantic Ocean during July and August of 1990. The experiment used ship‐deployed explosive charges to provide ensonification of the ocean surface and bottom over a range of low frequencies up to 1 kHz. Overall the surface scatter results agreed well with the Ogden–Erskine curves, matching the predicted dependence on wind speed, grazing angle, and frequency. At two sites northeast of the Grand Banks the surface scattering strengths appeared to be dominated by volume scattering due to fish. The bottom scattering strengths were observed to have considerable variation in level (up to 16 dB) between different sites. As a function of grazing angle, most of the bottom scattering strength curves paralleled the Mackenzie curve for grazing angles between 20° and 40°. A moderate frequency dependence of less than 3 dB was observed. Comparisons of the bottom scattering strengths with the Damuth 3.5‐kHz echo‐character province types yielded no consistent correlations. Comparisons with archival results for the same region yielded general agreement within 6 dB. Comparison of the ship‐based and airborne techniques showed that they yielded comparable scattering strengths to within 5 dB.

Azimuthal variation of low‐frequency Bragg scattering from the ocean surface

Low‐frequency sound, backscattered from an annulus of ocean approximately 9 km wide at a nominal 43‐km range, was measured on a 128‐hydrophone horizontal line array (HLA) during an experiment in the Gulf of Alaska. Fourier analysis of the backscattered signal permitted separation of the Bragg‐shifted returns from the 250‐Hz carrier frequency. The Bragg‐reflected energy was resolved in azimuth by beamforming the HLA data. Azimuthally varying sea surface backscattering strengths, corresponding to measured wind speeds, were obtained using an exact solution for the backscattered field applied to a two‐dimensional Donelan water wave spectrum. Normal mode model predictions using the calculated scattering strengths agreed with the up‐ and down‐shifted Bragg energy measurements in both level and azimuthal dependence. [Work supported by SPAWAR PMW 192.]

Low-frequency Arctic reverberation. III: Measurement of ice and bottom backscattering strengths from medium-range bottom-bounce returns

Backscattering strength measurements are usually extracted from short-range reverberation data using direct-path returns or from long-range reverberation data for which the dominant source of reverberation (e.g., a bottom seamount) is known. The small area ensonified by short-range direct paths provides only a relatively small data sample for backscattering strength estimates. While long-range data cover a much larger area, the returns from the surface and bottom are not always easily identified in the data. In the Arctic both the surface (ice) and bottom contribute significantly to the reverberation at low frequencies. The contributions of ice and bottom must be separated by signal processing means before their backscattering strengths can be measured separately. In this paper a method to separately measure the surface and bottom backscattering strengths using medium-range (5- to 20-km) reverberation data is investigated. The method of separation is based on the time versus vertical arrival angle contours associated with bottom-bounce propagation paths. Separation of surface and the bottom backscattering returns is most straightforward for a vertical array. It is noted that a (low tilt) vertical array has been shown to be a practical tool for collecting reverberation data in the Arctic. By extending analysis to medium-range reverberation data, backscattering data samples can be significantly increased. The ice backscattering strengths are estimated at 23 Hz from the CEAREX 89 reverberation data and the results are compared with the Burke–Twersky model, in which surface roughness is modeled by elliptic half-cylindrical protuberances on a pressure-release surface. Bottom backscattering strengths are also obtained at 23 Hz, and are found to be consistent with Lambert’s law within the experimental errors.