Backscattering Strength Research Articles

Acoustic scattering properties of a seagrass, Cymodocea nodosa: in-situ measurements

Abstract Marine prairies play various crucial roles in marine ecosystems. The seagrasses that compose them are one of the most important components engineering the marine coastal system, providing significant spatial niches. Some of the seagrasses found in marine prairies are protected, and it is not recommended to sample them with destructive methods. Non-destructive methods such as remote sensing have been proposed as important means of studying these protected species. In the present study, the acoustic scattering properties of Cymodocea nodosa were studied with two different in/ex situ experiments conducted on a Turkish Mediterranean coast using a scientific echosounder (206 kHz split beam transducer) in different months over the years 2011 and 2012. After a series of acoustic processes, correlations and regression equations were established between different acoustic parameters of the Elementary Distance Sampling Units and biometric traits of below/above ground parts of the seagrass. The relationships were logarithmically established producing first a Rayleigh zone, followed by a geometrical zone that occurred with increased biometrics. No seasonal difference occurred in the relationships for the above-ground parts. Unlike the leaves, seagrass sheaths demonstrated unstable echo energy, inconsistent relationships, and unexplained acoustic responses over the span of several months. Regarding leaf density changing in time, significant relationships were explained as a function of the acoustic zones. Four points were highlighted to explain the differences in the estimations between the two experiments; i) the backscattering strengths depended on strength of biomass and its fractions (leaf area, shoot density and volume) between the two experiments, ii) the first experiment measured backscattering strength from individual specimens, but the second experiment was performed on the total biomass of seagrass per unit area, iii) different frequency response to the biometrics occurred in the two experiments, and iv) the non-linear effect of the sheath could not be separated from that of the leaf during the second experiment. The present study was the first attempt to characterize relationships between the biometric and acoustic backscattering properties of C. nodosa, and will guide researchers in future use of non-destructive methods.

Acoustic Target Strength of Jellyfish, Nemopilema nomurai, Measured at Multi-Frequency and Multi-Orientation

The jellyfish Nemopilema nomurai occupies an important position in the Northwest Pacific ecosystem, and monitoring its biomass is necessary to ensure the protection of fishery resources and the safety of offshore industrial production. The acoustic method has been proposed for jellyfish flux estimation and bloom warning, in which the target strength (TS) of the jellyfish is a crucial parameter. However, varied swimming orientations of jellyfish aggregation result in different backscatter strengths. The acoustic echo characteristics in horizontal swimming orientations and multi-frequency broadband signals are yet to be revealed. This study aims to obtain the TS of jellyfish in various orientations and to comprehensively investigate the jellyfish acoustic echo features at various frequency broadband sounds. In an anechoic tank, we used wide-band echosounder and 70 kHz, 120 kHz, and 200 kHz split-beam transducers to measure the TS of jellyfish swimming omnidirectionally. The results show a difference of approximately 4 dB in the jellyfish’s normalized TS at 70 kHz (frequency range: 45 kHz to 95 kHz) and 200 kHz (frequency range: 160 kHz to 260 kHz) center frequency. The normalized TS of jellyfish varies by around 8 dB between horizontal and vertical swimming orientations. For jellyfish swimming horizontally, the TS and bell diameter have the following least squares fits: T S D 70 k H z = 20 log D − 89.36 r 2 = 0.83 ; T S D 200 k H z = 20 log D − 93.85 r 2 = 0.83 . The swimming orientation has significant effects on TS estimation and model construction.

A Preliminary Study of Suspended Matters Variation Associated with Hypoxia and Shoaling Internal Tides on the Continental Shelf of the Northern Andaman Sea

The northern Andaman Sea (AS) continental shelf is unique due to the diverse marine ecosystem and existences of both hypoxia and internal tides, but limited in situ observations restrict our understanding of the hydrographic dynamic process. Based on the turbidity, mean volumes of backscattering strength (MVBS), we qualitatively studied the mean distribution characteristics and temporal variation in suspended matters on the northern AS continental shelf and their relation to hypoxia and internal tides. The results of both MVBS and turbidity revealed that the suspended matters exhibited a three-layer vertical structure. The upper and lower layers exhibited high values, while the middle layer had low values. The upper boundary of the high-value region in the upper layer descended below the surface to a depth of 30 m after sunrise and returned to the surface after sunset, indicating a diel vertical migration of zooplankton and micronekton. Daytime migration depth was likely constrained by hypoxia’s upper boundary. In the lower layer, three MVBS enhancements and attenuations correlated with vertical upward and downward velocities, respectively, primarily driven by uplift or suppression. We proposed vertical velocity patterns resulted from internal bores, possibly triggered by shoaling semidiurnal internal tides.

Acoustic Characteristics of Largehead Hairtail (Trichiurus japonicus) Using Multi-Frequency Detection in the Southern Sea, South Korea

This study was conducted to investigate the acoustic characteristics of largehead hairtails (Trichiurus japonicus) in the Southern Sea of South Korea from April 2022, using 18, 38, 70, 120, and 200 kHz split-beam echosounders. At two stations, juvenile (preanal length: 3.2 cm–5.8 cm) and immature (preanal length: 13.0 cm–26.0 cm) hairtails were caught by midwater trawling. Juvenile hairtails were distributed in layers without forming schools, while immature hairtails were detected as forming small, clustered schools. The mean volume backscattering strength (MVBS) of immature hairtails was more than 20 dB stronger than that of juvenile hairtails at both the 38 kHz and 120 kHz detection frequencies. Furthermore, the SV of juvenile and immature hairtails was high at 18 kHz and decreased as the frequency increased. We concluded that juvenile and immature hairtails can be distinguished according to their morphological characteristics because the distribution characteristics detected in the echograms differed according to their growth stage. Finally, these findings hold remarkable potential for applications within the field of hairtail identification. Acoustic backscatter information, as presented, offers valuable insights into age determination in this fish species.

Influence of ocean dynamics on Cuvier’s beaked whale (Ziphius cavirostris): Presence and their prey

Cuvier’s beaked whales (Ziphius cavirostris; Zc) dive below 1000 m to forage predominantly on deep-sea squids. The impact of oceanographic conditions on Zc prey and the effects of anthropogenic sounds on Zc foraging remain unclear. Zc prey distribution is patchy and potentially influenced by mesoscale oceanographic features interacting with local bathymetry. Favorable environmental conditions lead to aggregations of lower trophic level species that attract squid, and potentially increase Zc presence. We collected simultaneous active and passive acoustic data in Tanner Basin, a southern California submarine canyon, intermittently between 2017 and 2022. Passive acoustic data revealed Zc were present throughout all years, with lower presence during late summer and fall. Subsurface Zc 3D track reconstructions indicated a preference for one side of the canyon, where Zc foraged within a layer of potential prey, detected from active acoustic data. Prey concentrations, quantified as volume backscatter strength below ∼1150 m depth, exhibited an increase following the passage of mesoscale features, which were measured using Finite-size Lyapunov Exponents (FSLE). These prey increases drove immediate increases in Zc presence. Understanding potential physical drivers of Zc predator-prey dynamics is a critical first step for assessing natural variability and potential impacts of sonar use on Zc foraging behavior.

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.

Acoustic scattering properties of seagrass: In/ex-situ measurements of Posidonia oceanica

The marine prairies are paramount to the marine ecosystem playing a crucial role in various ways. Owing to the global atmospheric events inducing hydrospheric changes, marine seaweeds have been negatively affected and are vulnerable. Conventional methods (SCUBA), which were previously used to collect seagrasses, have become a destructive method for deriving unrecoverable damages for their stocks and have been replaced with remote sensing methods. Considering the advantages of the acoustic methods, two different in/ex-situ experiments were conducted to ground-truth the common seagrass, P. oceanica, in time and space of the Turkish Mediterranean water in 2011-2012 using a scientific echosounder with a split beam transducer operated at a frequency of 206 kHz. After the separation of the seagrass from spurious scatterers, the acoustic parameters (Sa: area backscattering strength, Sv: volume backscattering strength, and TS: Target Strength) were correlated and regressed with the biometric variables (Leaf Area Index, biomass, volume, length, width, diameter, or thickness) of different parts (leaf, rhizome, and sheath) of the seagrass. Estimation of biometrics by acoustic methods has been considered a challenge up to now. Statistical relationships between biometrics and acoustics were precisely examined for the species. The relationships were linearly established in the acoustic geometric region. There was a seasonal difference in the relationships. Their rhizomes and sheaths were considered unstable non-linear parts and remained unexplained for the acoustic response. Acoustic response to the leaf density (d, g cm-3), which was a distinguished component in the reflection, was found to be dependent on the seasonal biological activities of P. oceanica. Posidonia, which has a d greater than 1 g/cm3, produced the geometric region. The present study was the first attempt to establish the relationships of the seagrass under protection, and can decrease usage of destructive methods for future studies.

Indications of marine benthos occurrence from multi-spectral multi-beam backscatter data: a case study in the North Sea

To facilitate the conservation of seafloor habitats and planning of offshore activities, there is a growing need for mapping marine benthos in an effective and efficient way. Acoustic data acquired by multi-beam echosounders (MBES) have been extensively used for large-scale and high-resolution seafloor characterization. A deeper understanding of the relationship between backscatter data and sediment compositions can help to identify the benthos occurrence from the MBES data. With two multi-spectral MBES datasets collected near the western Wadden Sea islands in the North Sea, we investigated the potential of mapping marine benthos through backscatter classification. Two unsupervised classification methods, i.e., Bayesian classification, which mainly exploits the backscatter strength from incident angles larger than 20°, and hierarchical clustering of the backscatter strength at different angular ranges, were employed and the results were compared. The classification results from both methods showed a good correspondence with sediment properties such as the median grain size. Moreover, based on a principal component analysis of bottom sample properties, the hierarchical clustering results indicated a better distinction between contributions from the gravel content and benthos occurrence, e.g., sand mason worm density, than Bayesian classification, through involving the backscatter angular variations. Classification for multiple frequencies, on the other hand, showed little difference regarding the relationship with bottom sample properties. Although the backscatter difference between frequencies was also found to positively correlate with certain sample properties, using multi-spectral features for acoustic classification in this study did not reveal additional information compared to single-frequency classification results.

The Preliminary Study on Backscattering Strength of High Frequency Sonar in Extremely Shallow Water

The nearshore water area is a very important part of maritime safety and application water area. Due to the complexity of sound propagation, and the influence of terrain, geology, tidal range and experiment’s difficulty, there has few researches in this area. Based on an existing engineering works, this article attempts to give some related calculations of the bistatic backscattered acoustic strength received and launched by a high frequency sonar in the extremely shallow water area with the depth less than 10 meters, and followed by in situ experiments test. It shows that the calculated results are in good agreement with the experimental tests. This work is a preliminary attempt in the study of extremely shallow water areas, and will provide a certain reference for the later sonar work in extremely shallow water areas.

Integrated use of Sentinel-1 and Sentinel-2 data and open-source machine learning algorithms for burnt and unburnt scars

This research compares the use of the SAR (Sentinel-1) and Optical (Sentinel-2) sensors in identifying and mapping burnt and unburnt scars are rising during a bushfire in southeastern Australia and Margalla Hills, Islamabad, Pakistan, in 2019 and 2020. In order to evaluate the backscatter strength along with the Polarimetric decomposition portion, the C-band dual-polarized Sentinel-1 data was investigated to determine the magnitude of the burnt areas of forest cover in the study area. We could derive texture measurements from locally-based statistics using the Grey Level Co-occurrence Matrix (GLCM) and the backscatter coefficient. This was because of how well it picked up on differences in texture between burned and unburned scars. In contrast, Sentinel-2 optical remote sensing was employed to evaluate the extent of the burnt intensity levels for both regions utilizing the differential Normalized Burnt Ratio (dNBR). A Support Vector Machine (SVM) and Markov Random Field (MRF) classifier were utilized to investigate the study’s context. The ideal smoothing parameter is the result of incorporating the image’s spectral characteristics and spatial meaning. Sentinel-2 images were used as a foundation for both the test and training datasets, which were built from images of both unburned and burned areas broken down pixel by pixel. In both types, including spectral sensitivity and sensitivity of Polarimetric for the two groups identified after classification, the experimental findings showed a clear association between them. The algorithm’s efficiency was evaluated using the kappa coefficient and F-score calculation. Except for Sentinel-1 data in Pakistan, all fire areas have more than 0.80 accuracies. The highest precision of both Sentinel-1 and Sentinel-2 was also provided by the performance of users’ and producers’ accuracy. The entropy alpha decomposition helped define the target given by the H-a plane based on its physical properties. After the burn, the entropy and alpha values diminished and formed a pattern. However, the findings in this field validate the effectiveness of SAR sensors data and optical satellite in forest applications. The related sensitivity is highly dependent on the composition of the landscape, the geographical nature of the study area, and the severity of the burn.

Fish Stock Status Assessment in Alue Naga Waters Using A 200 Khz Single Beam Echosounder

Monitoring fish stocks is an important part of sustainable management of fisheries resources. Conducting the current evaluation is very difficult since there are no reliable data on the potential for fisheries in the waters surrounding Banda Aceh city. The hydroacoustic method was used for the first time in the waters of Alue Naga, located north of Banda Aceh, to evaluate the condition of fish stocks. A single beam scientific echosounder with a frequency of 200 kHz was used to collect hydroacoustic data in the area. The collected data was then processed using the Sonar5-Pro software. Cell integration was carried out at an interval of 10 meters vertically with an elementary sampling distance (ESD) of 200 m. While the volume backscattering strength (SV) value tends to be the highest in intermediate layers, the target strength (TS) value was found to be maximum in deeper layer, reaching a maximum value of -49.46 dB at a depth of 51–60 m. Area density value (#.ha-1) was found to be the highest at a depth of 11-20 m, while Biomass (g.ha-1) at a depth of 21-30 m was the highest among other depth layers with a value of 1558 g.ha-1. According to the in-situ catches of Carangoides, Selaroides, Aphareus, Variola, and Priacanthidae, there are a number of potential reef fish resources in the waters of Alue Naga. The findings of this study strongly support the conclusion that the hydroacoustic method effectively provides comprehensive information on the horizontal and vertical distribution of fish in Alue Naga waters.

Characteristics of Sound Attenuation by Individual and Multiple Fishes

Fish biomass and stock assessment are estimated from acoustic volume backscattering strengths (Sv) obtained from various hydroacoustic equipment. Although sound attenuation due to fish schools and water influences the Sv value, only attenuation from water is considered during the acoustic data process. For these reasons, it is necessary to understand the characteristics of sound attenuation by fish. Unfortunately, little is known about sound attenuation from fish. In the present study, the attenuation from one to four fish specimens was precisely measured during ex situ experiments in a water tank. The scientific echo sounder of a split-beam 200 kHz transducer and a miniature hydrophone were used for the attenuation measurements. Results show that the maximum attenuation coefficient (α) was approximately 25 dB/cm when the 4 multiple fishes had high fish heights. The relationship between the attenuation coefficient (α200kHz) and the total fish height (Htotal) was approximately α200kHz=0.54∗Htotal ± 0.06 (r2=0.72). This work describes the sound attenuation characteristics to provide basic information for the compensation of Sv from fish schools or layers.

Target strength of seahorse (Hippocampus kuda) based on presence dorsal and ventral aspect of brood pouch: a experiment hydroacoustic using artificial neural network

Hydroacoustic technology is a tool can be used to study the behavior of organisms. The target strength of an object is main key in hydroacoustic technology. The value of target strength is strongly influenced by level of hardness of object, in this case the seahorse has a different level of hardness between dorsal and ventral aspect. The unique of male seahorse that experiences pregnancy and keeps it’s baby in brood pouch until it hatches. This can be used as a difference between males and females based on value of target strength. Acoustic data recording using single beam scientific echosounder SIMRAD EK-15 and Sonar-4 processing software. The results showed that mean TS value of pregnant male seahorses (with brood pouch) was smaller than of female seahorses (without brood pouch), there were differences in structure composition and anatomy of the brood pouch which backscattering strength to decrease. The artificial neural network shows that there is a difference in response between seahorse (having a brood pouch/male) compared seahorse (without a brood pouch/female). The results showed that was a difference in dorsal aspect of 87.5% and ventral aspect of 98.6%. The results of this study can be used as key to distinguish sex of seahorses using the hydroacoustic method.

Observed correlations between the sediment grain size and the high-frequency backscattering strength.

In March 2019, Teledyne RESON and the Applied Physics Laboratory at the University of Washington conducted surveys with a calibrated multibeam echosounder at ten sites in Sequim Bay, a shallow sheltered bay in Washington State, USA. For each site, the mean grain size was obtained from a diver core sample, and estimates of the backscattering strength at frequencies ranging between 200 and 350 kHz were calculated. The correlation between the backscattering strength and the normalized grain size have been investigated for the grazing angles 45° and 75°. For 45°, a correlation consistent with previous results has been found. It demonstrates the potential for simple seabed classification.

Sonar investigation of the old Nubian Villages in the bottom topography of Nasser Lake, Egypt

Underwater sonar survey using Side Scan Sonar (SSS) and Single beam Echo-sounder (SBES) was used for the investigation of the old Nubian archaeological villages buried within the bottom topography of Nasser Lake. The research objective involved emphasizing the efficiency of marine geophysical techniques in the detection of the submerged old Nubian Villages in Nasser Lake behind the High Dam, the study also aimed to study riverbed topography and determining the old course of the Nile River in the area. The resulted bathymetric maps showed that the two former banks of the Nile River in the study area were characterized by two distinct natures, while the shoreline has retreated with different extents starting from 250 m at the southern regions up to 3 km in the northern parts. The high-resolution sonar images of the bottom uncovered distinctive acoustic patterns according to difference in the returned backscatter strength. Submerged Nubian houses were detected along some areas of the eastern bank, which were found either standing over elevated rocky cliffs or built on multiple tiered levels. Precise target measurements were applied on the detected houses and different dimensions were obtained, which suggest that these houses were built according to the availability of suitable land to construct such dwellings.

Relating marine mammal distribution to water column prey structure derived from echosounding

Managing human impacts on marine mammal populations depends on understanding their distributions in space and time. Knowledge of these distributions is becoming increasingly important due to offshore energy development and climate-induced shifts in oceanic habitat. Most models assessing pelagic marine mammal abundance and distribution primarily use ocean surface or ocean floor variables as proxies for water column habitat. Here, we used a ship-based marine mammal sighting survey to test the utility of echosounder-based predictive variables for modeling marine mammal distribution and abundance. We assessed the distribution of 7 marine mammal taxa and 3 feeding guilds along the shelf break off the northeast USA relative to prey structure derived from acoustic data. We classified prey into 4 categories: (1) fish with swimbladders; (2) small resonant bubbles as phytoplankton, fish larvae, and gelatinous zooplankton; (3) fluid-like scatterers such as krill and copepods; and (4) fish with no swimbladder. We quantified the spatial structure of prey by calculating backscattering strength, location, dispersion, occupied areas, evenness, and aggregation. Spatial resolution along the survey track line was set to bins 1000 m in distance and either 50 or 200 m in depth. We then built generalized additive models (GAMs) using these acoustically derived variables to explain marine mammal distribution. The resulting GAMs explained between 9 and 38% of deviance, with model fit often reflecting aspects of foraging depth and prey preference. This approach could contribute to improved management through more accurate species distribution models that employ direct measurements of prey.

Measurement of Scattering Properties of Water Body by Using a Multibeam Echosounder System

We propose a backscattering compensation based procedure to enhance the performance of multibeam echosounder systems (MBESs). High resolution in a wide field of view is the main benefit of MBESs, and this property makes them increasingly important for measuring scattering in water body. The performance of these systems is vulnerable to the anisotropic characteristics of scatterers, and thus, the results provided by MBESs can be inaccurate. To enhance the performance of MBESs, we compensate for the echo data on the basis of backscattering strength. The compensation considers the reflected sound energy by introducing interlayer coefficients calculated from the amounts of energy that penetrate into and are reflected from each layer. To demonstrate the effectiveness of the proposed method, we perform numerical simulations with randomly positioned scatters having anisotropic properties. Furthermore, anechoic pool experiments are conducted using bubbles generated by air compressors and propellers. The after-compensation results reveal richer details, thereby confirming the effectiveness of the proposed method.

Design of a Low-Frequency Active Sonar Underwater Target Simulator Based on Bubble Clouds

According to the theory that bubble clouds have strong scattering properties in the water, effective medium theory (EMT) based on bubble clouds has been developed for a new kind of target simulator available for multi-static sonar experiments. According to the theory, the effects of the porosity of the bubble clouds and the size of the simulator on the backscattering strength of the target simulator are investigated. The optimal parameters for designing the target simulator are given by simulation analysis. Finally, by comparing with the traditional target simulator design scheme, it is shown that the target simulator designed based on EMT theory has the characteristics of low-frequency bandwidth, high target strength, small size, low cost, and application flexibility.

Imaging of Artificial Bubble Distribution Using a Multi-Sonar Array System

Bubble clusters present in seawater can cause acoustic interference and acoustic signal distortion during marine exploration. However, this interference can also be used as an acoustic masking technique, which has significant implications for military purposes. Therefore, characterizing the distribution of bubble clusters in water would allow for the development of anti-detection technologies. In this study, a sea experiment was performed using a multi-sonar array system and a bubble-generating material developed by our research group to obtain acoustic signals from an artificial bubble cluster and characterize its distribution. The acquired acoustic data were preprocessed, and reverse-time migration (RTM) was applied to the dataset. For effective RTM, an envelope waveform was used to decrease computation time and memory requirements. The envelope RTM results could be used to effectively image the distribution characteristics of the artificial bubble clusters. Compared with acoustic Doppler current profiler data, the backscattering strength of the boundary of the imaged artificial bubble cluster was estimated to range between −30 and −20 dB. Therefore, the three-dimensional distribution characteristics of bubble clusters in the open sea can be effectively determined through envelope RTM. Furthermore, the data obtained from this study can be used as a reference for future studies.

Relationship of Acoustic Volume Backscattering of Fish Aggregations and Density of Pelagic Fish in The Waters of Pulau Baai Bengkulu City

The waters of Pulau Baai are unique waters that are closed or protected which have small pelagic fish resources, used as pelagic fishing areas for local fishermen so that they can help the community's economy. One method that can be used to describe the distribution of pelagic fish density is the use of hydroacoustic technology. This study aims to analyze the value of acoustic backscattering (volume backscattering strength, SV) of pelagic fish and associated with the estimation of the density of pelagic fish in the waters of Pulau Baai, Bengkulu City, through in-situ acoustic measurements. The results showed that in the waters of Pulau Baai the highest average SV value was -50.47 dB, which was thought to have a fish density of 2.57 ind/m3 while for a low average SV with a value of -57.48 dB, it was assumed to have a density of 0.04 ind/m3. The density of pelagic fish is acoustically estimated at 26.47 ind/m3 in the water column of Pulau Baai. The SV value affects the estimated fish density by 69% and is categorized as having a high correlation and a positive correlation direction.