Sonar Equation Research Articles

Beyond counting calls: estimating detection probability for Antarctic blue whales reveals biological trends in seasonal calling

We explore the utility of estimating the density of calls of baleen whales for better understanding acoustic trends over time. We consider as a case study stereotyped ‘song’ calls of Antarctic blue whales (Balaenoptera musculus intermedia) on their Antarctic feeding grounds over the course of a year-long, continuous recording from 2014. The recording was made in the Southern Ocean from a deep-water autonomous hydrophone moored near the seafloor in the Eastern Indian sector of the Antarctic. We estimated call density seasonally via a Monte-Carlo simulation based on the passive sonar equation, and compared our estimates to seasonal estimates of detection rate, which are commonly reported in acoustic studies of Antarctic blue whales. The resulting seasonal call densities at our Antarctic site were strongly influenced by seasonally varying noise levels, which in turn yielded seasonal differences in detection range. Incorporating the seasonal estimates of detection area into our analysis revealed a pattern of call densities in accord with historic (non-acoustic) knowledge of Antarctic blue whale seasonal distribution and migrations, a pattern that differed from seasonal detection rates. Furthermore, our methods for estimating call densities produced results that were more statistically robust for comparison across sites and time and more meaningful for interpretation of biological trends compared to detection rates alone. These advantages came at the cost of a more complex analysis that accounts for the large variability in detection range of different sounds that occur in Antarctic waters, and also accounts for the performance and biases introduced by automated algorithms to detect sounds. Despite the additional analytical complexities, broader usage of call densities, instead of detection rates, has the potential to yield a standardized, statistically robust, biologically informative, global investigation of acoustic trends in baleen whale sounds recorded on single hydrophones, especially in the remote and difficult to access Antarctic.

Validation and verification of models for setting SONAR system requirements

When determining requirements for a SONAR system as a whole, typically very high-level requirements such as number of detections and total false alarms allowed per unit time must be converted into specific requirements for system components. These requirements may include array requirements like hydrophone count and position. Validation of probability of detection via sea test is fraught with complications, not least of which is environmental uncertainty. As a precursor, it is generally advisable to approach and attempt to validate models for each term in the SONAR equation one at a time. The approaches to validation vary depending on the term and can range in complexity from comparison to analytical solutions, to comparison with alternate available models, up through data collections at sea. Each of these approaches comes with its own challenges and limitations, which are discussed.

Target strength analysis using reflection coefficient of a submarine with low-frequency anechoic coatings

As the technological development of modern weapons systems progresses, attack and defense against the weapons systems of the past can become useless. In the case of underwater weapon systems, improvements in sonar and torpedo technology necessitate enhancements in submarine stealth performance. The stealth performance of a submarine is analyzed through the acoustic target strength (TS), which is indicated by the degree of sound wave reflection in the sonar equation. Various methods have been researched to analyze the TS of submarines, but methods considering the Anechoic coating attached to submarines are rare. Coating-related research has mainly focused on the absorption characteristics of coatings attached to flat surfaces. Therefore, there is an increasing need for research to analyze the sound absorption performance of anechoic coatings considering the curved surface of a submarine. In this study, we propose a method to verify the stealth performance of a submarine by analyzing the TS according to the azimuth angle by applying periodic inclusion of anechoic coating (PIAC) to the submarine. The PIAC based on the meta-structure is designed to form a bandgap in the low-frequency region by considering the resonance of the cylindrical Inclusion and the air cavity, applying the Bloch Theorem and the Brillouin zone, as well as the periodic arrangement characteristics. The PIAC was attached to the submarine model and modeled, and the absorption and reflection coefficients according to the azimuth angle were calculated using the finite element equation. The TS analysis included the process of determining the effective area by combining Geometrical Optics (GO) based on ray tracing and Physical Optics (PO) using the Kirchhoff approximation. The stealth performance was verified by comparing the TS analysis results according to the azimuth angle of the submarine equipped with the existing Alberich coating and the submarine equipped with PIAC at the same frequency.

Exploring diel vertical migration and spatiotemporal variation of zooplankton backscattering strength using an acoustic Doppler current profiler instrument in the Halmahera Sea, Indonesia

Given its critical role in marine ecosystems, this study comprehensively examined zooplankton distribution and behavior in the Halmahera Sea. The temporal and spatial dynamics of zooplankton acoustic backscatter values were analyzed using a 153.6 kHz vessel-mounted acoustic Doppler current profiler (ADCP). Analysis was supplemented by biological sampling with a bongo plankton net. Further evaluation included the analysis of oceanographic and bathymetric data. The acoustic, oceanographic, and biological sampling data were obtained from the Jala Citra I ?Aurora? survey expedition in 2021, while the bathymetry data were obtained from the General Bathymetric Charts of the Ocean (GEBCO). The raw ADCP data, represented as digital counts, were transformed into mean volume backscattering strength (MVBS) expressed in decibels (dB) using sonar equations to yield a measure proportional to zooplankton biomass. Temporal observations revealed a diel vertical migration (DVM) pattern in zooplankton aggregation, characterized by movements responding to the daily solar cycle. Spatial observations indicated a higher zooplankton density in semi-enclosed waters than in open water. The high values of acoustic backscatter are not attributed to a single species of zooplankton. Biological sampling identified that Oncaea spp. and Oithona spp., a species from the Cyclopoida order, exhibit the highest abundance. The study concludes that the ADCP, based on acoustic backscatter measurements and data sampling, is an effective tool for detecting the presence and behavior of zooplankton.

Analysis of Nuclear Explosion Detection Capability of IMS Hydroacoustic Network

The Comprehensive Nuclear Test Ban Treaty (CTBT) organization has established a hydroacoustic network to ensure the effective detection of nuclear explosions in the vast ocean and in the atmosphere over it in order to effectively guarantee the implementation of the treaty. To assess its effectiveness, according to the reciprocity of underwater acoustic field, this study employs the hydrophone position as the central sound source and utilizes the parabolic equation, along with seabed topography and sound velocity profile data, to estimate the transmission loss. Background noise levels of station evaluation are conducted by using historical data so as to estimate the underwater nuclear explosion sound source level via a passive sonar equation. Utilizing the full ship shock trial (FSST) results of the “Ford” aircraft carrier on June 18, 2021, as a case study, the detection capability of the HA10 station for nuclear explosions is investigated. Subsequently, the network’s overall detection capability was evaluated using the source level at 20 Hz corresponding to a 1 kiloton nuclear explosion yield as a reference. Findings indicate the network’s proficiency in identifying long‐distance propagation signals, particularly those with high explosion source levels within the sound fixing and ranging (SOFAR) channel. The sensitivity analysis demonstrates the network’s ability to detect explosions smaller than 1 kg equivalent in close proximity to a station, and larger explosions exceeding 1 kg or 1 ton at greater distances. This evaluation underscores the network’s significance in detecting and monitoring underwater nuclear explosions across extensive oceanic regions.

Quantitative Analysis on the Influence of the Oceanic Front on Underwater Acoustic Detection with Investigated Marine Data

At present, some shortcomings of the research on coupling modeling of the oceanic front–sound field may need attention: (1) Most of the acoustic propagation simulation is based on ideal front models, but the application of investigated marine data is lacking. (2) Most studies focus on the acoustic field characteristics, with the influence of fronts on acoustic propagation, but few studies aim at the direct quantitative analysis of the performance of underwater acoustic detection in oceanic fronts. To deal with the above problems, based on the measured data in the northwest Pacific Ocean, here, we first design different sound source layout schemes and calculate sound field characteristics in the sub-Arctic front using the ray theory. Then, the cumulative detection probability model is built based on the active sonar equation to evaluate the efficiency of underwater detection. Finally, the detection probability is calculated and expressed by regionalization, and the influence of the sub-Arctic front on underwater detection is quantitatively analyzed. The results show that the sub-Arctic front can significantly affect the underwater acoustic detection. The sound source located in the front in the cold-water mass has a better detection performance, especially detecting towards cold water (horizontal detection range > 60 km). In contrast, the sound source located in the warm-water mass has a poor detection performance (horizontal detection range in shallow sea < 10 km).

Flocculation Patterns Related to Intra-Annual Hydrodynamics Variability in the Lower Grijalva-Usumacinta System

Particle aggregation modifies sediment dynamics, which is a determining factor for morphodynamic and ecological processes in deltaic plains. Here, we investigated the link between intra-annual hydrodynamics variability and flocculation in the Grijalva-Usumacinta system. Monthly (2016–2017) and seasonal (2021–2022) river data was processed using analytical methods and the simplified sonar equation. Flocs were reformed and characterized in the laboratory, validating the in situ settling velocities (0.5–3.8 mm/s) and the existence of large low-density macro-flocs (>300 μm). We verified that flocculation prevailed, exhibiting seasonal patterns; (1) the highest aggregation rates matched the increase in total suspended solids at rising-flow (>100 mg/L), (2) periods of high-flow showed stable aggregation rates, and (3) an influence of marine conditions occurred at low-flow. Particulate phosphorous and organic fraction showed seasonal patterns linked to flocculation. Due to damming, the shear rates varied slightly (7–11 L/s) in the Grijalva, leading to high flocculation intensities affecting the diffusivity ratio. In the Usumacinta, aggregation was limited by shear rates that normally exceed 15 1/s. We found seasonal Rouse parameters representative of sediment dynamics.

Investigation and Numerical Simulation of the Acoustic Target Strength of the Underwater Submarine Vehicle

Modern weapon systems’ survival hinges on their detection capabilities more than anything else. In the active sonar equation, the acoustic target strength is crucial. Under the assumption of plane wave propagation, the standard target strength equation is used to forecast the reradiated intensity for the far field. The ability of a submarine to remain unnoticed while on patrol or accomplishing a mission is its primary defense. Sonar, sometimes known as sound navigation ranging, is a popular method for locating submarines. This is because saltwater effectively absorbs radio frequencies. Sonar technology is used in more than just the commercial fishing business; it is also used in undersea research. The submarine’s designers consider the reflection of acoustic waves to minimize the amount of space required for such reflections. The Target Strength (TS) metric is used to assess the sonar objects’ size. This manuscript explains and demystifies the Benchmark Target Echo Strength Simulation (BeTTSi) benchmark submarine’s TS analysis. This model’s Pressure Acoustic-Boundary Element Model (PA-BEM) interface has been stabilized, and the model itself is pretty huge acoustically.

Modeling sonar performance using J-divergence

This presentation demonstrates the use of J-divergence as a performance measure for detection in a sonar system. The inherent inaccuracies between system-level performance and the cell-level (PD,PF) detector operating point used in traditional analysis open the door to using approximate performance measures such as J-divergence. The properties of J-divergence making it an appealing choice are covered: summing to accrue J-divergence across multiple independent measurements (e.g., from multiple source signals, waveforms, or arrays), a data-processing inequality dictating that processing cannot improve J-divergence, and an asymptotic relationship to the traditional (PD,PF) operating point. Simple forward models of J-divergence are presented for matched filters and energy detectors when applied to the standard signal models in Gaussian noise. A “design” J-divergence, which is chosen by the desired qualitative performance level, is used in simple equations to obtain the “design” SNR require to achieve it. This provides a direct replacement for the detection threshold (DT) term in the sonar equation that is easier to apply and evaluate. Example applications of J-divergence are presented illustrating its utility in the modeling and adaptation of current systems as well as the design and analysis of new ones.

Rancang Bangun Driver PZT dan Filtering Data Akustik Pada Sonar Aktif

Acoustic transducer is a source level component in the sonar equation that is indispensable as an underwater acoustic energy source, with a certain emission pattern called beamforming. In this research, we designed the construction of a driver circuit to trigger a piezoelectric (PZT) transducer to produce an acoustic energy beam for the purpose of detecting underwater targets. The manufacture of an acoustic driver with a simple working principle has been carried out to provide transmit (TX) and receive (RX) commands. The RX module will receive the acoustic signal from the TX and the results are recorded for analysis. The mixing of the information signal with noise makes the information obtained cannot be directly verified so that a signal processing process is needed to be able to separate the information signal and noise. Calibration using a spherical target was carried out to determine the detection results and the acoustic intensity level. Signal analysis and visualization methods use Fast Fourier Transform (FFT) and wavelets.

The sonar equation versus reality: Inherent non-environmental variability in detection range

Acoustic Range Prediction uses the sonar equation to forecast the “detection range,” which is naturally understood by many to be the expected detection range in a closing-target encounter. Its variability is then the difference between actual and forecast detection ranges. The difference is often attributed to imperfect representation of the ocean environment (propagation and background levels). But the sonar equation’s original design-orientation and its terminology have at times caused misunderstanding. It is shown that the sonar equation’s detection range marks the sweet-spot of coverage, in which target detection is expected to be easy (fast and reliable), but which generally cannot be interpreted as the expected range or outer limits of detection in closing-target encounters. The classical and expected detection ranges concepts are disentangled here, giving each its appropriate roles in design, planning, and operations. It is also shown that, quite apart from environmental variability, there remains an inherent, significant variability of detection range simply because (1) the probability of detection increases slowly for a distant closing target, and (2) the target depth is unknown (unalerted detection). A significant degree of coverage variability therefore generally remains even if the environmental knowledge is perfect.

Model validation for simulated synthetic aperture sonar time series data

Synthetic aperture sonar (SAS) systems have grown in popularity due to their ability to create high-resolution sonar imagery with a high area coverage rate. Researchers continue to find new uses for SAS data products. To support the development of new uses and algorithms, it is important to be able to accurately simulate SAS systems with synthetic time series data. This simulated data can be used for many purposes, including estimating system signal-to-noise ratio and performance in a variety of environments, testing signal processing algorithms, and training algorithms to recognize objects in SAS imagery. For these processing tasks to be successful, the simulator must accurately represent physical phenomena in the oceanic underwater environment, which can be extremely complex. This presentation will outline a process for validation and verification of a high-frequency SAS time series simulator, and discuss several useful tests for validating model fidelity. Two fundamental concepts will be explored in depth: the sonar equation and spatial coherence theory. The sonar equation is important because it accounts for correct implementation of multiple lower level models such as geometry, beam patterns, propagation, and seafloor scattering strength. Spatial coherence is important because it is relied on to form coherent imagery.

Accounting for the Lombard effect in estimating the probability of detection in passive acoustic surveys: Applications for single sensor mitigation and monitoring.

The detection range of calling animals is commonly described by the passive sonar equations. However, the sonar equations do not account for interactions between source and ambient sound level, i.e., the Lombard effect. This behavior has the potential to introduce non-linearities into the sonar equations and result in incorrectly predicted detection ranges. Here, we investigate the relationship between ambient sound and effective detection ranges for North Atlantic right whales (Eubalaena glacialis) in Cape Cod Bay, MA, USA using a sparse array of acoustic recorders. Generalized estimating equations were used to model the probability that a call was detected as a function of distance between the calling animal and the sensor and the ambient sound level. The model suggests a non-linear relationship between ambient sound levels and the probability of detecting a call. Comparing the non-linear model to the linearized version of the same model resulted in 12 to 25% increases in the effective detection range. We also found evidence of the Lombard effect suggesting that it is the most plausible cause for the non-linearity in the relationship. Finally, we suggest a simple modification to the sonar equation for estimating detection probability for single sensor monitoring applications.

Estimation on the Underwater Explosion Equivalent Based on the Threshold Monitoring Technique

Underwater nuclear explosions can be monitored in near real-time by the hydroacoustic network of the International Monitoring System (IMS) established by the Comprehensive Nuclear-Test-Ban Treaty (CTBT), which could also be used to monitor underground and atmospheric nuclear explosions. The equivalent is an important parameter for the nuclear explosions’ monitoring. The traditional equivalent estimation method is to calculate the bubble pulsation period, which is difficult to obtain satisfactory results under the current conditions. In this paper, based on the passive sonar equation and the conversion process of acoustic energy parameters in the hydroacoustic station, the threshold monitoring technique used for underwater explosion equivalent estimation was studied, which was not limited to the measurement conditions and calculation results of the bubble pulsation period. Through the analysis of practical monitoring data, estimation on the underwater explosion equivalent based on the threshold monitoring technique was verified to be able to reach the accuracy upper boundary of current methods and expand the measurement range to further ocean space, along with the real-time monitoring capability of IMS hydroacoustic stations which could be estimated by this method.

Listening space reduction calculations

Listening space reduction analysis has been applied recently by several authors to evaluate the relative reduction of distance or space within which listeners can effectively detect and recognize important sound sources, due to increased acoustic masking caused by a known increase in masking noise. The approach balances the increased masking noise level by reducing source-listener distance so that propagation loss is commensurately reduced. Its recent applications have not considered all important parameters, including listener hearing sensitivity and directionality, listener cognitive signal processing gain, non-uniform acoustic propagation loss, and temporal and spatial distributions of masking noise variability. Here we derive the listening distance and space expressions from the sonar equation, accounting for the parameters above. Listening space reduction calculations are insensitive to some uncertain parameters, including source level and detection threshold (for non-human listeners) that are required for communication space calculations. The method is applicable to any sound type, including predator sounds, prey sounds, conspecific communications, and natural sounds. A modified version of the method is derived for echolocation sounds, and a corresponding echolocation space reduction factor is derived.

Comparison of Passive Sonar Narrowband Detection and Wideband Detection Performance

Conventional passive sonar uses wideband beam energy for target detection. In the case of low signal-to-noise ratio and strong target interference, the performance of wideband energy detection is rapidly degraded. In view of the relatively stable line spectrum of the radiated noise of underwater targets, the performance of narrowband detection and wideband detection is considered. In stable line spectrum underwater target detection, narrowband detection has performance advantages over wideband detection. However, its actual performance has disadvantages such as sensitivity to noise frequency bands. In this paper, based on the sonar equation, the theoretical methods of wideband detection and narrowband detection are analyzed and the main factors affecting detection performance are given. Simulation and sea trial data processing results verify the effectiveness of the method.

Use of Underwater Acoustics in Marine Conservation and Policy: Previous Advances, Current Status, and Future Needs

The interdisciplinary field of assessing the impacts of sound on marine life has benefited largely from the advancement of underwater acoustics that occurred after World War II. Acoustic parameters widely used in underwater acoustics were redefined to quantify sound levels relevant to animal audiometric variables, both at the source and receiver. The fundamental approach for assessing the impacts of sound uses a source-pathway-receiver model based on the one-way sonar equation, and most numerical sound propagation models can be used to predict received levels at marine animals that are potentially exposed. However, significant information gaps still exist in terms of sound source characterization and propagation that are strongly coupled with the type and layering of the underlying substrate(s). Additional challenges include the lack of easy-to-use propagation models and animal-specific statistical detection models, as well as a lack of adequate training of regulatory entities in underwater acoustics.

Quantifying Suspended Sediment using Acoustic Doppler Current Profiler in Tidung Island Seawaters

Tidung Island, located near Jakarta Bay, is a tourism and conservation area. It is necessary to keep these seawaters unpolluted. To calculate the level of pollution, it is necessary to know the sediment concentration. Quantifying concentration suspended sediment is important for knowledge of sediment transport. Researchers usually use water sample analysis and optical method for quantifying suspended sediment in seawater. Less accuracies of these methods are due to under sample of seawater and the existence of biological fouling. One promising method to measure concentration of suspended sediment is using Acoustic Doppler Current Profiler (ADCP). ADCP is usually used by oceanographer and hydrographer to measure ocean current. In this research, ADCP with 300 kHz operating frequency was used effectively to measure suspended sediment concentration (SSC) and ocean current simultaneously. The echo intensity received from suspended sediment was computed using sonar equations to quantify SSC. The empirical equation between echo intensity and SSC was found. The SSC value obtained by ADCP was also compared with in situ measurement. The result showed that quantified SSC value obtained by ADCP was nearly equal with SSC obtained from in situ measurement with coefficient correlation of 0.98. The high concentration ranged from 55 mg/L to 80 mg/L at the surface layer to a depth 12 m, moderate concentration ranged from 45 mg/L to 55 mg/L at a depth 12 m to 40 m, and low concentration less than 45 mg/L at a depth greater than 40 m. The distribution of SSC was correlated with ocean current condition. In small currents, suspended solids will settle faster so that the concentration in the water column will decrease. Conversely, if the velocity is high, suspended solids will continue to float carried by the current in the water column so that the concentration is high.

Acoustically measured transmission loss improves effectiveness of sonar search

The Sonar Equation is the heart of the process for predicting the effectiveness of an Antisubmarine Warfare (ASW) ship's sonar in the underwater detection of submarines. The equation's determination of Ranges associated with probabilities of detection and false alarms is the fundamental input for the selection of the most effective sonar modes in an individual ship and, at the broader tactical scale, in the selection of ASW search plans. The Sonar Equation's predictions of sonar performance, however, are subject to considerable error when employed tactically at sea in ASW operations. A major source of the error is the input, Transmission Loss, usually derived from XBT “snapshots” of temperature versus water depth. The resulting predictions of sonar performance are projected uniformly over space and time, leading to predictions that are stale and inaccurate. Accuracy of the Sonar Equation's output, however, can be improved by measuring Transmission Loss acoustically and continuously. A stream of pulses, periodically broadcast from a specially designed expendable sound source, is measured at the front end of the ship's sonar. A large population of Transmission Loss values is built up from which strong statistical inferences regarding detection can be drawn. The power of a continuously updated population of Transmission Loss values leads to more accurate inferences of the local acoustic environment.

Anthropogenic underwater sound emissions and other ecological effects on the marine environment

This presentation provides an overview of U. S. Navy and academia sponsored research to monitor, measure, model and mitigate the effects of active sonar sound transmissions on the health and well-being of marine mammals. Sound in the ocean has received considerable attention in the last several decades. Such intensive interest is in the sustainability and conservation of marine resources in ocean environments. Marine mammals in particular are sensitive to anthropogenic sounds. To capture the frequency-dependent nature of these sound effects on marine species, auditory computed weighting functions emphasize frequencies where a particular species is most susceptible to noise exposure. Researchers within academia and government continue development of functions like these and other metrics to characterize marine mammal responses to anthropogenic sound in the ocean. Underwater acousticians, marine biologists, oceanographers, engineers and computer scientists combine efforts to provide common standards of measuring and characterizing the anthropogenic sound effects on marine species. Further, computerized sonar equation models are used to: (1) provide standards of measuring and characterizing anthropogenic sound affecting various mammal species, and (2) determine metrics to guide policy makers' decisions in determining best practices for balancing marine environmental protection with national defense obligations.