Echo Statistics Research Articles

Adaptive beamformer with echo statistics

Delay-and-sum (DAS) beamforming is commonly used in commercial scanners and computationally efficient for real-time imaging. However, the ability to suppress off-axis signals is limited. A minimum variance (MV) beamformer realizes a superior performance in suppression of off-axis signals and improves lateral resolution significantly. On the other hand, MV degrades the contrast-to-noise ratio (CNR) compared to DAS because it alters speckle statistics. In this study, we developed a method for improvement of CNR in MV beamforming by evaluating envelope statistics of echo signals. It is well known that the envelope statistic of speckle echoes from a random medium obeys the Rayleigh distribution. The echo envelope statistics were evaluated using the shape parameter of the Nakagami distribution. The proposed beamformer is worked as DAS for speckle echoes and MV for non-speckle echoes by referring to the Nakagami shape parameter. In the phantom experiment, the lateral resolution of MV was 0.18 mm, which was significantly better than 0.53 mm obtained by DAS. However, CNR was degraded from 6.73 dB to 4.05 dB by MV. The proposed beamformer realized a lateral resolution of 0.25 mm, which was significantly better than DAS, with a CNR value of 6.03 dB, which was comparable to DAS.

Evaluation of Fish School Resources in Shallow Sea Based on Echo Statistic Method

In order to obtain the spatial-temporal distribution of fish resources together with its changes in sea ranching, and realize the acoustic monitoring and evaluation of fish resource proliferation in sea ranching, the acoustic evaluation technology of fish resources based on the echo statistics method is researched in this paper. Firstly, the simulation is conducted by constructing the fish school echo signal based on the Kirchhoff-ray model, and the simulation results show that the echo statistics method is suitable for acoustic monitoring of fish stocks in sea ranching. Then, the self-developed fish finder was used to survey the national sea ranching demonstration area in the western waters of Furong Island in Shandong Province by navigating in November 2020 and March 2021 respectively. The data obtained from the two voyages were post-processed using the echo statistics method, and the density changes and activities of the fish schools in different times and regions in the surveyed sea were obtained. The analysis results show that the echo statistics method is suitable for the monitoring and evaluation of fishery resources in the sea ranching area.

Clusters of Ultrasound Scattering Parameters for the Classification of Steatotic and Normal Livers

The study of ultrasound tissue interactions in fatty livers has a long history with strong clinical potential for assessing steatosis. Recently we proposed alternative measures of first- and second-order statistics of echoes from soft tissues, namely, the H-scan, which is based on a matched filter approach, to quantify scattering transfer functions and the Burr distribution to model speckle patterns. Taken together, these approaches produce a multiparameter set that is directly related to the fundamentals of ultrasound propagation in tissue. To apply this approach to the problem of assessing steatotic livers, these analyses were applied to in vivo rat livers (N=21) under normal feeding conditions or after receiving a methionine- and choline-deficient diet that produces steatosis within a few weeks. Ultrasound data were acquired at baseline and again at weeks 2 and 6 before applying the H-scan and Burr analyses. Furthermore, a classification technique known as the support vector machine was then used to find clusters of the five parameters that are characteristic of the different steatotic liver conditions as confirmed by histologic processing of excised liver tissue samples. With the in vivo multiparametric ultrasound measurement approach and determination of clusters, steatotic can be discriminated from normal livers with 100% accuracy in a rat animal model.

Two decades of progress in physics-based echo statistics

The echo from an aggregation of marine organisms naturally fluctuates from ping to ping as the echosounder is moved over that aggregation due to various interference phenomena and statistical processes. There is important information in the characteristics of the fluctuations, such as numerical density of the scatterers, which can be lost through averaging when calculating volume scattering strength. Physics-based echo statistics involves describing the fluctuations through use of the parameters of the echosounder (signal type, beamwidth), scattering properties of the marine organisms, interference effects of echoes from multiple scatterers, and effects due to the environment such as presence of boundaries and heterogeneities in the medium. Advances over the past 20 years will be reviewed which include describing the statistics for broadband signals, randomized elongated scatterers, and long-range sonars. This presentation will draw from the tutorial recently published by these authors which also contains open access software to calculate the statistics over a wide range of scenarios [J. Acoust. Soc. Am. 144, 3124–3171 (2018)].

Observed Characteristics Change of Tropical Cyclones During Rapid Intensification Over Western North Pacific Using CloudSat Data

Rapid intensification (RI) progress is the main challenge that precludes the improvement of tropical cyclone (TC) intensity estimates. In this paper, a composite study of the observable characteristics of 11-year TCs undergoing RI and non-RI in the Western North Pacific were conducted using CloudSat tropical cyclone (CSTC) dataset. 2B-GEOPROF, 2B-CLDCLASS, and 2B-CWC-RO products in the CSTC dataset were used to construct radar reflectivity, cloud class, and cloud water-content distributions for each category, respectively. The results show that radar echo statistics have an arc-like contour profile for all five categories, with two distinct modes of reflectivity distributions separated by the melting layer. RI has the highest frequency expanding through the whole reflectivity range along with the broadest coverage in the upper branch, and a “continuity” of reflectivity distribution from −10 to 10 dBZ at 6–11 km could be a vital indicator of TC intensification. A sharp slope can be observed in the lower branch due to heavy precipitation attenuation. Vertical distributions of cloud types show that all categories have similar cloud compositions, and deep convective clouds are apparently to play an important role in maintaining the TC intensification. Liquid water content (LWC) curve exhibits a bimodal distribution with two peaks at around 1.5 and 5 km, while ice water content (IWC) curve is much smoother and has one peak at around 9 km. It is suggested that TCs with larger LWC around freezing level or/and larger IWC at upper level tend to intensify within the next 24 hours.

Echo statistics associated with discrete scatterers: A tutorial on physics-based methods.

When a beam emitted from an active monostatic sensor system sweeps across a volume, the echoes from scatterers present will fluctuate from ping to ping due to various interference phenomena and statistical processes. Observations of these fluctuations can be used, in combination with models, to infer properties of the scatterers such as numerical density. Modeling the fluctuations can also help predict system performance and associated uncertainties in expected echoes. This tutorial focuses on "physics-based statistics," which is a predictive form of modeling the fluctuations. The modeling is based principally on the physics of the scattering by individual scatterers, addition of echoes from randomized multiple scatterers, system effects involving the beampattern and signal type, and signal theory including matched filter processing. Some consideration is also given to environment-specific effects such as the presence of boundaries and heterogeneities in the medium. Although the modeling was inspired by applications of sonar in the field of underwater acoustics, the material is presented in a general form, and involving only scalar fields. Therefore, it is broadly applicable to other areas such as medical ultrasound, non-destructive acoustic testing, in-air acoustics, as well as radar and lasers.

Stereophonic Acoustic Echo Suppression for Speech Interfaces for Intelligent TV Applications

In this paper, a unified framework for using speech interfaces with intelligent televisions (TVs) is proposed. The proposed framework is primarily based on suppressing the stereophonic acoustic echo of the TV audio sounds and adaptive tracking of the time varying acoustic transfer function (ATF) of the echo sounds. In order to effectively suppress the acoustic echoes from the TV speakers, an optimal filter with a multichannel subspace approach is imposed on noisy inputs and the second order statistics of the stereophonic echoes are adaptively estimated by tracking the time varying ATF using the normalized least mean square method. Through assessing the proposed method with a database collected in real TV watching environments, noticeable improvements were achieved in the signal to noise ratio gain, Itakura–Saito distance, and speech recognition rates.

Broadband classification and statistics of echoes from aggregations of fish measured by long-range, mid-frequency sonar.

For horizontal-looking sonar systems operating at mid-frequencies (1-10 kHz), scattering by fish with resonant gas-filled swimbladders can dominate seafloor and surface reverberation at long-ranges (i.e., distances much greater than the water depth). This source of scattering, which can be difficult to distinguish from other sources of scattering in the water column or at the boundaries, can add spatio-temporal variability to an already complex acoustic record. Sparsely distributed, spatially compact fish aggregations were measured in the Gulf of Maine using a long-range broadband sonar with continuous spectral coverage from 1.5 to 5 kHz. Observed echoes, that are at least 15 decibels above background levels in the horizontal-looking sonar data, are classified spectrally by the resonance features as due to swimbladder-bearing fish. Contemporaneous multi-frequency echosounder measurements (18, 38, and 120 kHz) and net samples are used in conjunction with physics-based acoustic models to validate this approach. Furthermore, the fish aggregations are statistically characterized in the long-range data by highly non-Rayleigh distributions of the echo magnitudes. These distributions are accurately predicted by a computationally efficient, physics-based model. The model accounts for beam-pattern and waveguide effects as well as the scattering response of aggregations of fish.

Acoustic observations of the fish assemblage during a multibeam hydrographic survey of a cod spawning area

A two-day hydrographic survey using a dual-head Kongsberg EM3002 multibeam echosounder was conducted to map the seafloor features within 9 square kilometers of the Gulf of Maine Cod Spawning Protection Area, where Atlantic Cod (Gadus morhua) were known to concentrate during their spring spawning season. Acoustic backscatter from the water column collected during the first day of this survey was used to describe the fish assemblage associated with a corridor bounded by elevated features when the area was closed to commercial fishing. During the survey, a bottom trawler under a research permit made six tows where fish were observed by the hydrographic survey vessel. The species assemblage was dominated (>80%) by Atlantic Cod. Acoustic observations from the second day, when the area was open to an active commercial fishery, were compared to those during the fishing area closure. Echo statistics and position in the water column were used to characterize categories of fish detections and associate them spatially to the seafloor features and temporally to fishing closure.

Effect of Pulse Duration on Echo Matched-Filter Statistics in a Shallow-Water Channel

Active sonar echo statistics (i.e., the probability density function of the match-filtered echo) are an important part of performance prediction. System design parameters such as pulse amplitude, duration and bandwidth are usually only assumed to alter the signal-to-noise-power ratio (SNR) at the detector and not the echo statistical model or its parameters. While this may be an accurate assumption for low bandwidth waveforms in a static propagation channel, it is not necessarily so when the echo is resolved or the channel is time varying. Echoes from a moored, vertical air hose were obtained during the 2013 Target and Reverberation Experiment (TREX-2013) and used to evaluate the impact of pulse duration on echo statistics. Using the scintillation index (SI) to represent the statistical character of the air-hose echo measurements, increasing pulse duration was seen to make the echoes more deterministic (i.e., to decrease SI). Assuming a time-varying Rician channel, a model was developed relating the pulse duration, channel correlation time, and ratio of coherent to incoherent power in the channel to the echo SI. Comparison with the TREX-13 air-hose measurements showed a good or excellent fit to five of the six 1-h measurement periods analyzed. Increasing pulse duration beyond the channel correlation time essentially increases the ratio of coherent to incoherent power in the echo, which improves detection performance when SNR is fixed (e.g., in a reverberation-limited scenario). If a single channel instantiation already produces a nearly deterministic echo, the performance improvement is minimal. However, when the channel is weakly Rician (e.g., a shallow-water channel at large ranges relative to water depth), the improvement can be significant.

Usefulness of combined BI-RADS analysis and Nakagami statistics of ultrasound echoes in the diagnosis of breast lesions

To develop a method combining the statistics of the ultrasound backscatter and the Breast Imaging-Reporting and Data System (BI-RADS) classification to enhance the differentiation of breast tumours. The Nakagami shape parameter m was used to characterise the scatter properties of breast tumours. Raw data from the radiofrequency (RF) echo-signal and B-mode images from 107 (32 malignant and 75 benign) lesions and their surrounding tissue were recorded. Three different characteristic values of the shape parameters of m (maximum [mLmax], minimum [mLmin] and average [mLavg]) and differences between m parameters (Δmmax, Δmmin, Δmavg) of the lesions and their surrounding tissues were assessed. A lesion with a BI-RADS score of 3 was considered benign, while a lesion with a score of 4 was considered malignant (a cut-off of BI-RADS 3/4 was set for all patients). The area under the receiver operating characteristic (ROC) curve (AUC) was equal to 0.966 for BI-RADS, with 100% sensitivity and 54.67% specificity. All malignant lesions were diagnosed correctly, whereas 34 benign lesions were biopsied unnecessarily. In assessing the Nakagami statistics, the sum of the sensitivity and specificity was the best for mLavg (62.5% and 93.33%, respectively). Only four of 20 lesions were found over the cut-off value in BI-RADS of 4a. When comparing the differences in m parameters, Δmavg had the highest sensitivity of 90% (only three of 32 lesions were false negative). These three lesions were classified as BI-RADS category 4c. The combined use of B-mode and mLmin parameter improve the AUC up to 0.978 (p=0.088), compared to BI-RADS alone. The combination of the parametric imaging and the BI-RADS assessment does not significantly improve the differentiation of breast lesions, but it has the potential to better identify the group of patients with mainly benign lesions that have a low level of suspicion for malignancy with a BI-RADS score of 4a.

Inspirations of past leaders in Acoustical Oceanography: Reflections of our roots in AO and bioacoustics

I had the privilege of knowing the leaders of Acoustical Oceanography (AO) during its early years, first as a topical entity, then when it was eventually formalized as a Technical Committee (TC). I first arrived at the University of Wisconsin in 1980 and did research there with Clarence Clay for eight years as a scientist. The 1977 book, “Acoustical Oceanography” by Clay and (Hank) Medwin, had recently been published and was required reading. At around the same time, Van Holliday, David Farmer, Mike Buckingham, and Medwin, established AO as a formal committee, first as a Specialty Group (Medwin as Chair), and later as a TC in 1991. Buckingham was elected first Chair of the AOTC and, given the anticipated challenges in starting a new TC, I was made Alternate Chair to work alongside Buckingham on many things. Clay and Holliday inspired much of the AO bioacoustics research that I have conducted, including developing acoustic scattering models, physics-based echo statistics, and broadband acoustic methods to use sound to characterize marine organisms such as fish and zooplankton. I will tell some stories and give examples of recent scientific advances that are traceable to Clay and Holliday.

Statistics of Broadband Echoes: Application to Acoustic Estimates of Numerical Density of Fish

A general numerical model has been developed to predict the probability density function (pdf) of the magnitudes of complex pulse-compressed broadband echoes (broadband echo pdf) due to arbitrary aggregations of scatterers that are detected with a single-beam echosounder. The model is based on physics principles and rigorously accounts for the broadband frequency-dependent characteristics of the system, signal, scatterers, and the beampattern modulation effects of the sonar/radar transceiver. A key aspect to modeling the statistics of broadband echoes is accounting for the scenarios where the pulse-compressed echoes may only partially overlap. The echo statistics under those conditions will be significantly different than those associated with a narrowband system with the same center frequency but whose echoes will completely overlap given the same density of scatterers. As a result, pulse-compressed broadband echoes will generally deviate more from the Rayleigh distribution (i.e., be more “heavy-tailed”) than narrowband echoes-a feature that is critical to analyzing real-world data. As in the case of narrowband signals, the shape of the broadband echo pdf is shown to vary from strongly non-Rayleigh to Rayleigh as the number of dominant scatterers in the beam increases. The model is applied to sonar in the ocean in which the numerical density of fish is inferred using broadband echoes (30-70 kHz). The results are compared with those from conventional echo energy methods. As with narrowband systems, statistics of broadband echoes can be used to estimate the numerical density of scatterers without the need for absolute calibration of the system.

Interpreting Echo Statistics of Three Distinct Clutter Classes Measured With a Midfrequency Active Sonar: Accounting for Number of Scatterers, Scattering Statistics, and Beampattern Effects

A recently developed published approach to predict echo statistics is applied to clutter data that were collected with a midfrequency sonar and published in a separate independent study. This method explicitly accounts for the (finite) number of unresolved scatterers, the statistics associated with the arbitrary scattering properties of the individual scatterers [but assumed to have identical echo probability density functions (pdfs) in this application], and beampattern effects which significantly affect the echo statistics due to each scatterer being randomly located in the sonar beam. The data had been categorized according to whether they were associated with bottom structures, diffuse compact clutter, and compact nonstationary (moving) clutter. In this paper, the recently developed method is incorporated in a two-component mixed pdf (mixed with a Rayleigh distribution to account for the diffuse background) to model the statistics of the three classes of clutter. This is the first such application of the model which had principally been validated only numerically. The degree to which the data are non-Rayleigh (heavy tailed) is reasonably predicted by the model and the number of scatterers per resolution cell is inferred for each type of clutter.

Non-Rayleigh Scattering by a Randomly Oriented Elongated Scatterer Randomly Located in a Beam

The echo statistics of a randomly rough, randomly oriented prolate spheroid that is randomly located in the beampattern are investigated from physics-based principles both analytically and numerically. This is a direct-path geometry in which reflections from neighboring boundaries are not a factor. The statistics and, in particular, the tails of the probability density function (pdf) and probability of false alarm (PFA) are shown to be strongly non-Rayleigh and a strong function of shape of scatterer. The tails are shown to increase above that associated with a Rayleigh distribution with increasing degree of elongation (aspect ratio) of the scatterer and when roughness effects are introduced. And, as also shown in previous studies, the effects associated with the scatterer being randomly located in the beam contribute to the non-Rayleigh nature of the echo.

Statistics of Echoes From Mixed Assemblages of Scatterers With Different Scattering Amplitudes and Numerical Densities

In this paper, an exact solution is derived, evaluated, and numerically validated for describing the statistics of echoes from mixed assemblages of scatterers. Here, a “mixed assemblage” involves the geometry in which there is more than one type of scatterer spatially interspersed and uniformly distributed within the analysis window, which is much larger than the resolution cell of the system (i.e., there are many independent samples per window). The scatterers are generally not resolvable and the signals are narrowband. The scattering geometry is in the backscattering direction in the direct-path case in which there are no interfering echoes from neighboring boundaries. The probability density functions (pdfs) of echo envelopes in such cases can be highly non-Rayleigh and possess heavy tails, and the shape of the pdf curves contains information for characterizing and discriminating the composition of mixed assemblages. The general formulation is based on characteristic functions (CFs; hereafter referred to as the CF-based mixed assemblage pdf) and incorporates effects due to the scatterers being randomly located in the beam. Comparisons are made between the performance of the CF-based mixed assemblage pdf and the commonly used mixture model for simulated cases involving two different types of scatterers, arranged either interspersed or segregated in the analysis window. Both models can be made to fit the shape of the echo pdf of simulated data in some conditions. However, mismatch between model assumptions and the actual physical scattering processes can lead to order of magnitude errors in the inferred numerical density and backscattering amplitude of each type of scatterers.

An approach to evaluate and monitor RISAT-1 SAR from level-0 raw data

Data quality evaluation of synthetic aperture radar (SAR) products is essential with respect to mission performance and reliability of the data products for users. The generation of any value-added SAR product starts with a level-0 raw data product, so it is vital firstly to analyse and monitor the raw data. In other words, any inconsistency or error occur because of data, the SAR processor requires to ‘quality evaluate’ the raw data first to ensure that the higher level of data products meets the user specifications. Many parameters can be evaluated from the raw data per se before SAR processing. Here, an independent approach is formulated to evaluate the long-term stability and performance of the SAR system from raw data. Quality parameters such as echo data statistics in terms of bias in the mean of Inphase (I) and Quadrature in phase (Q), power imbalance, phase imbalance, antenna pattern in elevation for the qualified swath, Doppler centroid estimation, and chirp phase and amplitude stability were evaluated and monitored for various scenes of an orbit and characterized in detail for different beams covering the near to far range. In the Radar Imaging Satellite (RISAT-1), the SAR beam is formed by a number of transmit/receive modules (TRMs) (total number of active TRMs depends on the look angle), and the consolidated effect of TRMs can be observed from the raw data instead of individual TRMs, which enables examination of the overall behaviour of beams, from near to far range in different orbits, for temporal stability. In this article, for the first time, a novel approach is taken to observe and evaluate the antenna pattern for the 3 dB qualified swath from range-uncompressed raw data and to relatively monitor the different beams from level-0 raw data for the qualified swath, for both extended homogeneous and non-homogeneous targets. Furthermore, the approach is justified in that it is not necessary to perform range compression, and also one can use scenes with heterogeneous targets to estimate and monitor the beamwidth parameter of the antenna pattern. This article focuses on the results obtained from RISAT-1 level-0 raw data from various beams covered in strip-map mode. The methodology to evaluate the RISAT-1 SAR system from raw data is discussed. Analysis is carried out with respect to different beams (60 beam data of different orbits either side, covering near to far nadir swath coverage) of fine resolution strip-map-1 (FRS-1) mode in circularly transmitted and linearly received polarization, with a defined swath of 25 km with 3 m × 2 m (azimuth × range) resolution. The results of evaluation and monitoring analysis show consistency in the identified parameters observed over time for different beams, and they are within specifications.

Echo statistics of individual and aggregations of scatterers in the water column of a random, oceanic waveguide.

The relative contributions of various physical factors to producing non-Rayleigh distributions of echo magnitudes in a waveguide are examined. Factors that are considered include (1) a stochastic, range-dependent sound-speed profile, (2) a directional acoustic source, (3) a variable scattering response, and (4) an extended scattering volume. A two-way parabolic equation model, coupled with a stochastic internal wave model, produces realistic simulations of acoustic propagation through a complex oceanic sound speed field. Simulations are conducted for a single frequency (3 kHz), monostatic sonar with a narrow beam (5° -3 dB beam width). The randomization of the waveguide, range of propagation, directionality of the sonar, and spatial extent of the scatterers each contribute to the degree to which the echo statistics are non-Rayleigh. Of critical importance are the deterministic and stochastic processes that induce multipath and drive the one-way acoustic pressure field to saturation (i.e., complex-Gaussian statistics). In this limit predictable statistics of echo envelopes are obtained at all ranges. A computationally low-budget phasor summation can successfully predict the probability density functions when the beam pattern and number of scatterers ensonified are known quantities.

Accounting for the non-Rayleigh echo statistics of individual elongated scatterers in an aggregation

The statistics of echoes from active sonar systems, such as the shape of the probability density function (pdf) of echo magnitude, can be used to estimate the numerical density of scatterers in aggregations. This study investigates the importance of non-Rayleigh echo statistics of individual scatterers on the shape of the echo pdf of aggregations in which the echoes from individuals may overlap. The signals are broadband, and the geometry involves direct paths between the sonar and the scatterers without interference from the boundaries. Echo pdf models are generated by varying the number of scatterers randomly distributed in a half-space shell while accounting for the frequency-dependent system response and beampattern effects. Individual scatterers are modeled using elongated spheroidal shapes with varying distributions of lengths and angles of orientation. The non-Rayleigh echo statistics of individual scatterers were found to contribute significantly to the non-Rayleigh characteristics of the echo pdf of aggregations of those individuals. This model is applied to estimate the numerical density of fish in aggregations observed using broadband signals (30–70 kHz) in the ocean. The results show the importance of incorporating realistic parameters for modeling individual scatterers in echo statistical analyses.

Deep-diving autonomous underwater vehicle provides insights into scattering layer dynamics

Organisms within deep scattering layers are often too densely packed to be ensonified as individuals using surface or seafloor based sensors, are too fast to be easily captured by research nets yet too small for most fishing gear, and are mixed with other individuals, making it difficult to interpret acoustic data from these ecologically important animals. To address this, we integrated a two-frequency, split-beam echosounder into an autonomous underwater vehicle (AUV) capable of flight at 600 m. As part of a study on whale foraging ecology off the Channel Islands, California, we flew the echosounders through scattering layers found at three depth ranges. Echoes were obtained from individual scatterers within layers being foraged upon by Risso's dolphins. Examining the echo statistics throughout layers revealed remarkable heterogeneity of echo strength and frequency response within layers that generally appeared homogeneous with respect to these same characteristics from ship-based echosounders. Some layers were internally layered but most features showed distinct, small patches of similar scatterers adjacent to those with different characteristics. The extensive horizontal coverage and near target sampling permitted by the AUV-based echosounders are providing a new understanding of the scales of biological organization within horizontally extensive scattering features.