Effect Of Beam Pattern Research Articles

Echo grid integration: A novel method for preprocessing multibeam water column data to quantify underwater gas bubble emissions

AbstractWater column imaging multibeam echo sounder systems (MBESs) are a promising technology for quantitative estimates of the gas bubble volume flow within large gas seepage areas. Considerable progress has been made in recent years toward applicable calibration methods for MBESs as well as developing inversion models to convert acoustically measured backscattering cross sections to gas bubble volume flow. However, MBESs are still not commonly used for quantitative gas flow assessments. A reason for this is the absence of published processing methods that demonstrate how MBES data can be processed to quantitatively represent bubble streams. Here, we present a novel method (echo grid integration) that allows for assessing the aggregated backscattering cross section of targets within horizontal water layers. This derived value enables quantifying bubble stream gas flow rates using existing acoustic inversion methods. The presented method is based on averaging geo‐referenced volume backscattering coefficients onto a high‐resolution 3D voxel‐grid. The results are multiplied with the voxel volume to represent measurements of the total backscattering cross‐section within each voxel cell. Individual gridded values cannot be trusted because the beam pattern effects cause the values of individual targets to “smear” over multiple grid‐cells. The true aggregated backscattering cross‐section is thus estimated as the integral over the grid‐cells affected by this smearing. Numerical simulation of MBES data acquisition over known targets assesses the method's validity and quantify it's uncertainty for different, realistic scenarios. The found low measurement bias (< 1%), and dispersion (< 5%) are promising for application in gas flow quantification methods.

Effect of clamped beam pattern on resonant frequency shift of microresonator under near-infrared laser irradiation

In this study, we investigated the effect of the resonator beam shape on the thermal stress and the resonance frequency change when providing heat to the resonator beam by near-infrared laser irradiation. This investigation provides crucial data for optical and heat sensing application using microresonators. The microresonator beam with an optical absorber for near-infrared region was fabricated, varying the numbers of beams and corners. When irradiating the optical absorber with a laser, the resonant frequency decreased. It is because the photothermal effect provides compressive stress in the beam, resulting in resonant frequency decrease. The resonant frequency change rate increased with decreasing the number of beams. According to the analytical results, we confirmed that the analytical thermal stress determined the resonant frequency change rate in the same beam length. The corner remarkably decreased the resonant frequency change rate. The straight beams bend to the orthogonal direction by the thermal expansion of another straight part of the beam, resulting in deformation of the beam in the axial direction. Therefore, the reaction force and compressive stress in the axial direction reduces.

Synthesising the intrinsic FRB population using frbpoppy

Context. Fast radio bursts (FRBs) are radio transients of an unknown origin whose nature we wish to determine. The number of detected FRBs is large enough for a statistical approach to parts of this challenge to be feasible. Aims. Our goal is to determine the current best-fit FRB population model. Our secondary aim is to provide an easy-to-use tool for simulating and understanding FRB detections. This tool can compare surveys, or provide information about the intrinsic FRB population. Methods. To understand the crucial link between detected FRBs and the underlying FRB source classes, we performed an FRB population synthesis to determine how the underlying population behaves. The Python package we developed for this synthesis, frbpoppy, is open source and freely available. frbpoppy simulates intrinsic FRB populations and the surveys that find them with the aim to produce virtual observed populations. These populations can then be compared with real data, which allows constraints to be placed on the underlying physics and selection effects. Results. We are able to replicate real Parkes and ASKAP FRB surveys in terms of detection rates and observed distributions. We also show the effect of beam patterns on the observed dispersion measure distributions. We compare four types of source models. The “complex” model, featuring a range of luminosities, pulse widths, and spectral indices, reproduces current detections best. Conclusions. Using frbpoppy, an open-source FRB population synthesis package, we explain current FRB detections and offer a first glimpse of what the true population must be.

A wideband acoustic method for direct assessment of bubble-mediated methane flux

The bubble-mediated transport and eventual fate of methane escaping from the seafloor is of great interest to researchers in many fields. Acoustic systems are frequently used to study gas seep sites, as they provide broad synoptic observations of processes in the water column. However, the visualization and characterization of individual gas bubbles needed for quantitative studies has routinely required the use of optical sensors which offer a limited field of view and require extended amounts of time for deployment and data collection. In this paper, we present an innovative method for studying individual bubbles and estimating gas flux using a calibrated wideband from the Bolin Centre for Climate Research database: http://bolin.su.se/data/.and split-beam echosounder. The extended bandwidth (16 – 26 kHz) affords vertical range resolution of approximately 7.5 cm, allowing for the differentiation of individual bubbles in acoustic data. Split-aperture processing provides phase-angle data used to compensate for transducer beam-pattern effects and to precisely locate bubbles in the transducer field of view. The target strength of individual bubbles is measured and compared to an analytical scattering model to estimate bubble radius, and bubbles are tracked through the water column to estimate rise velocity. The resulting range of bubble radii (0.68–8.40 mm in radius) agrees with those found in other investigations with optical measurements, and the rise velocities trends are consistent with published models. Together, the observations of bubble radius and rise velocity offer a measure of gas flux, requiring nothing more than vessel transit over a seep site, bypassing the need to deploy time-consuming and expensive optical systems.

Investigating bubble transport and fate in the watercolumn with calibrated broadband split-beam echosounder data

The transport and eventual fate of gas bubbles in the marine environment is a topic of interest to researchers in numerous fields. Acoustic systems are commonly used to study bubble ebullition sites as they provide synoptic measurements of the watercolumn. However, the visualization of individual bubbles has traditionally required the use of point-source equipment, such as vehicle-mounted cameras. Here, we present an acoustic methodology for studying individual bubbles using a calibrated broadband split-beam echosounder. The extended bandwidth (14–24 kHz) provides a vertical resolution on the order of 10 cm, which allows for the discrimination of individual bubbles in the echogram. Split-beam phase differentiation provides phase-angle data which can be used to compensate for beam-pattern effects and precisely locate bubbles within the watercolumn. Bubble target strength is measured and compared to analytical models to estimate bubble radius, and bubbles are tracked through the watercolumn to estimate rise velocity. The resulting range of bubble radii (1–6 mm in radius) is similar to those found in other investigations, and the rise velocities are consistent with published models. Together, the observations of bubble radius and rise velocity offer a measure of gas flux.

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.

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.

Effects of electron beam patterns on melting and refining of silicon for photovoltaic applications

An electron beam melting (EBM) system was fabricated to investigate its effectiveness in the refining of metallurgical grade silicon. The metallurgical refining of silicon is a promising process for reducing the production cost of silicon feedstock for solar cells. Especially, EBM is an excellent technology for evaporating not only metal impurities such as Mg and Ca, but also non-metal impurities such as P, C and O. In this study, a Cu cold crucible was placed in a high vacuum chamber equipped with a 100-kW electron-beam (e-beam) gun. Silicon was charged without any pretreatment such as crushing or acid-leaching. Among the various process parameters, the e-beam pattern was a key to determine the purification efficiency. Two patterns of e-beam were applied, circular and spiral, and their effects on the microstructure and impurity removal were investigated. A different interface between the solid and liquid silicon phase was formed during the solidification depending on the e-beam patterns. Those results were identical to those of the numerical analysis. When a spiral pattern was applied, the horizontal interface was formed and grains grew vertically and uniaxially, which enhanced the impurity removal efficiency. The charged silicon purity of 99.806% was enhanced up to 99.996% after only 44 min of EBM and refining.

A review of Clarence Clay's research contributions in the area of fisheries acoustics

This presentation provides an overview of the significant contribution that Clay and his students made to the understanding of the statistical nature of the acoustic signals scattered from fish and the methods for removing the effect of the beam pattern from the received echo statistics to measure the underlying fish backscattering statistics. By making measurements of the acoustic scattering from live fish, he showed that the probability density function of the envelope amplitude of the echo signal scattered from the fish could be modeled by a Ricean PDF. He further showed that as the ratio of the length of the fish to the acoustic wavelength became large, the PDF became Rayleigh distributed. Clay and his students were interested in using the measured echo statistics to obtain information about the size distribution of the fish producing the scattering. They developed a method for deconvolving the effect of the acoustic beam pattern from the received echo statistics to provide an estimate of the fish sca...

Estimating numerical density of scatterers in monotype aggregations using the statistics of broadband echoes: Applications to fish echoes

The statistics of echoes from active sonar systems can yield important information on aggregations of scatterers. This study explores the use of echo statistics for estimating the numerical density of scatterers in monotype aggregations. Here, “monotype” refers to scatterers with the same scattering amplitude distribution in the considered frequency range. The signals are broadband, and the geometry involves direct paths between the sonar and the scatterers without interference from boundaries. Model probability density functions (pdf's) of envelope amplitudes of matched-filter outputs are numerically generated by varying the number of Rayleigh scatterers randomly-distributed in a half-space shell while accounting for the frequency-dependent system response, scatterer response, and beampattern effects. The shape of the echo pdf as observed by the sonar receiver is highly non-Rayleigh when there are few scatterers in the beam, and gradually approaches the Rayleigh distribution when the number of scatterers increases. This model is applied to broadband fish echoes (30-70 kHz) collected in the ocean through a best-fit procedure. The inferred numerical density of fish is comparable to the density estimated using corresponding measurements of volume backscattering strength and modeled target strengths.

Echo statistics: Pursuing one of Clay's visions

One of the first papers that Clay gave to one of the authors (TKS) when he entered Clay's office in 1980 was one of Clay's papers involving echo statistics and, specifically, accounting for beampattern effects in using single beam echoes from resolved fish to estimate their target strength and abundance. That, and a multitude of conversations with Clay helped propel TKS, and later DC, into a career where echo statistics was an integral aspect of their work. We will review our work on echo statistics associated with a variety of scatterers--the seafloor, sea ice, fish, zooplankton, and machined objects. A key aspect of this work has been connecting the physics of the scattering process and sonar parameters with parameters of the statistical functions (such as shape parameter).

Statistics of echoes from mixed assemblages of scatterers with different scattering strengths and numerical densities

Mixed assemblages are defined to describe the cases in which more than one type of scatterer are present and are randomly located and spatially interspersed among one another in each sonar resolution cell. The probability density functions (pdfs) formed by the echo envelopes in such cases can be highly non-Rayleigh and possess heavy tails. The shape of the pdf curves contains information for characterizing and discriminating the composition of mixed assemblages. A general characteristic-function-based mixed assemblage pdf model is formulated in this study. The model, which takes into account beampattern effects, was validated using numerical simulations. Simulated data of two-component mixed assemblages with different relative scattering strengths, numerical densities, and spatial distributions were used to compare the performance of this new mixed pdf model and the widely used weighted multiple component mixture pdf model. It was found that use of the latter model can lead to orders of magnitude errors in estimating the composition of the mixed assemblages. This study is inspired in the context of acoustic studies of mixed biological aggregations in the ocean, but the theory is generally applicable to other types of mixed assemblages as well.

A method for characterizing the biodiversity in bat pinnae as a basis for engineering analysis

A quantitative analysis of the interspecific variability between beamforming baffle shapes in the biosonar system of bats is presented. The data set analyzed consisted of 100 outer ear (pinna) shapes from at least 59 species. A vector-space representation suitable for principal component analysis (PCA) was constructed by virtue of a transform of the pinna surfaces into cylindrical coordinates. The central axis of the cylindrical transform was found by minimizing a potential function. The shapes were aligned by means of their respective axes and their center of gravity. The average pinna of the sample was a symmetrical, obliquely truncated horn. The first seven eigenvalues accounted already for two-thirds of the variability around the mean, which indicates that most of the biodiversity in the bat pinna can be understood in a more low-dimensional space. The first three principal components show that most of the variability of the bat pinna sample is in terms of opening angle, left–right asymmetry, and selective changes in width at the top or the bottom of the pinna. The beampattern effects of these individual components have been characterized. These insights could be used to design bioinspired beamforming devices from the diversity in biosonar.

Analysis of mixed assemblages of fish using the statistics of echoes from a single beam broadband echosounder.

Broadband echoes (30–70 kHz) were collected from fish aggregations with a downward‐looking single beam echosounder over Georges Bank near Cape Cod, MA in the fall of 2008. The echoes were collected in a short‐range, direct path geometry. Echoes from sequential pings occurring within sampling windows that lie entirely in a given patch were grouped to obtain the distribution of the magnitude of matched filtered echoes. Non‐Rayleigh distributed echoes were observed in patches both near the surface and bottom. Two‐component mixed probability density functions (pdf’s) were formed by accounting for beampattern effects and assuming various proportions of sizes of fish with correspondingly different scattering strengths and are used to fit the measured echo distribution. For each patch, parameters of the best fitting pdf’s for a series of sampling windows within the given patch were used to infer the fish size composition of the patch. The best‐fit parameters for the surface patches were different from those from near the bottom. Results of the acoustically inferred fish assemblages are compared with the net‐tow data and are used to investigate the biological significance of the fish aggregations.

Non-Rayleigh Echoes From Resolved Individuals and Patches of Resonant Fish at 2–4 kHz

In this paper, the statistics of the echo envelope measured from fish is studied over a range of scattering geometries in the frequency band 2-4 kHz. In this frequency band, not only are these particular swimbladder-bearing fish resonant (at about 3.7 kHz) resulting in strong echoes, but also their target strength is narrowly distributed at these frequencies. In addition, their size distribution is narrow. Thus, it can be assumed that the echo variability as measured through the receiver of the sonar system is principally due to beampattern effects of the sonar, spatial variability of the fish, and interference between overlapping echoes. The measurements were conducted over Georges Bank near Cape Cod, MA, using a broadband downward-looking echosounder in a short-range, direct-path geometry. Echoes from sequential pings were grouped according to whether they 1) involved resolved echoes from individual fish, 2) were entirely within a single patch of fish, 3) spanned the edge of a single patch of fish, or 4) spanned multiple patches of fish. The resultant distributions of echoes were compared with theoretical probability density functions (pdfs). The echoes are determined to be strongly non-Rayleigh, with beampattern effects a major factor, for all geometries except for the case when all echoes are from within a single patch. Recommendations are made for the appropriate echo pdf for these scattering geometries.

Formulation for statistics of echoes due to a finite number of scatterers and patches of scatterers in a directional sonar beam.

A general theoretical formulation is developed for echo statistics associated with a finite number of scatterers and patches of scatterers, each being randomly located in a directional sonar beam. This is the case of a direct-path sonar where there are no echoes from the seafloor and sea surface. The formulation is derived by combining the equations of Ehrenberg and colleagues (single scatterer randomly located in sonar beam) and Barakat (sum of finite number of random variables). Each scatterer or patch of scatterers is independent and can have its own echo probability density function (PDF) (before beampattern effects). Examples show that the PDF of the echo (as received by the sonar) is strongly non-Rayleigh due to beampattern effects and finite number of scatterers or patches. Numerical simulations are made for validation. [Work supported by ONR.]

Scintillation index of ocean surface forward scattered HF acoustic signals: Beam pattern and pulse length effects

The intensity fluctuations of surface bounce path measured during a field test near San Clemente Island in August 2002 have been presented previously. Signals used during this test were 20- and 40-kHz continuous wave (CW) pulses with 0.14- and 1.0-ms durations (‘‘short’’ and ‘‘long’’ pulses, respectively). Experimental results have been compared to predictions of a model by Yang and McDaniel [Waves Random Media, 1, 419–439 (1991); J. Acoust. Soc. Am. 91, 1960–1966 (1992)]. Their initial model assumes CW signals and omni-directional projectors and receivers. We observed a good fit between model and experimental results for the long pulse data when the specular point was in the main lobe of both projector and receiver. The discrepancies observed in the other cases were attributed to beam pattern and pulse length effects. We have extended the model to incorporate these two effects. [Work supported by ONR Code 321US.]

Processing of high-frequency multibeam echo sounder data for seafloor characterization

Processing simultaneous bathymetry and backscatter data, multibeam echosounders (MBESs) show promising abilities for remote seafloor characterization. High-frequency MBESs provide a good horizontal resolution, making it possible to distinguish fine details at the water-seafloor interface. However, in order to accurately measure the seafloor influence on the backscattered energy, the recorded sonar data must first be processed and cleared of various artifacts generated by the sonar system itself. Such a preprocessing correction procedure along with the assessment of its validity limits is presented and applied to a 95-kHz MBES (Simrad EM 1000) data set. Beam pattern effects, uneven array sensitivities, and inaccurate normalization of the ensonified area are removed to make possible further quantitative analysis of the corrected backscatter images. Unlike low-frequency data where the average backscattered energy proves to be the only relevant feature for discriminating the nature of the seafloor, high-frequency MBES backscatter images exhibit visible texture patterns. This additional information involves different statistical distributions of the backscattered amplitudes obtained from various seafloor types. Non-Rayleigh statistics such as K-distributions are shown to fit correctly the skewed distributions of experimental high-frequency data. Apart from the effect of the seafloor micro-roughness, a statistical model makes clear a correlation between the amplitude statistical distributions and the signal incidence angle made available by MBES bathymetric abilities. Moreover, the model enhances the effect of the first derivative of the seafloor backscattering strength upon statistical distributions near the nadir and at high incidence angles. The whole correction and analysis process is finally applied to a Simrad EM 1000 data set.

Correction on school geometry and density: approach based on acoustic image simulation

Models of simulated schools have been used to determine the intrinsic variability in echotraces due to beam pattern effect. This work concerns only morphometric and energetic parameters that can be extracted from echotraces. It appears that dST, the difference between school density and processing threshold is a key parameter, which directly influences the concerned detection angles. Relations, taking into account dST but also, Nb i, the relative school length image compared to the beam width, have been settled for the calculation of length and density corrections. In most cases, corrected values are obtained with errors less than respectively 5% for length and 0.5 dB for density (reverberation index), provided that the Nb i value is 1.5 or more. When Nb i is less than 1, it seems impossible to bring some pertinent corrections. The school energy does not need any correction. It is recommended to use threshold values not too low, to avoid detection through the side lobes. However this setting must be determined in order to obtain dST values greater than 10 dB. Thresholds between –60 and –65 dB seem adequate, at least for schools volume backscattering strength (VBS, Sv) values commonly encountered in the Bay of Biscay.

An evaluation of a steerable sidescan sonar for surveys of near-surface fish

A towed 330 kHz sector scanning sonar was evaluated for surveys of near-surface migratory salmonids ( Oncorhynchus sp.) and other fish in the Strait of Georgia near the mouth of the Fraser River near Vancouver, BC (Canada). Detection of large near-surface fish presents a serious challenge to conventional net trawls or echo-sounder surveys due to vessel avoidance behaviour by the fish. The sonar was towed beside a research vessel on a custom-built towfish at a nominal depth of 18 m. The optimum configuration was determined to be with the sonar scanning a 30° vertical sector oriented perpendicular to the tow direction, at a ping rate of 5 Hz and a sector scan period of 5 s. Fish were detected up to a range of 100 m, limited by systemic noise and backscattered reverberation from the sea surface. Some evidence of vessel-avoidance behaviour by the fish was observed. Near surface temperature and salinity gradients are shown to induce downwards refraction of the sonar beam, resulting in biases in apparent depth and target strength of the fish. A simple model of the sonar performance in this scanning mode, including beam-pattern effects, is used to generate curves of fish target detectability and beam-pattern induced bias as a function of range and fish target strength. After correction for refraction effects, beam-pattern bias, and detectability, the survey results show reasonable agreement with net trawl and riverine escapement data.