Backscatter Amplitude Research Articles

Suspended sediment flux statistics in unidirectional flow, using a pulse-coherent acoustic Doppler profiler

An experiment was conducted in the St. Anthony Falls Lab main channel flume, involving sand sediment (d50 = 0.4 mm) in unidirectional flow. The flow conditions were 1 m/s flow rate and 1 m depth, and the bed state consisted of quasi-linear sand dunes with ~1 m wavelength and 10-20 cm height. A pulse-coherent acoustic Doppler instrument (MFDop) measured high-resolution near-bed vertical profiles of velocity and backscatter amplitude at various positions spanning the dune profile. These measurements are used to obtain probability distributions of instantaneous suspended sediment concentration and flux, which are compared to predictions from two stochastic theories. While the stochastic theories were previously developed and validated for low transport rates over a flat bed, the acoustic measurements enabled measurements at a much higher transport rate; despite this, the new measurements remain in remarkably good agreement with the theories.

Validation of automated supervised segmentation of multibeam backscatter data from the Chatham Rise, New Zealand

Using automated supervised segmentation of multibeam backscatter data to delineate seafloor substrates is a relatively novel technique. Low-frequency multibeam echosounders (MBES), such as the 12-kHz EM120, present particular difficulties since the signal can penetrate several metres into the seafloor, depending on substrate type. We present a case study illustrating how a non-targeted dataset may be used to derive information from multibeam backscatter data regarding distribution of substrate types. The results allow us to assess limitations associated with low frequency MBES where sub-bottom layering is present, and test the accuracy of automated supervised segmentation performed using SonarScope® software. This is done through comparison of predicted and observed substrate from backscatter facies-derived classes and substrate data, reinforced using quantitative statistical analysis based on a confusion matrix. We use sediment samples, video transects and sub-bottom profiles acquired on the Chatham Rise, east of New Zealand. Inferences on the substrate types are made using the Generic Seafloor Acoustic Backscatter (GSAB) model, and the extents of the backscatter classes are delineated by automated supervised segmentation. Correlating substrate data to backscatter classes revealed that backscatter amplitude may correspond to lithologies up to 4 m below the seafloor. Our results emphasise several issues related to substrate characterisation using backscatter classification, primarily because the GSAB model does not only relate to grain size and roughness properties of substrate, but also accounts for other parameters that influence backscatter. Better understanding these limitations allows us to derive first-order interpretations of sediment properties from automated supervised segmentation.

Segment‐scale variations in seafloor volcanic and tectonic processes from multibeam sonar imaging, M id‐ A tlantic R idge Rainbow region (35°45′–36°35′N)

Along-axis variations in melt supply and thermal structure can lead to significant variations in the mode of crustal accretion at mid-ocean ridges. We examine variations in seafloor volcanic and tectonic processes on the scale of individual ridge segments in a region of the slow-spreading Mid-Atlantic Ridge (35o45'-36o35'N) centered on the Rainbow non-transform discontinuity (NTD). We use multibeam sonar backscatter amplitude data, taking advantage of multifold and multi-directional coverage from the MARINER geophysical study to create a gridded compilation of seafloor reflectivity, and interpret the sonar image within the context of other data to examine seafloor properties and identify volcanic flow fields and tectonic features. Along the spreading segments, differences in volcanic productivity, faulting, eruption style and frequency correlate with inferred magma supply. Regions of low magma supply are associated with more widely spaced faults, and larger volcanic flow fields that are more easily identified in the backscatter image. Identified flow fields with the highest backscatter occur near the ends of ridge segments. Their relatively smooth topography contrasts with the more hummocky, cone-dominated terrain that dominates most of the neovolcanic zone. Patches of seafloor with high, moderately high, and low backscatter intensity across the Rainbow massif are spatially correlated with observations of basalt, gabbro and serpentinized peridotite, and sediment respectively. Large detachment faults have repeatedly formed along the inside corners of the Rainbow NTD, producing a series of oceanic core complexes along the wake of the NTD. A new detachment fault is currently forming in the ridge segment just north of the now inactive Rainbow massif. This article is protected by copyright. All rights reserved.

Pemrosesan Citra Side Scan Sonar untuk Pemetaan Dasar Laut Pelabuhan Benoa

Detection of seabed can be obtained with a high resolution using side scan sonar instrument. This instrument also provides geospatial information of underwater target. This paper aims are to visualize and to interpret side scan sonar imagery both qualitatively and quantitatively, to analyse acoustic backscatter of seabed, and to perform processing characteristics of sonar signal using wavelet transform. Side scan sonar data are processed through the geometric correction to establish the actual position in the image pixel. The location of research study is Benoa Port, Bali Indonesia. The sonar surveys consist of several track lines survey with operating frequency of 325 kHz. The seabed texture consists of gravel with backscatter value range from 13824 mV to 23680 mV and 8832 mV to 13312 mV for sand, sediment ripples of 17792 mV to 32640 mV, dredging seabed of 8000 mV to 19330 mV, texture of mound area has 32640 mV, and rocky outcrop has 14845 mV to 23552 mV. The results analysis of the signal characteristics using wavelet transform generally defines the location of peaks and valleys of the backscatter amplitude for each texture. The peak magnitude of each texture in red color indicates the intensity of the seabed texture is more rough, complex and solid, while the seabed texture is relatively flat and soft physic material has a low magnitude the value in blue color.

Urban Areas Enhancement in Multitemporal SAR RGB Images Using Adaptive Coherence Window and Texture Information

In this paper, we present a technique for improving the representation of built-up features in model-based multitemporal synthetic aperture radar (SAR) RGB composites. The proposed technique exploits the multitemporal adaptive processing (MAP3) framework to generate an a priori information which is used to implement an adaptive selection of the coherence window size. Image texture is used to support the coherence information in case of decorrelation. The coherence information, powered by texture analysis, and combined with backscattering amplitude, provides a unique representation of built-up features. This allows for an immediate detection of urban agglomerates by human operators, and is an advantaged starting point for urban area extraction algorithms.

Quantifying Fish Backscattering using SONAR Instrument and Kirchhoff Ray Mode (KRM) Model

Sonar instrument was used to study backscattering from tuna fish. Extraction of target strength, incidence angle, and frequency dependence of the backscattered signal for individual scatterer was important for biological information. For this purpose, acoustic measurement of fish backscatter was conducted in the laboratory. Characteristics and general trends of the target strength of fish with special reference to tuna fish were investigated by using a Kirchhoff Ray Mode (KRM) model. Backscattering strength were calculated for the KRM having typical morphological and physical parameters of actual fish. Those backscattering amplitudes were shown as frequency, body length, backscattering patterns, the density and sound speed dependences, and orientation dependence. These results were compared with experimentally measured target strength data and good agreement was found. Measurement and model showed the target strength from the fish are depend on the presence of swimbladder. Target Strength increase with increasing the frequency and fish length.

Colour Sonar: Multi-Frequency Sidescan Sonar Images of the Seabed in the Inner Sound of the Pentland Firth, Scotland

The backscatter response of a seabed to an incident sonar signal is dependent on the carrier wave frequency: i.e., the seabed is acoustically colourful. Colour is implemented in a prototype three-frequency sidescan sonar system deployed in the Pentland Firth, north Scotland. Sonar amplitude data as a function of frequency are processed to render them an unconfounded effect of the seabed normalized to the response at a reference inclination angle, for colour to be a meaningful property of the seabed. Methods for mapping data at sonar frequencies to optical primary colours for human visualisation are explored. We recommend methods that in our opinion generate colour characteristics harmonious with human vision in which: shadow is white; saturation black; colour shade darkness is proportional to backscatter strength; and shades of red, green and blue are seen in proportion to the backscatter amplitudes of the low-, mid- and high-frequency sonar data. Frequency equalisation is applied to achieve a balance in colour responses in images. The seabed in the survey area is acoustically colourful. Using the “negative BGR” colour mapping method: a weakly backscattering sand dune in the north of the survey area appears as shades of light blue and purple; a strongly backscattering halo of cobbles around the dune appears as shades of hazel brown; a strongly backscattering gravel ridge across the south of the survey area appears as shades of royal blue; and exposed rock as textures ranging in colour from light brown to light blue/green. There is evidence for colour anisotropy (a dependence of colour on the direction of ensonification). Similarities between anthropic colour sonar and the natural sonar of Microchiropteran bats are noted. Bats’ sonar satisfies the information criteria for acoustic colour, and it is hypothesized that it informs a colourfully-perceived world view.

Segment-scale volcanic episodicity: Evidence from the North Kolbeinsey Ridge, Atlantic

The upper oceanic crust is produced by magmatism at mid-ocean ridges, a process thought to be characterized by cyclic bouts of intense magmatic activity, separated by periods when faulting accommodates most or even all of the plate motion. It is not known whether there is a distinct periodicity to such magmatic–tectonic cycles. Here we present high-resolution sidescan sonar data from the neovolcanic zone of the North Kolbeinsey Ridge, a shallow slow-spreading ridge where high glacial and steady post-glacial sedimentation rates allow relative flow ages to be determined with a resolution of around 2 kyr using backscatter amplitude as a proxy for sediment thickness and hence age. We identify 18 lava flow fields covering 40% of the area surveyed. A group of 7 flow fields showing the highest (and similar) backscatter intensity are scattered along 75 km of axial valley surveyed, suggesting that at least this length of the segment was magmatically active within a 1.2 kyr time window. Based on conservative age estimates for all datable flows and estimated eruption volumes, the post-glacial volcanic activity imaged is insufficient to maintain crustal thickness, implying that episode(s) of enhanced activity must have preceded the volcanism we image.

Observations of the surface of Titan by the Radar Altimeters on the Huygens Probe

Results from the radar altimeters on board the Huygens probe are reported, noting the content of data archived on the NASA Planetary Data System and its ESA counterpart. These instruments provide unique high-resolution information on the topography and electrical properties of the Titan surface over a ∼15km track across a boundary between a bright highland and the dark dissected alluvial terrain on which the probe landed. The highland appears ∼100m higher than the dark terrain. The dark terrain has a fairly high nadir radar backscatter, consistent with terrestrial lakebeds and alluvial surfaces, and shows small (5–10m) elevation fluctuations. Possible signatures of surface or volume scattering in the backscatter amplitude and intermediate frequency spectrum are discussed. A previously-undocumented characteristic of the Automatic Gain Control (AGC) on the flight altimeter is noted.

Measuring two-phase particle flux with a multi-frequency acoustic Doppler profiler.

A methodology is developed and tested for simultaneously extracting time-resolved one-dimensional profiles of the mass-concentration and velocity of two different particle types in a mixed suspension, using a multi-frequency pulse-to-pulse coherent Doppler instrument. The technique involves inversion of a model for frequency-dependent acoustic backscatter amplitude and phase. Results are presented from a laboratory settling column experiment, measuring a mixture of polystyrene beads (slowly-settling, strongly-scattering) and glass beads (quickly-settling, weakly-scattering) in a vertical pipe section.

A Simplified Formulation for Rough Surface Cross-Polarized Backscattering Under the Second-Order Small-Slope Approximation

We present simplified expressions for the cross-polarized backscatter of a randomly rough surface predicted by the second-order small-slope approximation (SSA2). The simplification is based on appropriate polynomial approximations of the SSA2 kernel function. We obtain numerically efficient expressions for the cross-polarized backscattering amplitude of a deterministic surface in the form of a single space integral involving only the surface elevation and the second (mixed) derivative of the surface elevation. The ensemble average normalized radar cross section is then derived under a Gaussian random process assumption for the surface. The resulting expression has the form of a Kirchhoff integral involving the roughness correlation function and its second- and fourth-order cross-derivatives. Further simplification is achieved for off-nadir observations using a high-frequency approximation; the result is an analytical formula involving only the resonant curvature and the radar-filtered mean square slope in the out-of-plane direction. A numerical validation of the simplified expressions is provided by comparison with exact SSA2 predictions in representative test cases. The dependence of cross-polarized backscattering on the incidence angle as well as wind speed and direction is then investigated for the case of a directional sea surface model. At near nadir incidence, a clear maximum in azimuth of the cross-polarized backscatter is observed for radar look directions 45 $^{\circ}$ from the wind direction.

Backscatter-difference Measurements of Cancellous Bone Using an Ultrasonic Imaging System.

Backscatter-difference measurements may be used to detect changes in bone caused by osteoporosis. The backscatter-difference technique measures the power difference between two portions of an ultrasonic backscatter signal. The goal of this study is to evaluate the feasibility of using an ultrasonic imaging system to perform backscatter-difference measurements of bone. Ultrasonic images and backscatter signals were acquired from 24 specimens of human cancellous bone. The signals were analyzed in the frequency domain to determine the normalized mean backscatter-difference (nMBD) and in the time domain to determine the normalized backscatter amplitude ratio (nBAR). The images were analyzed to determine the normalized pixel value difference (nPVD), which measures the difference in average pixel brightness between regions of interest placed at two different depths in the image. All three parameters were found to increase with bone mineral density. The signal-based parameters, nMBD and nBAR, correlated well with bone mineral density, yielding linear correlation coefficients that ranged from 0.74 to 0.87. The image based parameter, nPVD, performed somewhat less well, yielding correlation coefficients that ranged from 0.42 to 0.81. These results suggest that ultrasonic imaging systems may be used to perform backscatter-difference measurements for the purpose of ultrasonic bone assessment.

Estimation of total suspended solids concentration from aDcp backscatter and hydraulic measurements

ABSTRACTThis paper explores use of acoustic Doppler current profiler backscatter amplitude as a surrogate for total suspended solids concentration. Total suspended solids concentration and acoustic Doppler current profiler data were collected at the Lower Nelson River and Red River in Manitoba, Canada, during both ice-covered and open water periods such that acoustic Doppler current profiler backscatter data could be calibrated to predict TSS concentrations. Several novel calibration schemes were evaluated, including multiple linear regression models that involved flow parameters determined from acoustic Doppler current profiler data in addition to acoustic backscatter intensity. The inclusion of the average of the friction velocities of the channel bed and ice cover, calculated from time-averaged velocity data, in a multiple linear regression was found to improve model performance in both studied rivers and during both ice-covered and open water periods.

New methods for ADV measurements of turbulent sediment fluxes – application to a fine sediment plume

ABSTRACTNew methods have been developed to extract turbulent fluxes of momentum and fine sediments from Acoustic Doppler Velocimeter (ADV) data. The methods were validated with turbidity plume experiments. The ADV's backscatter amplitude signal was used to determine the sediment concentration and its turbulent fluctuations. However, different kinds of noise are found in the backscatter amplitude and the velocity signals, which are polluting the turbulent fluxes results. Therefore, spectral noise correction methods have been developed, which allow more accurate quantification of turbulent velocity and sediment concentration fluctuations. The techniques are applied to two benchmark cases of a vertical sediment-laden jet. Reynolds stresses, turbulent intensity of velocity and sediment concentration as well as turbulent sediment fluxes are shown to agree well with two-fluid plume measurements reported in the literature. The methods presented in this paper can be applied to the processing of measurements in cohesive sediment plumes, turbidity currents or mixing layers in the absence of, or in stable, flocculation.

Multi-sensor mapping of West African land cover using MODIS, ASAR and TanDEM-X/TerraSAR-X data

Land cover information plays an elementary role for regional water and land management, and is an essential variable for the assessment of ecosystem services and regional climate impact. This paper describes the generation of a regionally optimized land cover dataset for West Africa with a spatial resolution of 250m, which is based on earth observation data from three optical and radar instruments. The choice of sensors is based on their individual strengths and weaknesses in assessing specific land surface types. Annual profiles of the optical Moderate Resolution Imaging Spectroradiometer (MODIS) are analyzed for the classification of vegetated classes including agriculture. The classification approach builds on random forest classification with learning data extracted from higher resolution land cover maps. Envisat Advanced Synthetic Aperture Radar (ASAR) Wide Swath (WS) time series are used, in combination with MODIS data, to delineate permanent and seasonal water bodies. Here, an approach integrating threshold classification and morphological operations is applied. Built-up areas of different densities are identified based on a seamless coverage of radar imagery collected by the satellites TanDEM-X and TerraSAR-X. The detection of settlements is based on an unsupervised classification scheme which exploits texture metrics and backscattering amplitudes of the fine resolution radar sensors. The accuracy assessment of the multi-sensor land cover map yields an overall accuracy of 80% at legend level 1 (9 classes) and 73% at the more detailed legend level 2 (14 classes). Comparisons with available wall-to-wall datasets of the region demonstrate the valuable information content of the presented West African land cover map.

Acoustic scattering from submerged laminated composite cylindrical shells

The scattering of an oblique plane progressive monochromatic acoustic field upon a laminated composite cylindrical shell is studied based on the three-dimensional exact equations of anisotropic elasticity. Each layer of laminated structure is made of helically filament wound (fiber reinforced) homogeneous material whose degree of anisotropy is considered as monoclinic type. An approximate laminate model along with the local transfer matrix solution is used to solve the state space governing formulation within each layer. Considering the perfect bonding between the adjacent layers, the global transfer matrix is constructed as the product of the local transfer matrices employed to solve for the unknown scattering coefficients. The so-called back-scattered form function amplitude is evaluated for two cases of (strong anisotropy) Graphite/Epoxy and (weak anisotropy) Glass/Epoxy multilayered cylindrical shells, for different number of layers and different stacking sequence. Furthermore, the axisymmetric and non-axisymmetric dynamic behavior of the composite shells are studied by evaluating the directivity patterns of the far-field form function amplitude at the selected resonance frequency corresponding to the first overtone of n=2 vibration mode. Some limiting cases are considered and good agreements with the solutions available in the literature are obtained.

Comparison of supervised and unsupervised automatic classification methods for sediment types mapping using multibeam echosounder and grab sampling

Multibeam echosounder (MBES) systems are effective seabed mapping tools due to their simultaneous bathymetry and backscatter data acquisition. The acoustic response of the seafloor can be used to infer some of the physical characteristics of the sediment. However, further development and formalization of the methodology is still required. This investigation evaluates the ability to perform an automatic classification of sediment types with MBES data by using both unsupervised and supervised (with in situ sediment samples) digital image classification algorithms. The case-study focussed on an area near the Basque coast, northern Spain (SE Bay of Biscay). The seafloor morphological aspects were calculated from a digital elevation model derived from datasets acquired with a SeaBat 7125 MBES, while the backscatter information was recorded with an EM3002D MBES. The parameters considered for the automatic classification were seabed rugosity, slope, backscatter amplitude mean level and backscatter variance. A total of 58 sediment grab samples were used for supervised automatic classification training and validation. Results showed that supervised classification obtained higher precision than the unsupervised classification (76.9% and 30.8%, respectively) and higher reliability (0.7 and 0.2, respectively). According to these results, the unsupervised classification could be considered useful as a first estimate of the spatial distribution of seafloor types, but should only be used in studies where in situ samples are not available. In contrast, supervised classification demonstrated its ability to discriminate more sedimentary facies than the unsupervised classification and was especially effective in areas where the seabed displayed heterogeneous features and multiple sediment types. The result of this investigation confirms the potential of MBES and automatic classification algorithms for the production of classified maps of sedimentary types, with sufficient reliability for different applications, including management purposes.

EXAFS spectroscopic refinement of amorphous structures of evaporation-deposited Ge–Se films

EXAFS spectroscopic analysis is employed to elucidate the amorphous structure of evaporation-deposited GexSe100−x (x=30, 49 or 69 in at%) films. The difficulty in the determination of local structure resulting from the similar backscattering amplitude of Ge and Se atoms has been overcome through comparative analysis of film specimens subjected to controlled heat treatment. Our results indicate that the 4(Ge):2(Se) structural model, which satisfies the (8−N) rule, is valid for all of the compositions studied. However, the nearly equiatomic Ge49Se51 composition with 4(Ge):2(Se) amorphous structure crystallizes into orthorhombic GeSe crystalline phase with 3(Ge):3(Se) structural arrangements and quasi-crystalline Ge clusters. This significant difference in the local structure of amorphous and crystalline states is a distinctive signature of phase change compositions.

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.

Extended X-ray absorption fine structure data analysis of copper (II) hydroxamic acid mixed ligand complexes

The X-ray absorption spectra of copper mixed ligand complexes, having hydroxamic acid as one of the ligands, have been recorded at the K-edge of copper at BL-8 Dispersive EXAFS beamline at the 2.5 GeV INDUS-2 Synchrotron, RRCAT, Indore, India. For the analysis of EXAFS data, crystallographic data of the complex or of its analog is required, which is not available. Hence, for the analysis of EXAFS data, theoretical EXAFS data of the studied complexes have been generated using the EXAFS equation employing computer software program Mathcad. Firstly, the experimental data has been processed using the computer program Athena to obtain the normalized absorption versus energy data. From the experimental EXAFS data, the phase shift parameter (an energy independent constant 5) has been computed using Lytle, Sayers and Stern's (LSS) method. The backscattering amplitude has been taken from the available theoretical tabulations and other parameters have been taken from crystallographic data of the copper metal. Fourier transforms of both the experimental and theoretical data have been computed, and the two Fourier transforms are found to agree with each other for all the complexes. The position of the first peak in the Fourier transform gives the value of the first shell bond length, which is shorter than the actual bond length as a result of energy dependence of the phase factor (5(k)) in the sine function of the EXAFS equation. Since, the Fourier transform method and LSS method both are uncorrected for phase and other parameters of the EXAFS equation, the present method gives phase uncorrected bond length of the first coordination shell.