High-resolution Acoustic Imaging Research Articles

High-resolution acoustic imaging method based on block sparsity constraint

Acoustic imaging method is a critical task in various applications since it can locate the sound sources. However, the resolution of the method becomes low at low frequencies. This paper proposes a novel method to realize high-resolution acoustic imaging based on block sparsity constraint. By dividing the focusing area into blocks, the block sparse property of sound sources can be utilized to improve the resolution of the result. In this study, a block orthogonal matching pursuit method is introduced to deal with source localization problem with block sparsity constraint. To evaluate the performance of the proposed method, it is tested through numerical simulations and an experiment. The results of simulations prove that the proposed acoustic imaging method based on block sparsity constraint can localize sound sources accurately and performs better than that based on sparsity constraint. The results of the experiment further validate these findings.

Image analysis of acoustic data and interpretation of rock stress orientations for geothermal exploration in Gothenburg borehole GE-1, SW Sweden

Abstract This study contributes to geothermal exploration in 1660–1520 Ma old, reworked bedrock in Sweden. Our primary objectives are to constrain the orientation of horizontal stresses, and to discuss implications for geothermal exploration. High-resolution acoustic televiewer image data reveal the downhole distribution of stress indicators (borehole breakouts, drilling-induced fractures and petal centreline fractures) and pre-existing structures (natural fractures, foliation). About 135 m of stress indicators are measured from 0.2–1.0 km. The results suggest a uniform NNW–SSE mean maximum horizontal stress orientation. A total of 1525 pre-existing structures (natural fractures, foliation) are mapped in borehole GE-1. The prevailing stress regime controls whether natural fractures and foliation are well-oriented for stimulation. For strike-slip and normal faulting stress regimes, well-oriented fractures steeply dip towards the WSW. For a reverse faulting stress regime, shallow dipping fractures are well-oriented for stimulation. The downhole distribution of stress indicators and other stress measurements in the region and other parts of Fennoscandia tentatively suggest a strike-slip stress regime, but additional studies are needed to constrain the complete stress field at study depth and towards engineered geothermal systems reservoir target depths. Our secondary objective is to highlight that interpretation of high-resolution acoustic data, particularly in metamorphic crystalline rocks, is subjective and that more guidelines for data interpretation are needed. The interactive interpretation of the images is based on visual analyses of complex pre-existing structures and stress indicators with highly variable shapes. The application of three methods for data analyses in the GE-1 borehole proposes that drilling-induced fractures are little influenced by the method applied. Interpretations on individual borehole breakout azimuths may, however, result in over 10° differences in orientation.

High-resolution acoustic imaging method based on equivalent source method and reweighted l1 minimization

Sparse regularization has been successfully applied to equivalent source method (ESM) in order to improve the acoustic imaging resolution. However, the application is not always feasible, especially at low frequencies. To overcome the problem, this paper proposes a high-resolution acoustic imaging method. In this method, reweighted l1 minimization is introduced to ESM to deal with the ill-posed inverse problems. Then the obtained equivalent source strengths are used to locate the sound sources. Compared to the sparse regularization-based ESM, the proposed method can provide a low side lobe and higher spatial resolution of acoustic imaging. Meanwhile, by arranging equivalent sources in three-dimensional space, the proposed method can also realize the acoustic imaging in three-dimensional sound field with high resolution. The results of the simulation and experiment demonstrate the validations.

A study of model selection techniques for modeling synthetic aperture sonar backscatter statistics

High resolution acoustic imaging of the seafloor, such as with synthetic aperture sonar, can reveal complex environments due to the ability to resolve scatterers of different types. The statistical distribution of the backscattered field is appropriately modeled using a mixture distribution, which consists of a sum of a K pdf for each scatterer type, weighted by the relative frequency with which they occur. In this type of modeling, the number of distributions must be selected before parameters can be fit, and as the number of components increases, so does the danger of overfitting the data. Several methods of model selection are explored here. Two methods, the Bayesian information criterion and the Akaike information criterion are based on point parameter estimates, and a penalty due to the number of model parameters. The other two, the deviance information criterion and the Watanabe-Akaike information criterion, are based on Monte-Carlo sampling of the model parameter space. These four model selection criteria are compared to each other, and to the authors manual selection of the number of components.

Waiting for the right time and tide: The fine-scale migratory behavior of river herring in two coastal New England streams

Abstract Objective Alewife Alosa pseudoharengus and Blueback Herring A. aestivalis, collectively referred to as river herring, are anadromous clupeids inhabiting the North American Atlantic coast. Although the seasonal patterns of their spawning migration are well studied and predictable, the fine-scale movements are still poorly resolved. Methods This study used high-resolution acoustic imaging to study the fine-scale behavior of river herring during their spring spawning migration in two coastal rivers in Massachusetts, United States. Result Observations suggest that seasonal patterns of migration appear to be influenced by temperature, while fine-scale patterns in migration were associated with the time of day, tidal state, and their interaction. River herring were consistently observed during both day and night, with elevated peaks of activity in the early morning and late afternoon. Daytime movement consisted of the intermittent passage of large, dense schools, while nighttime movement consisted of the frequent passage of individuals or small groups. The highest numbers of herring observations were associated with flood and high tides. Additionally, the interactions between time of day and tidal state resulted in synergistic positive effects during crepuscular incoming tides, which were favorable to movement, and synergistic negative effects during midday low tides, which inhibited fish movement. Conclusion The changing relationship between the time of day and tidal state within the season manifested in changing periodicity in fish movement to correlate with favorable movement conditions. Since movements may be related to both time of day and tide, previous run size estimates from visual counts made only during the day may be too low. Sampling methodologies that collect information during all 24 h would likely produce the most accurate run size estimates. This study highlights the need to view river herring migration in a holistic context predicated by multiple environmental and biological factors that vary at several temporal scales (i.e., seasonally, daily, and hourly).

Seismic and medical ultrasound imaging of velocity and density variations by nonlinear vectorial inverse scattering.

We present an iterative nonlinear inverse scattering algorithm for high-resolution acoustic imaging of density and velocity variations. To solve the multi-parameter nonlinear direct scattering problem, the acoustic wave equation for inhomogeneous media in the frequency domain is transformed into a vectorial integral equation of the Lippmann-Schwinger type for the combined pressure and pressure-gradient field. To solve the multi-parameter nonlinear inverse scattering problem, we use the Newton-Kantorovich method in conjunction with matrix-free representations of the Fréchet derivative operators and their adjoints. The approximate Hessian information that is accounted for in our iterative solution of the (nonlinear) multi-parameter inverse scattering problem is essential for the mitigation of multi-parameter cross talk effects. Numerical examples related to seismic and medical ultrasound breast imaging illustrate the performance of the new algorithm for multi-parameter acoustic imaging.

Correlating Surface and Downhole Perforation Entry Hole Measurements Leads to Development of Improved Perforating Systems

Summary Studies have shown that achieving a consistent perforation hole size in casing (i.e., entry hole) and zero-phase perforation gun orientation led to improved treatment distribution among multiple perforation clusters in plug-and-perf limited entry treatments. In addition to reducing variation in the perforation entry hole by establishing uniformity in gun clearance and the angle of incidence of the perforation jet at the wall of the casing, oriented perforating has been shown to minimize the tendency of proppant to separate from the fracturing fluid while traveling across the perforated intervals (inertial effect) and mitigate nonuniform entry hole erosion due to gravity-induced proppant stratification. The primary goal of this study was to determine the controllable perforating gun elements and accessories that effect the accuracy of gun orientation and entry hole dimensions. Surface tests were conducted at manufacturing facilities for determining the characteristics of the entry holes in pipe produced by shaped explosive charges using various system configurations and the robustness of various gun orientation devices. Promising perforating systems were then used in wellbores to create calibration entry holes (downhole tests) that were measured for equivalent diameter and orientation accuracy using high-resolution acoustic imaging before conducting treatments. This process enabled components of the perforating system influencing entry hole size and gun orientation to be evaluated and modified, as necessary. Elements of the perforating system and downhole environment that influenced entry hole size and consistency included casing type, cement sheath characteristics, perforating gun clearance and orientation, perforating charge type and density, packing arrangement of multiple charges, charge tube and charge carrier design, gun detonation system, hydrostatic pressure, and locking devices. Achieving tight control of these elements significantly reduced variation in entry hole size. Deviations from surface and downhole testing results were commonly attributed to using perforating system elements in the field that differed from those used by the manufacturer in surface testing. Factors affecting gun orientation accuracy and consistency included weight bar type, gun string length, weight, and stiffness, the presence of modified standoff bands, progressive gun deformation during firing, wellbore tortuosity, and self-orienting devices. Several orientation systems were found that achieved orientation within the target 20°-window. To assess the value of this workflow process, the paper includes information on the results of diagnostic tests for evaluating the accuracy of the ultrasonic measuring device, the derivation process used for determining coefficients for a two-component perforation erosion model, and the use of the derived erosion rate coefficients for computing the mass of proppant that enters each perforation and perforation cluster during a fracturing treatment.

First Hexa-Combo Logging-While-Drilling Run in Kuwait: A Case Study

Directional wells through the 6-in. production-hole sections in the Marrat Reservoir of the Jurassic formations have traditionally required several wireline-logging and hole-conditioning runs for comprehensive petrophysical interpretation and completion design. As the planned well inclinations increase to maximize reservoir exposure and sweep efficiency, wireline deployment poses significant challenges due to the increased risk of losing the bottomhole assembly (BHA) in the hole. Over time, logging-while-drilling (LWD) tools have become preferable for the asset team, where the tools are run either in the drilling BHA or during a dedicated wiper trip after the section has been drilled to total depth (TD). Using LWD tools in this application also reduces well delivery times and costs. A comprehensive logging solution was required to drill the 6-in. reservoir section of a study well. The complex LWD string, consisting of gamma ray, resistivity, neutron porosity, azimuthal density, azimuthal sonic, and nuclear magnetic resonance (NMR) tools, was deployed on a motorized rotary steerable system (MRSS) BHA. In addition, a prototype high-resolution acoustic imaging and caliper tool, designed to be run in both water-based mud (WBM) and oil-based mud (OBM), was also included. The acquired logging data were used for enhanced formation evaluation. Fracture and borehole breakout interpretation from the image data played a key role in the successful completion design. This ultimately led to Kuwait’s first successful “hexa-combo” LWD drilling run and the country’s first LWD ultrasonic imaging tool run in OBM in this hole size, with 13.3 ppg OBM with a maximum downhole temperature of 275°F.

Achieving high-resolution 3D acoustic imaging in a large-aspect-ratio cabin by the non-synchronous measurements

The resolution of conventional acoustic imaging methods in a large-aspect-ratio cabin is reduced seriously by array parameters limitation and reverberation. The non-synchronous measurements technique breaks through the limitation of array shape and the number of microphones. This method can approximate the result of synthetic large-aperture array acoustic imaging. The increase in the number of microphones and low-rank constraint assumption in non-synchronous measurements suppress the reverberation interference. The low-rank constraint filters out the eigenvalues of the reverberation sound other than the main noise sources, which is acted as an eigenvalue filter. It has been demonstrated that non-synchronous measurements with 3D beamforming can improve the resolution of acoustic imaging in cubic space, but the related research on the large-aspect-ratio cabin is still missing. The non-synchronous measurements method is applied to achieve high-resolution acoustic imaging in a large-aspect-ratio cabin in this paper. The array measurement scheme is investigated comprehensively, including the number of non-synchronous measurements, the array step distance and appropriate spatial basis functions. The experimental results show that the non-synchronous measurements can achieve high-resolution 3D acoustic imaging in the cabin of an urban rail vehicle.

Grid-moving equivalent source method in a probability framework for the transformer discharge fault localization

The transformer discharge fault localization is important for the power supply reliability. The audible sound radiated by the transformer can reflect the extensive operation information. Thus, discharge faults can be localized by high spatial resolution acoustic imaging techniques to facilitate the inspection and maintenance of power equipment. In this paper, the grid-moving equivalent source method (GMESM) is proposed to visualize the discharge fault on a transformer, where the equivalent source grids can approach the discharge fault position adaptively. A probability framework is established in GMESM. The noise power can be firstly estimated by the Bayes rule to avoid an additional measurement of the background noise. Then, the expectation maximization algorithm is used to optimize the proposed model. Two experiment cases are used to validate the proposed method. It turns out that the proposed GMESM can visualize the discharge fault on the transformer with high spatial resolution and accuracy.

Unsupervised learning of platform motion in synthetic aperture sonar.

Synthetic aperture sonar (SAS) provides high-resolution acoustic imaging by processing coherently the backscattered signal recorded over consecutive pings as the bearing platform moves along a predefined path. Coherent processing requires accurate estimation and compensation of the platform's motion for high quality imaging. The motion of the platform carrying the SAS system can be estimated by cross-correlating redundant recordings at successive pings due to the spatiotemporal coherence of statistically homogeneous backscatter. This data-driven approach for estimating the motion of the SAS platform is essential when positioning information from navigational instruments is absent or inadequately accurate. Herein, the problem of platform motion estimation from coherence measurements of diffuse backscatter is formulated in a probabilistic framework. A variational autoencoder is designed to disentangle the ping-to-ping platform displacement from three-dimensional (3D) spatiotemporal coherence measurements. Unsupervised representation learning from unlabeled data offers robust 3D platform motion estimation. Including a small amount of labeled data during training improves further the platform motion estimation accuracy.

Super-resolution acoustic focusing based on the particle swarm optimization of super-oscillation

The spatial resolution of conventional waves is restricted by the diffraction limit of half wavelength. Hence, how to construct super-resolution acoustic beams with a smaller focal radius is one of the major challenges in recent studies. In the present paper, the super-resolution acoustic focusing method is proposed based on the superposition of multi-frequency super-oscillation beams and the Particle Swarm Optimization (PSO), which can improve the spatial resolution concurrently with good controllability. Based on the diffraction effect of traditional ultrasound fields, the acoustic lens of Fresnel zone plane (FZP) at the center frequency is designed by the half-wave zone method. Multiple acoustic beams at several preset frequencies within the transducer bandwidth are sent out to build the super-oscillation focal area by the pressure superposition. The radius of the super-resolution focal spot constructed by the PSO algorithm with optimized amplitudes and phases is less than the half wavelength at the center frequency, which is even smaller than the focal radius at the highest frequency. Furthermore, the focal radius is also proved to decrease with the increase of the number of multiple frequencies and the center frequency. The favorable results demonstrate the feasibility of super-resolution acoustic focusing based on the PSO of super-oscillation, and provide an applicable strategy for the high-resolution acoustic imaging and manipulation.

Acousto-optical metasurfaces for high-resolution acoustic imaging systems

This work introduces a paradigm for acoustic imaging in which a metasurface converts the acoustic waves scattered by remote scenes into coherent light focused into images by conventional optical cameras. The metasurface is composed of acousto-optical unit cells that sense the local acoustic pressure and use the resulting signal to modulate the amplitude of the electric field produced by a laser on an optical aperture. We derive the general design requirements for the image reconstruction in the optical domain and validate the concept through acoustic field measurements of the ultrasound scattered from an object submerged in water followed by numerical simulations predicting how this field is processed by the metasurface and camera. We show that this approach has two main advantages compared to traditional acoustic imaging systems. First, the acoustic-to-optic wavelength down-conversion leads to effective acoustical apertures very large compared to the physical size. Second, the unit cells are not synchronized electronically and thus the complexity of the metasurface increases only linearly with the number of unit cells, which is a significantly slower increase compared to conventional synchronized arrays. This work shows that these advantages lead to compact acoustic cameras providing image resolutions higher than possible with conventional acoustic imaging methods.

Motion Compensation for OFDM Inverse Synthetic Aperture Sonar Imaging Based on Compressed Sensing

Inverse synthetic aperture sonar (ISAS) is the major instrument to observe moving underwater vehicles with high-resolution acoustic images. In this article, we consider the cyclic prefix (CP)-based orthogonal frequency-division multiplexing (OFDM) ISAS imaging for a noncooperative underwater vehicle. Compared to the ISAS using the conventional linear frequency-modulated (LFM) waveform, the OFDM signal is more sensitive to Doppler shifts/errors. The fluctuation of acoustic velocity in the propagating medium brings random phase errors and leads to heavily blurred images. Existing ISAS imaging methods are based on the translational motion of the target, and these methods suffer from defocusing in severe ocean conditions. In this article, we propose the motion compensation algorithm for the OFDM ISAS imaging to minimize the effect of velocity variations caused by ocean turbulence. The sparsity of the ISAS image is utilized as a global metric for the phase error estimation and image generation by constructing an <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> -norm optimization. It is demonstrated that inter-range-cell interference (IRCI)-free ISAS images can be obtained no matter whether there are phase errors and/or strong noise. Simulation results indicate the effectiveness of the proposed method.

Acoustical ghost imaging

Ghost imaging, which is an indirect imaging technique that uses correlations between two beams to retrieve the information of an object, is applicable to any wavelength. It has been performed with a growing range of light sources, including x-rays, the visible, and the infrared, and has also been extended to atoms and electrons recently. In this Letter, we demonstrate acoustical ghost imaging using the correlation between random acoustic patterns, which are generated by the phase and delaying manipulation of a transducer array and the total transmitted intensity signals. Furthermore, the reconstruction quality and resolution can be further improved by compressed sensing. This study shows a demonstration of ghost imaging using mechanical waves. Moreover, this approach requires fewer computational resources and offers great promise for high-speed and high-resolution acoustical imaging.

A High-Resolution and Low-Frequency Acoustic Beamforming Based on Bayesian Inference and Non-Synchronous Measurements

Beamforming is a powerful technique to achieve acoustic imaging in far-field. However, its spatial resolution is strongly blurred by the point spread function (PSF) of phased microphone array. Due to the limitation of array aperture and microphone density, the PSF is far from Dirac delta function, so that it is difficult to obtain a high-resolution beamforming image at low-frequencies (e.g.500-1500Hz). This paper proposes a Bayesian inference method based on Non-synchronous Array Measurements (Bi-NAM) so as to refine the PSF and break through the beamforming limitation for low-frequency source imaging. Firstly, by sequentially moving prototype array at different positions, the non-synchronous measurements can get a sizeable synthetic aperture and high density of microphones. The synthetic cross-spectrum matrix (CSM) can significantly improve the beamforming performance. To confine the approximation error of synthetic CSM and the uncertainty of forward model, as well as the noise interference, a Bayesian inference based on joint maximum a posterior (JMAP) is proposed to solve an ill-posed inverse problem. A Student-t prior is employed to enforce the sparse property of acoustic strength distribution. The background noise can be adaptively modeled by the Student-t distribution, which is related to some of the typical symmetric distributions. Then the hyper-parameters in JMAP inference are efficiently estimated by the Bayesian hierarchical framework. Through experimental data, the proposed Bi-NAM approach is confirmed to achieve high-resolution acoustic imaging at 1000Hz and 800Hz, respectively, even under the Laplace noise interference.

Synchronous 3-D Imaging and Velocity Estimation of Underwater Targets Using Pulse-Pair Acoustical Imaging Technique

Velocity, position, and geometry are key parameters for sonar applications in underwater target detection, classification, and tracking. Current underwater 3-D acoustical imaging techniques can provide much more geometric details of the underwater targets by high-resolution acoustic images. However, they cannot conduct target velocity estimations simultaneously. Kinematic information (i.e., Doppler shifts) of the targets has been lost during imaging since these techniques are essentially developed for obtaining static images. Therefore, this article proposes a “pulse-pair” acoustical imaging (PPAI) technique to acquire geometry, velocity, and distance information of underwater targets simultaneously in a single ping. Instead of transmitting a single pulse to collect echoes from the underwater environment like the conventional acoustical imaging techniques, a “pulse-pair” transmission scheme is proposed to realize 3-D underwater imaging in the PPAI method. The radial velocities of underwater targets are estimated in all target-existing volumetric cells by a pulse-pair autocorrelation method. Considering that a pulse-pair transmission should cause duplication and a mixture of the original image (image blurring), a deconvolution algorithm is adopted to correct the mixed image. The performance of the proposed method is evaluated with a high-resolution 3-D PPAI prototype in a simulation underwater 3-D scenario. Simulation results show that 3-D geometry and velocity features, as well as distances of underwater targets, can be effectively acquired in a single ping with the proposed method. It is significant for sonar applications to avoid possible changes in target features over time due to the relatively long propagation time of sound.

Compressive synthetic aperture sonar imaging with distributed optimization.

Synthetic aperture sonar (SAS) provides high-resolution acoustic imaging by processing coherently the backscattered acoustic signal recorded over consecutive pings. Traditionally, object detection and classification tasks rely on high-resolution seafloor mapping achieved with widebeam, broadband SAS systems. However, aspect- or frequency-specific information is crucial for improving the performance of automatic target recognition algorithms. For example, low frequencies can be partly transmitted through objects or penetrate the seafloor providing information about internal structure and buried objects, while multiple views provide information about the object's shape and dimensions. Sub-band and limited-view processing, though, degrades the SAS resolution. In this paper, SAS imaging is formulated as an ℓ1-norm regularized least-squares optimization problem which improves the resolution by promoting a parsimonious representation of the data. The optimization problem is solved in a distributed and computationally efficient way with an algorithm based on the alternating direction method of multipliers. The resulting SAS image is the consensus outcome of collaborative filtering of the data from each ping. The potential of the proposed method for high-resolution, narrowband, and limited-aspect SAS imaging is demonstrated with simulated and experimental data.

Calvin F. Quate (1923-2019).

Nanoscience pioneer who invented advanced microscopes

Theoretical and experimental study on multibeam synthetic aperture sonar.

High-resolution imaging method is one of the researching focuses of underwater acoustic detection. Underwater small-target detection also requires detailed imaging technology. Multibeam echo sounders (MBESs) and synthetic aperture sonar (SAS) are the effective instruments widely researched to obtain underwater acoustic images. Constrained by the theory, the along-track resolution of MBES decreases with distance and the gaps problem of SAS always exists and both inevitably limit the quality of acoustic imaging. In this paper, a two dimensional multibeam synthetic aperture sonar (MBSAS) model is designed to overcome the shortcomings of conventional underwater imaging instruments. MBSAS can provide a three dimensional (3D) high-resolution acoustic image without a gap problem. An echo model and transducer array manifold are designed to meet the requirements of engineering applications. Imaging theory and target simulations prove the feasibility of the MBSAS model. The performance of the proposed model is demonstrated with a tank experiment. A detailed image is obtained through an experiment that can indicate the shapes of targets and has the ability to separate adjacent targets. The simulations and experimental results indicate that MBSAS can obtain a more detailed 3D full-scan image than conventional MBES and SAS system with a better energy focusing ability.