Sonar Imaging System Research Articles

Adaptive sonar call timing supports target tracking in echolocating bats.

Echolocating bats dynamically adapt the features of their sonar calls as they approach obstacles and track targets. As insectivorous bats forage, they increase sonar call rate with decreasing prey distance, and often embedded in bat insect approach sequences are clusters of sonar sounds, termed sonar sound groups (SSGs). The bat's production of SSGs has been observed in both field and laboratory conditions, and is hypothesized to sharpen spatiotemporal sonar resolution. When insectivorous bats hunt, they may encounter erratically moving prey, which increases the demands on the bat's sonar imaging system. Here, we studied the bat's adaptive vocal behavior in an experimentally controlled insect-tracking task, allowing us to manipulate the predictability of target trajectories and measure the prevalence of SSGs. With this system, we trained bats to remain stationary on a platform and track a moving prey item, whose trajectory was programmed either to approach the bat, or to move back and forth, before arriving at the bat. We manipulated target motion predictability by varying the order in which different target trajectories were presented to the bats. During all trials, we recorded the bat's sonar calls and later analysed the incidence of SSG production during the different target tracking conditions. Our results demonstrate that bats increase the production of SSGs when target unpredictability increases, and decrease the production of SSGs when target motion predictability increases. Furthermore, bats produce the same number of sonar vocalizations irrespective of the target motion predictability, indicating that the animal's temporal clustering of sonar call sequences to produce SSGs is purposeful, and therefore involves sensorimotor planning.

Thinned Array Design With Minimum Number of Transducers for Multibeam Imaging Sonar

A large number of transducers employed in the multibeam imaging sonar array significantly increases the hardware complexity, costs, and computational load. Although a great amount of work has been done to minimize the number of transducers in the array, one can verify that these methods do not provide optimal results when applied to the case of multiple beams. In this paper, a novel thinning method is proposed for multiple-beam conditions where the number of transducers is minimized by an iterative process. Optimal common transducers’ positions are selected by a wind-driven optimization (WDO) algorithm, and transducers’ weights are optimized by convex optimization in order to suppress the peak sidelobe level (PSLL). The performance of the proposed method is evaluated via simulations and experiments. Results show that the method is applicable to the minimization of the number of transducers under hundreds of beam conditions, and consequently reduces the complexity of the imaging sonar system with imaging performance still acceptable.

Sonar imaging in extremely shallow inshore environments from an unmanned surface vehicle

Sonar imaging systems and autonomous platforms are becoming smaller and more affordable. Consequently, their applications are expanding further into the civilian domain. A good example and the focus of this work is an autonomous survey system for aiding police dive teams during underwater crime scene investigations. In this paper, we describe our ultra portable and low-cost unmanned surface vehicle (USV) currently under development for the London Metropolitan Police Service. In particular, we concentrate on the challenges associated with operating the USV's sonar payloads in the extremely shallow waters associated with relevant operational scenarios; these include, canals, lakes, and reservoirs with water depths commonly in the range of only 0.5 m to 3 m. In these cases, a key challenge is the corrupting influence of multipath reflections between the floor and water surfaces. We have taken a pragmatic approach to this multipath problem that uses in-situ assessment of the sonar performance for optimal data acquisition and interpretation. Results are presented from experimental trials conducted in operational canal environments.

Detecting a nearshore fish parade using the adaptive resolution imaging sonar (ARIS): An automated procedure for data analysis

Recent developments in sonar imaging provide an efficient way of obtaining near video quality of free-swimming fish in hard to reach areas, e.g., permanent structures like docks and piers, and hard to see environments, e.g., highly turbid waters. However, processing large volumes of data output by sonar imaging systems remains a major challenge. In the present study, we developed an automated image processing procedure to process footage recorded for 59 consecutive hours using an Adaptive Resolution Imaging Sonar, ARIS Explorer 3000 (Sound Metrics INC) deployed at a fixed location. Our approach successfully counted large free swimming fish at a precision rate >94% and estimated sample volume with manual and automatic calculations being highly correlated (r=0.96). An auto-regressive time series model (of the sixth or higher order) with a zero-inflated Poisson distribution was used to estimate local abundance. Fish counts were low to zero during the first 31h of sampling, and a major influx of fish occurred in the last 28h. The observed pattern was co-incidental with local weather patterns: intermittent thunderstorms in the first 32h and relatively calm weather in the last 24h. Though thunderstorms limited our conclusions on fish-pier usage with tidal and diurnal cycles, it is apparent that weather conditions play a role in pier usage by large fish. Overall, the ARIS3000 deployment, automated data processing and statistical analyses used in this study proved successful in studying fish affiliations with piers in shallow habitats.

A Low-Complexity Real-Time 3-D Sonar Imaging System With a Cross Array

The development of real-time 3-D underwater imaging is restricted by the huge hardware cost and the computational burden associated with a large number of transducers. In this paper, a low-complexity real-time 3-D sonar imaging system with a cross array is proposed. The low complexity, in both hardware cost and computational load, is achieved by an innovative method of signal processing in the far field and the advantages of the cross array in minimal transducer numbers. The method consists of two parts: a multifrequency (MF) algorithm for the transmitting process and a parallel subarray (PS) algorithm for the receiving beamforming. The MF algorithm solves the real-time problem of the cross array by reducing the scanning time that is proportional to the number of transmissions. The PS algorithm improves the computational efficiency using a two-stage parallel and pipeline framework. The PS algorithm is an approximate method, and its precision is analyzed in this paper. The computational efficiency of the proposed method is compared with the direct method (DM) beamforming in both cross and planar arrays. A prototype, based on the innovative method, was designed and tested in pool and lake trials. The results demonstrate that the low-complexity system can satisfy the real-time requirement of 3-D underwater imaging applications with an acceptable imaging quality.

Three-dimensional space representation by echolocation in bats

Echolocating bats are equipped with a biological sonar system that permits spatial navigation and target tracking in complete darkness. By actively controlling the directional aim, timing, frequency content, and duration of echolocation signals to “illuminate” the environment, the bat directly influences the acoustic input available to its sonar imaging system. Detailed analyses of the bat’s sonar behavior suggest that the animal’s actions play into a rich 3-D representation of the environment, which then guides motor commands for subsequent call production, head aim, and flight control in an adaptive feedback system. Studies of the bat’s sonar behavior have motivated neurophysiological studies of the midbrain superior colliculus (SC), a structure implicated in sensorimotor integration and spatial orientation. Using multichannel silicon probe recordings from the freely echolocating big brown bat, we characterized response profiles of single neurons, as the animal tracked moving targets from a stationary position on a platform and in free flight. Our data show premotor activity prior to sonar vocalizations and responses to echoes from objects. Echo responses depended on both the azimuth and delay of sonar returns. These findings demonstrate both auditory and premotor specializations in the bat SC, which contribute to the animal’s 3-D representation of space.

Application Research of 3D Imaging Sonar System in Salvage Process

In the salvage process, data about the shape of shipwreck, sink sitting posture, buried degree and other related information should be required in order to develop salvage program. Three-dimensional imaging sonar is used to provide such data for salvage work by underwater acoustic detection and three-dimensional imaging technology. An application example is described and the results show the effectiveness.

Gain and Phase Autocalibration of Large Uniform Rectangular Arrays for Underwater 3-D Sonar Imaging Systems

In this paper, a new calibration method for gain and phase errors in large uniform rectangle arrays is proposed for underwater 3-D sonar imaging systems. It requires only one calibrator source at an unknown position in the far field. An efficient and speedy three-step-iteration algorithm is performed first to provide a robust direction-of-arrival (DOA) estimator in the presence of gain and phase errors. It then follows with the gain and phase errors estimation using a spatial matched filter. Finally, a maximum a posteriori (MAP) exercise is executed to further adjust the estimated phase parameters and the source direction. The statistical performance of the proposed algorithm will be demonstrated and compared to representative methods. It will be shown that the proposed method is computationally effective and asymptotically efficient in Monte Carlo simulations.

Observer bias and subsampling efficiencies for estimating the number of migrating fish in rivers using Dual-frequency IDentification SONar (DIDSON)

Fixed-location, side-looking, multibeam, sonar techniques offer a practical approach to estimate the numbers of migrating fish in rivers that are too large or occluded for traditional sampling methods, such as weir trapping, visual observation techniques, and netting. While this technology has been used to enumerate salmonid escapement in coastal river systems of western North America, little use and evaluation has occurred in inland waters such as the Great Lakes, where rivers and runs of fish are considerably smaller than those along the Pacific coast. We use a “Dual-frequency IDentification SONar” (“DIDSON”) imaging sonar system to investigate the error and variability among nine people performing fish counts. There was no significant difference found among observers’ estimates of fish abundance per DIDSON file; however, the total count of all fish differed from the benchmark value by as much as 26%. Post-processing simple fish counts from DIDSON raw data is labour-intensive and costly. Three subsampling methods of fish passage estimations were developed and evaluated for their accuracy and precision for daily and seasonal time frames. The random and systematic subsampling methods had similar seasonal and daily accuracy and precision with few exceptions. Automation-assisted counting was much more accurate and efficient for seasonal estimates. A ratio of approximately 2:1 was found for the automated to manual fish counts and this varied little among years. The DIDSON multibeam sonar unit is useful in estimating potamodromous fish migrations for large tributaries of the Great Lakes. DIDSON image processing costs can be minimized through suitable subsampling approaches. The automation-assisted method is the most cost-effective means of estimating moderate levels of fish passage over longer study periods. Multiple individuals can be used interchangeably for the manual post-processing of DIDSON data.

Active listening in a complex environment

Spatially guided behaviors in echolocating bats depend upon the dynamic interplay between auditory information processing and adaptive motor control. The bat produces ultrasonic signals and uses information contained in the returning echoes to determine the direction and distance of objects in space. With this acoustic information, the echolocating bat builds a 3-D auditory representation of the world, which it uses to guide a suite of coordinated motor behaviors, including head and pinna movements, as well as the timing, duration, frequency characteristics, and directionality of sonar signals. Adaptive echolocation behaviors shape the acoustic information available to the bat’s sonar imaging system and provide a window to its perception of complex scenes. In a complex environment, an echolocating bat encounters multiple reflecting surfaces that return a cascade of echoes from each sonar transmission. The work presented here will focus on adaptive echolocation behaviors of the big brown bat as it tracks a selected prey item in the presence of multiple objects, both obstacles and other prey. Data suggest that bats can successfully segregate streams of echoes from closely spaced objects through finely tuned adaptive sonar signal control.

Seafloor measurements using synthetic aperture sonar

The past decade has seen considerable growth in the use of synthetic aperture sonar (SAS) imaging systems in both the civilian and military domains. Although SAS systems are almost always uncalibrated, they can still yield information about the seafloor given an understanding of the mechanisms affecting the statistical properties of the images produced by these systems. This talk will describe our recent efforts to link SAS image statistics to seafloor properties through the use of seafloor scattering models. Sample results from several SAS systems encompassing frequencies ranging from 6 to 300 kHz will be shown.

Adapting the minimum variance beamformer to a graphics processing unit for active sonar imaging systems

The MVDR beamformer has been shown to improve active sonar image quality compared to conventional methods. Unfortunately, it is also significantly more computationally expensive because a spatial covariance matrix must be estimated and inverted for each image pixel. We target this challenge by altering and mapping the MVDR beamformer to a GPU, and suggest three different solutions depending on the system size. For systems with relatively few channels, we suggest arithmetic optimizations for the estimation step, and show how a GPU can be used to yield image creation rates of more than 1 Mpx/s. For larger systems we show that frequency domain processing is preferable, as this promotes high processing rates at a negligible reduction in image quality. These GPU implementations consistently reduced the runtime by 2–3 orders of magnitude compared to our reference C and Matlab implementations. For even larger systems we suggest employing the LCA beamformer. It does not calculate a weightset, but merely computes the beamformer output for each of a predefined set of weights, and selects the one that best fulfils the MVDR criterion. The LCA creates images with a quality comparable to MVDR, and it is perfectly suited for a GPU.

Optimized Simulated Annealing Algorithm for Thinning and Weighting Large Planar Arrays in Both Far-Field and Near-Field

In this paper, an optimized simulated annealing algorithm is proposed for thinning and weighting large planar arrays in both far-field and near-field of underwater 3-D sonar imaging systems. This optimized algorithm is designed for the large planar array with a fixed sidelobe peak and a fixed current taper ratio under a narrowband excitation. It applies the approximation for time delay in the near-field beam pattern, and extends the simulated annealing algorithm to both far-field and near-field by defining a new “energy” function. One example of large planar array was used to evaluate the accuracy and efficiency of the optimized method.

Structure Optimization of Phase-Coded Sounding Signals

Pulse compression with a small sidelobe level is one of challenges in designing of modern radar, sonar or ultrasound imaging systems. It may be performed by means of matched filter, that is by correlating the received sounding signal with a stored replica of the transmitted signal. The advantage of the pulse compression method is the increase of the average transmission power while retaining the range resolution corresponding to a short pulse. Phase coding is one of the early techniques for pulse compression of radar signals. Polyphase sequences, which have low sidelobe levels, ensure an easily detectable peak in the output of a matched filter, in other words an easy detection of a received sounding signal. In this paper, an evolutionary algorithm combined with a local optimizer is used to search for polyphase codes with a small sidelobe level of an aperiodic autocorrelation function. The evolutionary algorithm is based on a floating-point representation and the Gaussian mutation is used to produce ospring for the next generation. The self-adaptation mechanism is used to control the mutation operator during

Structure of the Nankai Trough accretionary prism revealed by IZANAGI oceanfloor imaging sonar system

Structure of the Nankai Trough accretionary prism revealed by IZANAGI oceanfloor imaging sonar system

3-D motion estimation by integrating visual cues in 2-D multi-modal opti-acoustic stereo sequences

Object reconstruction and target-based positioning are among critical capabilities in deploying submersible platforms for a range of underwater applications, e.g., search and inspection missions. Optical cameras provide high-resolution and target details, but their utility becomes constrained by the visibility range. In comparison, high-frequency (MHz) 2-D sonar imaging systems introduced to the commercial market in recent years can image targets at distances of tens of meters in highly turbid waters. Where fair visibility permits optical imaging at reasonable quality, the integration with 2-D sonar data can enable better performance compared to deploying either system alone, and thus enabling automated operation in a wider range of conditions. We investigate the estimation of 3-D motion by exploiting the visual cues in optical and sonar video for vision-based navigation and 3-D positioning of submersible platforms. The application of structure from motion paradigm in this multi-modal imaging scenario also enables the 3-D reconstruction of scene features. Our method does not require establishing multi-modal association between corresponding optical and sonar features, but rather the tracking of features in the sonar and optical motion sequences independently. In addition to improving the motion estimation accuracy, another advantage of the proposed method includes overcoming the inherent ambiguities of monocular vision, e.g., the scale-factor ambiguity and dual interpretation of motion relative to planar scenes. We discuss how our solution can also provide an effective strategy to address the complex opti-acoustic stereo matching problem. Experiment with synthetic and real data demonstrate the advantages of our technical contribution.

Optimized simulated annealing algorithm for thinning and weighting large planar arrays

This paper proposes an optimized simulated annealing (SA) algorithm for thinning and weighting large planar arrays in 3D underwater sonar imaging systems. The optimized algorithm has been developed for use in designing a 2D planar array (a rectangular grid with a circular boundary) with a fixed side-lobe peak and a fixed current taper ratio under a narrow-band excitation. Four extensions of the SA algorithm and the procedure for the optimized SA algorithm are described. Two examples of planar arrays are used to assess the efficiency of the optimized method. The proposed method achieves a similar beam pattern performance with fewer active transducers and faster convergence ability than previous SA algorithms.

Optimization of the Digital Near-Field Beamforming for Underwater 3-D Sonar Imaging System

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper proposes a novel approximation for the delays of digital near-field beamforming based on a planar array. An optimized algorithm for the underwater 3-D sonar imaging system based on the novel approximation will also be described. The algorithm is applied to both an evenly spaced planar array and an unevenly spaced planar array. A data path for the optimized algorithm will be illustrated. Comparisons of the memory and the computational requirements between the optimized algorithm and the direct method (DM) beamforming will be presented. Comparisons of the proposed approximation and the Fresnel approximation will also be demonstrated. This paper illustrates that the proposed approximation enlarges the validity region of the system's view scene as compared with the Fresnel approximation. The optimized algorithm has the advantage of reducing the memory requirements for parameter storage comparing with DM beamforming. </para>

Compensating for intensity loss in a large-aperture MIMO sonar imaging system

To reduce the computation burden of a large-aperture multiple-input multiple-output(MIMO) sonar imaging system,the phase-shift beamformer(PSBF) is used at the cost of bringing the intensity loss(IL).The cause of the IL is analyzed in detail and a variable termed as IL factor is defined to quantify the loss amount.To compensate for the IL,two methods termed as intensity compensation for the PSBF(IC-PSBF) and the hybrid beamforming(HBF),respectively,are proposed.The IC-PSBF uses previously estimated IL factors to compensate for output intensities of all PSBFs;and the HBF applies the IC-PSBF to the center beam region and the shifted-sideband beamformer(SSBF) to the side beam region,respectively.Numerical simulations demonstrate the effectiveness of the two proposed methods.

三次元水中映像取得装置の開発

In port areas, a wide-angle underwater imaging system has been in strong demand for wide-range inspections of undersea structures for periodic maintenance and for rapid checks in case of disasters. However, seawater in such areas is often turbid due to urban activities, which makes underwater imaging more difficult. We developed a three-dimensional imaging sonar system (hereinafter called System 08) for surveying of portside underwater constructions based on a prototype named System 07, which was for off-line viewing of 3-D images made in tank experiments. System 08 could be printed on-line 3-D acoustic image at 1/8 screen/0.125 sec. and surveyed height of a slope within ± 5 % or less in water tank measurement.