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Bathymetric estimation from multiple synthetic aperture sonar (SAS) images is challenging for shallow water environments with low frequencies. This is due to the multipath echoes corrupting the desired direct-path echoes. In an attempt to suppress some of these multipaths, the MUD synthetic aperture sonar developed at TNO has a sixteen element vertical hydrophone array. In this paper we consider a number of different bathymetry estimation techniques using multielement vertical arrays. We introduce an ad-hoc multilook variant of the matrix pencil method and find that it outperforms beamformed interferometry, extending the range of useful bathymetry until the problematic multipath mode produces a similar angle of arrival that cannot be resolved. © 2012 ACM.

Background reverberation can obscure useful features of the target echo response in broadband low-frequency sonar images, adversely affecting detection and classification performance. This paper describes a resolution and phase-preserving means of separating the target response from the background reverberation noise using a coherence-based wavelet shrinkage method proposed recently for de-noising magnetic resonance images. The algorithm weights the image wavelet coefficients in proportion to their coherence between different looks under the assumption that the target response is more coherent than the background. The algorithm is demonstrated successfully on experimental synthetic aperture sonar data from a broadband low-frequency sonar developed for buried object detection. © 2014 Acoustical Society of America.

Synthetic aperture side-scan sonar (SAS) is a mature technology for high-resolution sea floor imaging [1]. Interferometric synthetic aperture sonars (InSAS) use additional hydrophones in a vertical array for bathymetric mapping [2]. This has created high-resolution bathymetry in deep water environments [3] but results in shallow water have been poor due to sea-surface multipath reducing the coherence [4]. This is exacerbated at low frequencies due to small vertical apertures constrained by the size of the sonar platform (e.g., towfish or AUV). In this paper, we analyse the performance of broadband minimum variance (MV) beamforming techniques for the reduction of some of the dominant multipath echoes from a low frequency synthetic aperture sonar operating in shallow water. We start in Section 2 discussing the MUD sonar since this is significantly different from other synthetic aperture sonars with its low frequency and wide beamwidth. We then consider the effects of multipath in Section 3. The signal processing techniques we used are described in Section 4 and the results are discussed in Section 5. Finally, conclusions and recommendations for future research are presented in Section 6.

Anisotropic materials are being used increasingly in high performance industrial applications, particularly in the aeronautical and nuclear industries. Some important examples of these materials are composites, single-crystal and heavy-grained metals. Ultrasonic array imaging in these materials requires exact knowledge of the anisotropic material properties. Without this information, the images can be adversely affected, causing a reduction in defect detection and characterization performance. The imaging operation can be formulated in two consecutive and reciprocal focusing steps, i.e., focusing the sources and then focusing the receivers. Applying just one of these focusing steps yields an interesting intermediate domain. The resulting common focus point gather (CFP-gather) can be interpreted to determine the propagation operator. After focusing the sources, the observed travel-time in the CFP-gather describes the propagation from the focus point to the receivers. If the correct propagation operator is used, the measured travel-times should be the same as the time-reversed focusing operator due to reciprocity. This makes it possible to iteratively update the focusing operator using the data only and allows the material to be imaged without explicit knowledge of the anisotropic material parameters. Furthermore, the determined propagation operator can also be used to invert for the anisotropic medium parameters. This paper details the proposed technique and demonstrates its use on simulated array data from a specimen of Inconel single-crystal alloy commonly used in the aeronautical and nuclear industries. © 2012 American Institute of Physics.

A method is presented for separating the broadband acoustic target response from the background seafloor reverberation based on the multi-look synthetic aperture sonar (SAS) image coherence. The method computes the coherence in the wavelet domain and uses this to distinguish between the target and background, which are assumed to be comparatively coherent and incoherent respectively. A wavelet shrinkage technique is applied to perform the separation whilst preserving phase and image resolution. The method is demonstrated on experimental data from a low-frequency broadband sonar developed for detecting buried objects. We also present preliminary results using a similar approach to obtain a coherence-based metric for seafloor complexity. Results are shown from a high-frequency imaging sonar.

Anisotropic materials are being used increasingly in high performance industrial applications, particularly in the aeronautical and nuclear industries. Some important examples of these materials are composites, single-crystal and heavy-grained metals. Ultrasonic array imaging in these materials requires exact knowledge of the anisotropic material properties. Without this information, the images can be adversely affected, causing a reduction in defect detection and characterization performance. The imaging operation can be formulated in two consecutive and reciprocal focusing steps, i.e., focusing the sources and then focusing the receivers. Applying just one of these focusing steps yields an interesting intermediate domain. The resulting common focus point gather (CFP-gather) can be interpreted to determine the propagation operator. After focusing the sources, the observed travel-time in the CFP-gather describes the propagation from the focus point to the receivers. If the correct propagation operator is used, the measured traveltimes should be the same as the time-reversed focusing operator due to reciprocity. This makes it possible to iteratively update the focusing operator using the data only and allows the material to be imaged without explicit knowledge of the anisotropic material parameters. Furthermore, the determined propagation operator can also be used to invert for the anisotropic medium parameters. This paper details the proposed technique and demonstrates its use on simulated array data from a specimen of Inconel single-crystal alloy commonly used in the aeronautical and nuclear industries.

In nondestructive testing, being able to remotely locate and size defects with good accuracy is an important requirement in many industrial sectors, such as the petrochemical, nuclear, and aerospace industries. The potential of ultrasonic guided waves is well known for this type of problem, but interpreting the measured data and extracting useful information about the defects remains challenging. This paper introduces a Bayesian approach to measuring the geometry of a defect while providing at the same time an estimate of the uncertainty in the solution. To this end, a Markov-chain Monte Carlo algorithm is used to fit simulated scattered fields to the measured ones. Simulations are made with efficient models where the geometries of the defects are provided as input parameters, so that statistical information on the defect properties such as depth, shape, and dimensions can be obtained. The method is first investigated on simulations to evaluate its sensitivity to noise and to the amount of measured data, and it is then demonstrated on experimental data. The defect geometries vary from simple elliptical flat-bottomed holes to complex corrosion profiles.

Since 2010, TNO has conducted a number of experiments on passive diver detection in Dutch waters. We have demonstrated detection, localization and tracking of divers wearing open circuit and closed circuit underwater breathing apparatus as well as boats. However, until now, we have not conducted any such experiments in warm waters. The soundscape in warm, coastal waters can be very different due to complex bathymetry profile, the presence of breaking waves and biological noise. In collaboration with Aruba Ports Authority, TNO conducted a series of measurements to evaluate the feasibility of passive diver detection and harbor protection in such an environment. Although the ambient noise level recorded is lower than in the Netherlands, it is dominated by snapping shrimp noise. These snaps constitute a non-Gaussian background noise, for which conventional signal processing techniques are not suited. In this paper, the effect of the snapping shrimp noise on the correlation, as well as the effect of de-noising techniques applied in the temporal and in the delay domain are demonstrated.

A PHD particle filter implementation has been detailed for the fusion of measurements from multiple passive sonar nodes. It has been demonstrated on simulated metadata and on experimental passive acoustic data of divers and small boats collected in an operational port environment. Fusion at the metadata level lowers the necessary data bandwidth, which has practical benefits for long baseline systems. Moreover, the data fusion resolves sensor ambiguities permitting the localisation of targets, and results in an overall improvement in detection performance and a reduction of false alarms. The simulated and experimental results show great promise for our approach to data fusion. However, several improvements can be made and this will be one of the focuses of future work, including: - Improved implementation based on random finite sets (e.g., the Gaussian-mixture PHD filter for efficiency [9] and the cardinalised PHD filter for better performance [10]); - Improvements to the assumed sensor and target models and priors; - Inclusion of more measurements from the acoustic data (e.g., signature features) and possible integration with additional sensor types; - Self assessment of system performance and adaptation (e.g., adjusting the assumed clutter rate based on the conditions, such as during periods of rain or heavy shipping traffic).

Surveillance of waterside locations for protection against threats from small fast surface vessels and underwater intruders is a very relevant but challenging problem. For this reason, the Netherlands Organisation for Applied Scientific Research (TNO) is developing SOBEK – a family of passive sonar technologies for waterside surveillance. The advantages of passive sonar are several-fold: it is low-cost, it allows covert operation, it is not harmful to underwater life, and the reverberation, which can pose significant challenges for active sonar in confined water, is absent. Furthermore, passive acoustic signatures provide a wealth of information for target classification. This paper presents recent developments in our SOBEK passive sonar research, including new results from a 2011 sea trial in the Dutch Navy harbour of Den Helder, and an overview of future system concepts.

Divers constitute a potential threat to waterside infrastructures. Active diver detection sonars are available commercially but present some shortcomings, particularly in highly reverberant environments. This has led to research on passive sonar for diver detection. Passive detection of open-circuit UBA (underwater breathing apparatus) has been demonstrated. This letter reports on the detection of a diver wearing closed-circuit UBA (rebreather) in an operational harbor. Beamforming is applied to a passive array of 10 hydrophones in a pseudo-random linear arrangement. Experimental results are presented demonstrating detection of the rebreather at ranges up to 120m and are validated by GPS ground truth

Ultrasonic array images are adversely affected by errors in the assumed or measured imaging parameters. For non-destructive testing and evaluation, this can result in reduced defect detection and characterization performance. In this paper, an autofocus algorithm is presented for estimating and correcting imaging parameter errors using the collected echo data and a priori knowledge of the image geometry. Focusing is achieved by isolating a known geometric feature in the collected data and then performing a weighted least-squares minimization of the errors between the data and a feature model, with respect to the unknown parameters. The autofocus algorithm is described for the estimation of element positions in a flexible array coupled to a specimen with an unknown surface profile. Experimental results are shown using a prototype flexible array and it is demonstrated that (for an isolated feature and a well-prescribed feature model) the algorithm is capable of generating autofocused images that are comparable in quality to benchmark images generated using accurately known imaging parameters.

A boom is a line of floats that constitutes a physical barrier for providing abovewater stopping power. However, it does not provide any situation awareness. In order to overcome this shortcoming, an integrated “booms and sensors” solution using passive sonar was conceived and tested experimentally by suspending hydrophones under a boom. The conducted study investigated the self-noise of the boom assembly, the motion of the boom and of the hydrophones, and the detectability of divers and boats. Experiments were conducted in late 2011 / early 2012 in Den Helder and in the Port of Rotterdam. The self-noise investigation showed that the boom produces transient signals that dominate the ambient noise on the hydrophone and limits the range of diver detection to about 100 m. This is sufficient to enable detection of an intrusion (i.e. a diver passing under the boom). Mitigation measures were implemented during the deployment of a 96 m boom equipped with 8 suspended hydrophones. The mitigation measures were effective and more than doubled the detection range. Two fixed acoustic sources were used to track the hydrophones motion and allowed localization of divers at 200 m. Tracking of the hydrophones was required to account for their large motion under the boom (up to several meters in a few seconds). A follow up study also demonstrated that the hydrophone tracking can be achieved with the requested accuracy using (surface) RTK GPS, thus enabling a fully acoustically passive operation of the system.

Divers, including closed circuit (rebreather) divers constitute a potential threat to waterside infrastructures. Active diver detection sonars are available commercially but present some shortcomings, particularly in highly reverberant environments. This has led to research on passive sonar for diver detection. Passive detection of open-circuit UBA (underwater breathing apparatus) has been demonstrated, but detection of rebreathers is more challenging due to their quiet operation. A study was conducted using an array of broadband hydrophones (100 kHz bandwidth). 10 hydrophones were arranged in a random array of 1.5 m baseline in order to achieve a favorable tradeoff between angular resolution (requiring a large array), side-lobe rejection (requiring low hydrophone spacing), and cost (requiring a limited number of sensors). The array was deployed from a pier in a basin that is nearby a busy shipping lane in the Port of Rotterdam. A diver equipped with an Inspiration Classic rebreather from Ambient Pressure Diving (a popular recreational rebreather) approached the array, towing a surface GPS that was used for validating detections. The collected data was processed using conventional beamforming and 4th order cumulant based beamforming. The detection of the rebreather at range up to 120 m in presence of shipping traffic was reported and validated against GPS ground truth. This result illustrates the potential of higher order statistics to improve passive acoustic detection. The relatively large detection range suggests that quieter military rebreathers may be detectable at closer range. Furthermore, the limited number of hydrophones used (10) compared to commercial active diver detection sonar, that include typically hundreds of sensors, indicates a strong potential for improvement.

The inspection of wall loss corrosion is difficult at pipe support locations due to limited accessibility. However, the recently developed ultrasonic Multi-Skip screening technique is suitable for this problem. The method employs ultrasonic transducers in a pitch-catch geometry positioned on opposite sides of the pipe support. Shear waves are transmitted in the axial direction within the pipe wall, reflecting multiple times between the inner and outer surfaces before reaching the receivers. Along this path, the signals accumulate information on the integral wall thickness (e.g., via variations in travel time). The method is very sensitive in detecting the presence of wall loss, but it is difficult to quantify both the extent and depth of the loss. If the extent is unknown, then only a conservative estimate of the depth can be made due to the cumulative nature of the travel time variations. Multi- Skip tomography is an extension of Multi-Skip screening and has shown promise as a complimentary follow-up inspection technique. In recent work, we have developed the technique and demonstrated its use for reconstructing high-resolution estimates of pipe wall thickness profiles. The method operates via a model-based full wave field inversion; this consists of a forward model for predicting the measured wave field and an iterative process that compares the predicted and measured wave fields and minimizes the differences with respect to the model parameters (i.e., the wall thickness profile). This paper presents our recent developments in Multi-Skip tomographic inversion, focusing on the initial localization of corrosion regions for efficient parameterization of the surface profile model and utilization of the signal phase information for improving resolution.

The quality of an ultrasonic array image, especially for anisotropic material, depends on accurate information about acoustic properties. Inaccuracy of acoustic properties causes image degradation, e.g., blurring, errors in locating of reflectors and introduction of artifacts. In this paper, for an anisotropic austenitic steel weld, an autofocus imaging technique is presented. The array data from a series of beacons is captured and then used to statistically extract anisotropic weld properties by using a Monte-Carlo inversion approach. The beacon and imaging systems are realized using two separated arrays; one acts as a series of beacons and the other images these beacons. Key to the Monte-Carlo inversion scheme is a fast forward model of wave propagation in the anisotropic weld and this is based on the Dijkstra algorithm. Using this autofocus approach a measured weld map was extracted from an austenitic weld and used to reduce location errors, initially greater than 6mm, to less than 1mm. © 2012 American Institute of Physics.

Situation awareness is an important component of security that justifies the equipment of sites with monitoring systems. For shore/harbour security, these systems are mostly composed of camera and radar, to monitor the land and the water surface, and of active sonar(s) when the underwater situation is monitored. An alternate approach for underwater monitoring is to use a network of passive sonars that detect targets by the sound they radiate. Unlike active sonar, a single passive sonar element cannot estimate the range of a target in the far field, only its direction; but it can characterize the sound radiated by a target, with obvious applications to target tracking and classification. Passive sonars, with frequency characteristics suitable for the target type of interest, have to be deployed as a network so that the source location can be estimated by appropriately crossing the estimated directions. TNO and the Stevens Institute of Technology are investigating this approach for diver detection and have conducted in 2010 a trial with the support of the Royal Netherlands Navy. A system involved in the trial presented a wide sampling dynamic and frequency range, making it suitable not only for diver detection but also for other forms of processing. Experimental results demonstrate that the deployed system can be used for diver detection and that its larger-than-required dynamic range enables other applications such as boat tracking and sound speed estimation, that can not only augment but improve the initial diver detection capability.

Divers and small vessels are increasingly recognized as a potential threat to high value assets. Harbour and waterside surveillance systems that are used to counter the threat of divers are usually based on active sonar, whose performance can be limited by reverberation in a harbour environment. Passive techniques are based on the detection of the sound emitted by the target. They are covert and yield information on the sound radiated by the detected source that can be used for classification. These advantages justify considering passive sonar as the basis for a waterside surveillance system, or as a supplement to a system based on active sonar. Experimental results illustrating some capabilities of passive techniques, such as the detection and tracking of divers and surface vessels using a network of passive nodes, are presented. The experiment was performed in collaboration with the Stevens Institute of Technology (NJ, USA). Possible uses of these techniques in either purely passive systems or to augment active systems are discussed.

The effectiveness and efficiency of mine-hunting operations with autonomous underwater vehicles (AUVs) are greatly influenced by environmental conditions, such as seabed, turbidity, currents, and tides. Therefore accurate environmental information is needed for the planning and evaluation of mine-hunting operations. Because environmental conditions can change rapidly and frequently with time and location, a priori planning is not optimal and can hinder the achievement of the mission objectives. It is therefore important to have a capability to monitor whether environmental conditions deviate from those used for the mission planning, and to evaluate the consequences on the effectiveness and efficiency of the mission. Based on this information, decisions can be made whether the mission plan needs to be modified, or whether changes to the AUV’s behaviour are required. We develop a technique for monitoring the mission progress based on in situ information. This information is subsequently used to perform optimization of the individual mine-hunting tasks. Such a process is referred to as utility-based optimization and is expected to be critical in achieving robust mine-hunting missions with AUVs with a predictable performance

The problem of estimating wind velocities from limited flight data recordings is considered, with application to sailplane flights in high-altitude mountain waves. Sailplane flight recorders routinely measure only GPS position and, infrequently, also airspeed and the problem is underdetermined. Maximum likelihood and maximum a posteriori estimators are developed for these kinds of data and tested by simulation and by application to sailplane flight data.