Roy Edgar Hansen

• Orescanin, Marko; Olson, Derek R.; Duvio, Dalton; Jennings, Haley; Geilhufe, Marc & Hansen, Roy Edgar [Show all 7 contributors for this article] (2026). Physically Motivated Distributions for Bayesian Deep Learning of Synthetic Aperture Sonar Imaging Artifacts. IEEE Geoscience and Remote Sensing Letters. ISSN 1545-598X. 23. doi: 10.1109/LGRS.2026.3715375. Full text in Research Archive Show summary

  We demonstrate that the choice of weight prior in Bayesian neural networks (BNNs) is a consequential and nontrivial design decision for synthetic aperture sonar (SAS) imaging artifact classification. Motivated by established statistical models of acoustic seafloor backscatter, we replace the standard univariate Gaussian prior on the first convolutional layer with physically motivated alternatives: a multivariate normal (MVN) prior that captures empirical pixel covariance, and asymmetric priors comprising the Gumbel model, which corresponds to a log-transformed Weibull distribution. All priors are implemented via local reparameterization with closed-form Kullback–Leibler (KL) divergence, preserving computational tractability. On a dataset of 54 geographically diverse SAS images comprising 3×105 300×300 pixel patches, the Gumbel prior yields 97.6% macro F1 and 0.7% expected calibration error (ECE) on ResNet20, versus 87.1% F1 and 9.7% ECE for the Gaussian baseline. On ResNet50, Gumbel achieves 99.2% accuracy, surpassing the deterministic model. Therefore, prior specification deserves domain-specific consideration in BNN deployments.

• Gereb, Gabor; Lønmo, Tor Inge Birkenes; Hansen, Roy Edgar; Näsholm, Sven Peter & Austeng, Andreas (2025). Clairvoyant performance bounds for adaptive beamforming in pulse-echo imaging. Journal of the Acoustical Society of America. ISSN 0001-4966. 157(1), p. 459–470. doi: 10.1121/10.0034836. Full text in Research Archive Show summary

  In adaptive beamforming, the array signal processing adjusts its sensor delays and weights based on the incoming data. In conventional beamforming, these parameters are instead given from a predefined model. Adaptive beamformers can improve measurement precision by dynamically rejecting spatial interference. While an established theory is available on the behavior of adaptive beamformers in textbook scenarios, their expected performance on realistic pulse-echo imaging scenes is still mostly uncharted. Imaging performance can be evaluated by individual pixel precision and aggregated metrics such as resolution and contrast. The achievable gain is strongly related to the sparsity of the scene and the availability of data to appropriately estimate the spatial covariance matrix. In pulse-echo measurements, the nonstationary interference poses a special problem for adaptive beamforming, which is a current research question of academic and industrial interest. The current work establishes a performance bound for adaptive beamforming in simulated realistic pulse-echo scenarios. This is derived and numerically implemented as the clairvoyant minimum variance distortionless response beamformer. The proposed framework allows for an a priori assessment of the applicability of adaptive beamforming, for a given scenario. The performance of the implemented algorithms can be directly compared with the theoretical limit in a simulated environment.

• Lorentzen, Ole J.; Sæbø, Torstein Olsmo; Austeng, Andreas; Hunter, Alan J. & Hansen, Roy Edgar (2025). Seafloor Slope Estimation and its Theoretical Accuracy in Interferometric Synthetic Aperture Sonar. IEEE Transactions on Geoscience and Remote Sensing. ISSN 0196-2892. 63. doi: 10.1109/TGRS.2025.3559213. Full text in Research Archive Show summary

  Estimating the seafloor slope from interferometric synthetic aperture sonar (SAS) measurements is useful for many applications. The fundamental approach to estimating the slope in a gridded depth map is straightforward, but its statistical accuracy is also desired. We provide expressions for the slope, incidence angle, and normal vector estimates. Then, we derive the Cramér-Rao lower bounds (CRLBs) of these estimates based on error propagation from the CRLBs of the depth estimates. Simulated SAS data supports our results by approaching the theoretical CRLB, and real data examples demonstrate that our results are applicable in practice.

• Orescanin, Marko; Olson, Derek; Harrington, Brian; Geilhufe, Marc; Hansen, Roy Edgar & Duvio, Dalton [Show all 7 contributors for this article] (2025). Classification of Imaging Artifacts in Synthetic Aperture Sonar With Bayesian Deep Learning. IEEE Journal of Oceanic Engineering. ISSN 0364-9059. doi: 10.1109/JOE.2025.3538948. Full text in Research Archive
• Lorentzen, Ole Jacob; Sæbø, Torstein Olsmo; Hunter, Alan Joseph & Hansen, Roy Edgar (2024). Synthetic Aperture Sonar Interferogram Filtering by Intensity Image Segmentation. IEEE Journal of Oceanic Engineering. ISSN 0364-9059. 49(4), p. 1516–1529. doi: 10.1109/JOE.2024.3374465. Full text in Research Archive Show summary

  Synthetic aperture sonar interferometry relies on the interferogram of two single look complex images to estimate bathymetry. The phase difference measurements have variance, which is typically reduced by spatial smoothing at the cost of horizontal resolution. The high resolution intensity image is related to the bathymetry because of the observation geometry. We therefore suggest an approach that constrains the filtering around edges found by intensity image segmentation. We demonstrate our suggested method on simulated data and show quantitative and qualitative improvements in both the horizontal resolution and the shape resolvability of small objects. We demonstrate a 30% improvement in RMSE of the bathymetric estimate, and observe that the estimated bathymetry more closely renders the real object shape for a small, but elevated object. We demonstrate our suggested method on real data and show similar results.

• Hansen, Roy Edgar & Sæbø, Torstein Olsmo (2024). Optimising collection geometry for long-range synthetic aperture sonar interferometry. IET Radar, Sonar & Navigation. ISSN 1751-8784. doi: 10.1049/rsn2.12607. Full text in Research Archive Show summary

  Interferometric synthetic aperture sonar (SAS) is a technique to image and map theseabed in very high resolution. For large area coverage rate systems with hundreds ofmetres swath width, the achievable performance varies significantly over the swath. Theperformance is a function of system, collection geometry, and seabed type. A model issuggested to optimise the collection geometry for maximising area coverage rate givencertain optimisation criteria, such as observation geometry, signal‐to‐noise ratio, depthmeasurement accuracy, and coverage within swath. The model is fitted to measurements(or calibrated) through a simple procedure. Specifically, the effect of vehicle altitudeduring the interferometric SAS data collection is studied. A novel model on data collectedby a HUGIN Superior autonomous underwater vehicle carrying a HISAS 1032 Dual Rxinterferometric SAS is demonstrated. The authors show that optimising the collectiongeometry, and in this case specifically the vehicle altitude, significantly improves theoverall performance.

• Hansen, Roy Edgar (2023). Challenges in synthetic aperture sonar imaging of shipwrecks for change detection. Underwater Acoustics Conference & Exhibition (UACE). Full text in Research Archive
• Sæbø, Torstein Olsmo; Hansen, Roy Edgar & Lorentzen, Ole J. (2023). Long-range interferometric synthetic aperture sonar. In Heald, Gary (Eds.), Synthetic Aperture Sonar and Synthetic Aperture Radar 2023. Institute of Acoustics. ISSN 9781906913441. Full text in Research Archive
• Hansen, Roy Edgar (2023). MIMO synthetic aperture sonar: Benefits and limitations. Electronics Letters. ISSN 0013-5194. doi: 10.1049/ell2.12723. Full text in Research Archive Show summary

  The use of Multiple Input Multiple Output (MIMO) orthogonally coded waveforms sharing the same bandwidth, has the potential to increase area coverage rate in synthetic aperture sonar (SAS). The disadvantage is cross-talk between the waveforms. In this letter, the benefits and limitations are listed for a best case use of MIMO in SAS. A scheme is developed to emulate a MIMO system with realistic cross-talk that can be tested on non-MIMO SAS data. The impact of cross-talk is evaluated on simulated data and real data collected by a HISAS interfereometric SAS carried by a HUGIN autonomous underwater vehicle. The finding is that the negative impact of cross-talk on SAS is significant.

• Orescanin, Marco; Harrington, Brian; Olson, Derek R.; Geilhufe, Marc; Hansen, Roy Edgar & Warakagoda, Narada Dilp (2023). A Study on the Effect of Commonly Used Data Augmentation Techniques on Sonar Image Artifact Detection Using Deep Neural Networks. IEEE International Geoscience and Remote Sensing Symposium proceedings. ISSN 2153-6996. doi: 10.1109/IGARSS52108.2023.10282626. Full text in Research Archive Show summary

  This paper presents an empirical study that evaluates the impact of different types of augmentations on the performance of Deep Learning (DL) models for detecting imaging artifacts in Synthetic Aperture Sonar (SAS) imagery. Despite the popularity of using DL in the SAS community, the impact of augmentations that violate the geometry and physics of SAS has not been fully explored. To address this gap, we developed a unique dataset for detecting imaging artifacts in SAS imagery with DL and trained a Bayesian neural network with a ResNet architecture using widely used augmentations in DL for computer vision, as well as common augmentations used in the SAS literature. The study shows that augmentations that violate the geometry and imaging physics of SAS can negatively impact supervised classification, but can sometimes improve performance. Overall, the study provides important insights into the impact of different types of augmentations on the performance of DL models in SAS applications.

• Orescanin, Marco; Harrington, Brian; Olson, Derek R.; Geilhufe, Marc; Hansen, Roy Edgar & Warakagoda, Narada Dilp (2023). Uncertainty quantification with deep learning through variational inference with applications to synthetic aperture sonar. Underwater Acoustics Conference & Exhibition (UACE). Full text in Research Archive Show summary

  Deep learning (DL) has gained popularity in applications and research within the active sonar community both in academic and commercial settings due to the ability of such models to learn complex non-linear relationships between the input features and the labels, a data driven approach. This has led to significant improvements in automatic target recognition and seafloor texture understanding with Synthetic Aperture Sonar (SAS). Most of the DL models reported in literature are deterministic and do not provide estimates of uncertainty of their predictions limiting the utility for the downstream tasks such as ATR and change detection.\n\nIn this work we demonstrate the ability to quantify uncertainty in deep learning predictions by utilizing variational inference to develop Bayesian Neural Networks. Further, we explore decomposition of obtained uncertainty into aleatoric and epistemic components.We showcase the importance of this decomposition on classifier performance and interpretation of the results. We introduce and compare several state-of-the art methods in variational inference on the task of classifying imaging artifacts in SAS. We conduct this on a novel dataset developed for this classification task through introduction of physical perturbations in the image formation stage, namely : 1) sound speed error of 40 m/s, 2) navigation error through perturbation in yaw of 0.35° and 3) Gaussian noise over the imaging channels prior to pulse compression (lowering the average\nimage SNR to 5 dB). \n\nOverall, we demonstrate that our best model, a mean-field variational inference via flipout ResNet architecture, achieves 92% accuracy with calibrated uncertainty. By rejecting 10% of the data with highest uncertainty we achieve additional 4% improvement in accuracy.

• Thon, Stine Hverven; Austeng, Andreas & Hansen, Roy Edgar (2023). Point detection in textured ultrasound images. Ultrasonics. ISSN 0041-624X. doi: 10.1016/j.ultras.2023.106968. Full text in Research Archive Show summary

  Detection of point scatterers in textured ultrasound images can be challenging. This paper investigates how four multilook methods can improve the detection. We analyze many images with known point scatterer locations and randomly textured backgrounds. The normalized matched filter (NMF) and multilook coherence factor (MLCF) methods are normalized methods that do not require any texture correction prior to detection analysis. They are especially propitious when optimal texture correction of the ultrasound images is difficult to obtain. The results show significant improvement in detection performance when the MLCF method is weighted with the prewhitened and texture corrected image. The method can be applied even when we do not have prior knowledge about the optimal prewhitening limits. The multilook methods NMF and NMF weighted (NMFW) are very favorable methods to apply on images where acoustic noise dominates the speckle background.

• Sæbø, Torstein Olsmo & Hansen, Roy Edgar (2023). Adaptive phase estimation filter in long range synthetic aperture sonar interferometry. Electronics Letters. ISSN 0013-5194. doi: 10.1049/ell2.12819. Full text in Research Archive Show summary

  Synthetic aperture sonar (SAS) interferometry is a technique for very high resolution imaging and mapping of the seabed. In SAS interferometry, the seabed depth estimation performance is a function of the system, the geometry, the signal-to-noise ratio (SNR) and the filter size, equivalent to the achieved horizontal resolution. A strong driver for SNR is the imaging range and grazing angle. The variation of these parameters over a typical SAS swath gives rise to a large variation in the depth estimation performance. To mitigate the negative effect of this, we suggest to use an adaptive phase estimation filter size, such that the standard deviation of the depth estimate is proportional to the horizontal resolution. We demonstrate the suggested adaptive filter size method on long range data collected using a HUGIN Superior autonomous underwater vehicle (AUV) equipped with a HISAS 1032 Dual receiver interferometric SAS. Our technique increases the valid area coverage when the SNR is marginal, at the expense of reduced horizontal resolution.

• Thon, Stine Hverven; Hansen, Roy Edgar & Austeng, Andreas (2022). Point Detection in Ultrasound Using Prewhitening and Multilook Optimization. IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. ISSN 0885-3010. 69(6), p. 2085–2097. doi: 10.1109/TUFFC.2022.3167923. Full text in Research Archive Show summary

  We investigate methods to improve the detection of point scatterers in ultrasound imaging using the standard delay-and-sum (DAS) image as our starting point. An optimized whitening transform can increase the spatial resolution of the image. By splitting an image’s frequency spectrum into many subsets using the multilook technique, we can exploit the coherent properties of a point scatterer. We present three new multilook methods and evaluate their effect on point detection. The performances are compared to DAS using synthetic aperture Field II simulations of a point scatterer in uniform speckle background. The results show that optimized prewhitening of the images can significantly improve the point detection. The multilook methods have the potential to improve the detection performance further when a sufficient number of looks are used. If prior knowledge about the optimal spectrum limits is unavailable and a nonoptimal prewhitening is applied, applying that the new multilook methods can considerably improve the point detection.

• Hansen, Roy Edgar; Geilhufe, Marc; Synnes, Stig Asle Vaksvik; Sæbø, Torstein Olsmo & Thon, Stine Hverven (2022). Detection of Coherent Scatterers in Synthetic Aperture Sonar Using Multilook Coherence, EUSAR 2022. VDE Verlag GmbH. ISSN 9783800758234. Full text in Research Archive Show summary

  Synthetic aperture sonar (SAS) is the reference technique for high-resolution acoustic imaging of the seabed. Unlike synthetic aperture radar (SAR), SAS suffers from insuffcient accuracy of position, speed of sound, and terrain knowledge. This leads to the need for automated assessment of SAS image quality. Multilook coherence is a technique for detecting coherent scatterers which can be used to assess image quality. In this paper, we describe different ways to calculate multilook coherence. We test the techniques on real data collected by a HUGIN autonomous underwater vehicle carrying a HISAS interferometric SAS. We rate the different coherence techniques by evaluating the suitability of the candiate coherent scatterers they produce. The candidate technique that produces best suited candiates is the multilook coherence factor using a large number of looks per dimension.

• Sæbø, Torstein Olsmo; Hansen, Roy Edgar & Lyons, Anthony P. (2022). Synthetic aperture sonar multi-aspect differential interferometry for detection of internal waves, EUSAR 2022. VDE Verlag GmbH. ISSN 9783800758234. p. 277–282. Full text in Research Archive Show summary

  Refractive effects due to breaking internal waves can be present in synthetic aperture sonar (SAS) images and SAS bathymetries. These effects cause large linear structures (tens of meters by several meters) in the SAS images and may be misinterpreted as actual bathymetric features on the seafloor. Such misinterpretations limit our ability to acquire accurate knowledge of the seafloor and are therefore important to resolve. Detection of internal waves in SAS images using repeated passes has been suggested, but a robust detection method for single passes is still missing in the literature. We suggest a novel method which uses multi-aspect differential interferometry to detect these features caused by refractive effects. We present results from the MANEX’13 experiment, where internal wave effects where present in the SAS data and originally detected using multiple passes. We also include a reference dataset from outside Horten, Norway, where there are no indications of internal waves.

• Lyons, Anthony P.; Olson, Derek R. & Hansen, Roy Edgar (2022). Modeling the effect of random roughness on synthetic aperture sonar image statistics. Journal of the Acoustical Society of America. ISSN 0001-4966. doi: 10.1121/10.0013837. Full text in Research Archive Show summary

  A model has been developed to predict the effect of random seafloor roughness on synthetic aperture sonar (SAS) image statistics, based on the composite roughness approximation–a physical scattering model. The continuous variation in scattering strength produced by a random slope field is treated as an intensity scaling on the image speckle produced by the coherent SAS imaging process. Changes in image statistics caused by roughness are quantified in terms of the scintillation index (SI). Factors influencing the SI include the seafloor slope variance, geo-acoustic properties of the seafloor, the probability density function describing the speckle, and the signal-to-noise ratio. Example model-data comparisons are shown for SAS images taken at three different sites using three different high-frequency SAS systems. Agreement between the modeled and measured SI show that it is possible to link range-dependent image statistics to measurable geo-acoustic properties, providing the foundation necessary for solving problems related to the detection of targets using high-frequency imaging sonars, including performance prediction or adaptation of automated detection algorithms. Additionally, this work illustrates the possible use of SAS systems for remote sensing of roughness parameters such as root mean square slope or height.

• Bryan, Oscar; Hansen, Roy Edgar; Haines, Tom S. F.; Warakagoda, Narada Dilp & Hunter, Alan Joseph (2022). Challenges of Labelling Unknown Seabed Munition Dumpsites from Acoustic and Optical Surveys: A Case Study at Skagerrak. Remote Sensing. doi: 10.3390/rs14112619. Full text in Research Archive Show summary

  The disposal of unexploded ordnance (UXOs) at sea is a global problem. The mapping and remediation of historic UXOs can be assisted by autonomous underwater vehicles (AUVs) carrying sensor payloads such as synthetic aperture sonar (SAS) and optical cameras. AUVs can image large areas of the seafloor in high resolution, motivating an automated approach to UXO detection. Modern methods commonly use supervised machine learning which requires labelled examples from which to learn. This work investigates the often-overlooked labelling process and resulting dataset using an example historic UXO dumpsite at Skagerrak. A counterintuitive finding of this work is that optical images cannot be relied on for ground truth as a significant number of UXOs visible in SAS images are not in optical images, presumed buried. Given the lack of ground truth, we use an ordinal labelling scheme to incorporate a measure of labeller uncertainty. We validate this labelling regime by quantifying label accuracy compared to optical labels with high confidence. Using this approach, we explore different taxonomies and conclude that grouping objects into shells, bombs, debris, and natural gave the best trade-off between accuracy and discrimination.

• Lorentzen, Ole Jacob; Hansen, Roy Edgar; Sæbø, Torstein Olsmo; Synnes, Stig Asle Vaksvik & Geilhufe, Marc (2021). 3D Rendering of Shipwrecks from Synthetic Aperture Sonar. Proceedings of Meetings on Acoustics (POMA). doi: 10.1121/2.0001483. Full text in Research Archive Show summary

  Synthetic aperture sonar (SAS) bathymetric mapping provides centimeter-scale horizontal resolution of theseafloor. Typically, SAS data has been visualized as 2D images or gridded 2.5D surfaces, but it can also beplotted as 3D point clouds. Even if the imaging system is not fully volumetric, the imaging geometry leavesthe possibility of having data points that reside in the same lateral positions in a ground coordinate system.Visualizing these without geometric distortion or loss of data is challenging. In this paper we investigate theuse of full-fledged 3D rendering with artificial lighting and shading techniques in order to convey more ofthe available information. We aim to produce better 3D visualization of shipwrecks. We provide anoverview of which data products are available from state-of-the-art SAS processing, and explore existingvisualization methods. We discuss the use of surfaces and point clouds for 3D rendering, and variousshading and rendering techniques. Our goal is to display SAS bathymetry in a way that is both intuitive andcan convey more supplemental information to the observer. Finally, we show image and video examples ofrendering large shipwreck scenes using data collected by a HISAS1032 SAS carried by a HUGINautonomous underwater vehicle

• Synnes, Stig Asle Vaksvik; Hansen, Roy Edgar & Sæbø, Torstein Olsmo (2021). Spatial coherence of speckle for repeat-pass synthetic aperture sonar micronavigation. IEEE Journal of Oceanic Engineering. ISSN 0364-9059. doi: 10.1109/JOE.2021.3060812. Full text in Research Archive Show summary

  Accurate positioning of autonomous underwater vehicles is a major challenge. The long-term drift is problematic if global position updates are not available, and for applications such as repeat-pass interferometry and coherent change detection, millimeter accuracy is needed. Repeat-pass synthetic aperture sonar (SAS) micronavigation is one potential technique for countering both challenges. While single-pass SAS micronavigation enabled successful coherent processing within one track, the potential is that repeat-pass SAS micronavigation can support coherent processing between passes. Both techniques are based on recognizing the speckle pattern in the seafloor return, but repeat-pass SAS micronavigation has additional challenges with the larger temporal and spatial separations between the observations. In this study, we investigate the spatial correlation of speckle as observed from SAS systems. We divide the different contributions to spatial decorrelation into three groups: 1) speckle decorrelation; 2) footprint mismatch; and 3) stretching. We examine each contribution separately and develop simplified formulas for their decorrelation baselines. When correlating synthetic aperture images, decorrelation from stretching dominates. When correlating single-element data recorded at low grazing angles common to SAS, speckle decorrelation dominates. We validate our findings on experimental data, and by combining elements into larger effective elements, we demonstrate increasing the across-track baseline for repeat-pass SAS micronavigation updates from less than 1 to 10 m.

• Thon, Stine Hverven; Hansen, Roy Edgar & Austeng, Andreas (2021). Detection of Point Scatterers in Medical Ultrasound. IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control. ISSN 0885-3010. doi: 10.1109/TUFFC.2021.3129619. Full text in Research Archive Show summary

  We present an overview of the detection of point scatterers in ultrasound images and suggest strategies for evaluating and measuring the detection performance. We use synthetic aperture Field II simulations of a point scatterer in speckle background and evaluate how common imaging techniques affect point target detectability. We discuss how to compare different methods and calculate confidence intervals. In general, applying speckle reduction methods reduces the point detection performance. However, the results show that it is possible to smooth the speckle background and preserve relatively high performance with a suitable and optimized method. The different detection performances of the advanced beamforming methods Coherence Factor (CF), Phase Coherence Factor (PCF), and Capon’s Minimum Variance (MV) are presented and benchmarked with standard Delay-and-Sum (DAS). The results show that CF achieves slightly better detection performance than DAS for weak point scatterers, whereas PCF and MV perform worse than DAS. Choice of apodization window and adaptive aperture size affects the probability of detection. Results show that methods that preserve spatial resolution have better detection performance of point scatterers.

• Blachet, Antoine; Austeng, Andreas; Aparicio, Joaquin; Hunter, Alan Joseph & Hansen, Roy Edgar (2021). Multibeam Echosounder With Orthogonal Waveforms: Feasibility and Potential Benefits. IEEE Journal of Oceanic Engineering. ISSN 0364-9059. doi: 10.1109/JOE.2021.3052568. Full text in Research Archive Show summary

  Modern multibeam echosounders (MBE) employ frequency-division techniques (FDT) to ensonify multiple sectors within the same ping cycle. This leads to improved performance in coverage rate, and yaw and pitch stabilization. However, it introduces a bias among sectors because MBES systems are frequency dependent. It also reduces the maximum pulse bandwidth compared to a single-sector sonar. In this study, we consider the code-division technique (CDT) as a solution to this problem. A set of orthogonal coded pulses are received within the same frequency band, and each sector is separated with a matched filter. We assess the feasibility of the technique through two stages. 1) First, we formulate an analytical model describing the power and crosstalk budgets of any multisector MBES. The model can then be used to design transmission sequences fitting these budgets. 2) Then, we display the practical usage of the technique for MBES imaging and mapping through simulated case studies. For the same total time-bandwidth budget, we compare the performance of FDT, CDT, and multicarrier CDT (MC-CDT), a hybrid method employing CDT and FDT, which is robust to strong dynamic backscatter. This study considers only bottom detection based on signal amplitude. Our results show that it is possible to share a larger frequency bandwidth between multiple sectors while maintaining an acceptable bottom detection performance similar to FDT. Our choice of time-bandwidth product with CDT offers a crosstalk suppression of − 25 dB between sectors, but may display low-magnitude residual artefacts in the water-column data. MC-CDT provides a significant gain of pulse bandwidth while it offers interband separation performance comparable to FDT and reduces significantly the water-column artefacts.

• Lønmo, Tor Inge Birkenes; Austeng, Andreas & Hansen, Roy Edgar (2020). Data-Driven Autocalibration for Swath Sonars. IEEE Journal of Oceanic Engineering. ISSN 0364-9059. doi: 10.1109/JOE.2020.3036184. Full text in Research Archive Show summary

  Sidelobes in swath sonar water column imagery can obscure targets of interest and create erroneous bottom detections. Differences between ideal and actual element responses limit the achievable sidelobe level for practical arrays. Therefore, removing or reducing the errors by calibration may reduce the sidelobe level. We examine an autocalibration technique for swath sonars that use data collected while mapping. Our method is based on the generalized interferometric array response (GIAR). The GIAR values are used to find pointlike signals and maximized to estimate the amplitude and phase errors for each element. On simulated data, this reduces the sidelobe level to below − 50 dB. In the field, we find the calibration estimate stable over ping groups and that most sidelobes improved by 2 dB or more. This suggest that GIAR calibration can work well when the calibration matches the swath sonar system. However, some sidelobes are also unchanged or degraded by a few decibels. An extended calibration model seems necessary to achieve further improvements in the field.

• Geilhufe, Marc; Hansen, Roy Edgar; Midtgaard, Øivind & Synnes, Stig Asle Vaksvik (2019). Through-the-sensor sharpness estimation for synthetic aperture sonar images. In Stahr, Fritz (Eds.), Proceedings of OCEANS 2019 MTS/IEEE SEATTLE. IEEE (Institute of Electrical and Electronics Engineers). ISSN 9780578576183. doi: 10.23919/oceans40490.2019.8962812. Full text in Research Archive
• Myers, Vincent; Quidu, Isabelle; Zerr, Benoit; Sæbø, Torstein Olsmo & Hansen, Roy Edgar (2019). Synthetic Aperture Sonar Track Registration With Motion Compensation for Coherent Change Detection. IEEE Journal of Oceanic Engineering. ISSN 0364-9059. doi: 10.1109/JOE.2019.2909960. Full text in Research Archive
• Lønmo, Tor Inge Birkenes; Austeng, Andreas & Hansen, Roy Edgar (2019). Improving Swath Sonar Water Column Imagery and Bathymetry With Adaptive Beamforming. IEEE Journal of Oceanic Engineering. ISSN 0364-9059. doi: 10.1109/JOE.2019.2926863. Full text in Research Archive Show summary

  Modern swath sonar is a mature technology today and has reached a very high level of sophistication including techniques to increase area coverage rate, data quality, and resolution. There is, however, often a need to explore features at the limit of what is resolvable. It is, therefore, of interest to consider alternative signal processing techniques for a given physical system. For the traditional delay-and-sum (DAS) beamformer there is a tradeoff between angular resolution and sidelobe suppression. Using either the Capon or the low complexity adaptive (LCA) beamformer, the water column edge definition, sidelobe level, and resolution are improved compared to a moderately weighted DAS beamformer. These improvements are similar to the recent results for sector-scanning sonar. This leads to improved performance for the amplitude-based center-of-gravity bottom detector. The Capon beamformer performs best, while the LCA beamformer has a large part of the improvement with higher robustness and easier implementation. We use simulations to show the improvements of the adaptive beamformers from nadir up to 42∘ across track, and validate the results using field data. The improvements in the water column increase the separation between features and noise, and collapse the apparent size of features down to more realistic dimensions. These improvements allow the adaptive beamformers to reveal features that are hidden for the DAS beamformer.

• Lorentzen, Ole J. & Hansen, Roy Edgar (2019). Fusion Digital Terrain Models for Synthetic Aperture Sonar Interferometry. In Stahr, Fritz (Eds.), Proceedings of OCEANS 2019 MTS/IEEE SEATTLE. IEEE (Institute of Electrical and Electronics Engineers). ISSN 9780578576183. doi: 10.23919/OCEANS40490.2019.8962838. Full text in Research Archive Show summary

  High resolution, centimeter-scale synthetic aperture sonar (SAS) imaging of the seafloor relies on a priori knowledge of the bathymetry in order to focus the image correctly. Common approaches to estimate the bathymetry includes using flat or sloped surfaces relative to the sonar altitude measurements or the estimated sidescan bathymetry (SSB), or using the SSB estimates directly. More advanced approaches include iterative processing and various methods of searching for better candidate solutions. In this paper we investigate the use of an additional digital terrain model (DTM) from a multibeam echo sounder (MBES). This external DTM can be used by itself or it can be combined with SSB or SAS bathymetry for a fusion DTM. We compare the results of the various methods by investigating the interferograms and using the number of valid measurements, the interferometric coherence and the phase difference as performance metrics. We also show SAS bathymetry examples of the differences between these approaches.

• Hansen, Roy Edgar; Geilhufe, Marc; Bakken, Erik Makino & Sæbø, Torstein Olsmo (2019). Comparison of synthetic aperture sonar images and optical images of UXOs from the Skagerrak chemical munitions dumpsite. Underwater Acoustics Conference & Exhibition (UACE). Full text in Research Archive Show summary

  After World War II chemical munitions were loaded onto large ships and sunkin the Skagerrak Strait in the Norwegian trench, in deep waters between Norway and Denmark. In collaboration with the Norwegian Coastal Administration, Norwegian Defence Research Establishment (FFI) conducted two separate missions in 2015 and 2016, where around450 square kilometers were mapped using a HUGIN autonomous underwater vehicle (AUV)equipped with a HISAS interferometric synthetic aperture sonar (SAS). 54 wrecks were found,and 36 of these are believed to be part of the chemical munitions dumpsite. Many shipwreckshave suffered severe damage, and their cargo is spread out around the shipwrecks. In January2019, FFI revisited the Skagerrak dumpsite for additional data collection using the HUGINAUV with SAS and an optical camera. Multiple wrecks were revisited, and SAS images andoptical images of unexploded ordnances (UXO) were gathered. In this paper, we compareSAS images with optical images of UXOs. We assess the retrievable information present inhigh resolution large area coverage rate SAS images by comparing separate objects and theirconditions as seen with optical camera.

• Sternlicht, Daniel D.; Hayes, Michael P. & Hansen, Roy Edgar (2018). Seabed Mapping SAS: The First 50 Years of Synthetic Aperture Sonar Imaging Technology. Sea Technology. ISSN 0093-3651. 59(11), p. 10–14. Full text in Research Archive
• Sternlicht, Daniel D.; Hayes, Michael P. & Hansen, Roy Edgar (2018). Historical development of seabed mapping synthetic aperture sonar. In Heald, Gary (Eds.), Proceedings of 4th International Conference on Synthetic Aperture Sonar and Radar. Institute of Acoustics. ISSN 9781906913304. p. 1–10. Full text in Research Archive
• Sæbø, Torstein Olsmo; Hansen, Roy Edgar & Myers, Vincent (2018). Comparison of co-registration techniques for synthetic aperture sonar images from repeated passes. In Heald, Gary (Eds.), Proceedings of 4th International Conference on Synthetic Aperture Sonar and Radar. Institute of Acoustics. ISSN 9781906913304. p. 31–38. Full text in Research Archive
• Lorentzen, Ole J.; Synnes, Stig Asle Vaksvik; Hansen, Roy Edgar & Sæbø, Torstein Olsmo (2018). Investigating layover effects in cross-correlation based interferometry for synthetic aperture sonar, EUSAR 2018; 12th European Conference on Synthetic Aperture Radar. VDE Verlag GmbH. ISSN 9783800746361. p. 338–343. Full text in Research Archive
• Ødegård, Øyvind; Hansen, Roy Edgar; Singh, Hanumant & Maarleveld, Thijs (2018). Archaeological use of Synthetic Aperture Sonar on deepwater wreck sites in Skagerrak. Journal of Archaeological Science. ISSN 0305-4403. 89, p. 1–13. doi: 10.1016/j.jas.2017.10.005. Full text in Research Archive Show summary

  Marine archaeological surveying in deep waters has so far been challenging, mainly due to operational and technological constraints. The standard tool has been Side Scan Sonar (SSS) towed behind a surface vessel. Synthetic Aperture Sonar (SAS) technology is not subject to the traditional range/resolution trade-off, and produces results of considerably higher quality than traditional SSS. In 2015 and 2016 a comprehensive mapping of wrecks in Skagerrak, a large deepwater area off the south coast of Norway was undertaken, using an interferometric SAS system deployed on an autonomous underwater vehicle. By examining data from two passes of one of the many historical wrecks that were detected in the survey area, we demonstrate how SAS can be used to produce very high resolution imagery and bathymetry of wreck sites. Furthermore, post processing techniques are applied to exploit the high information content inherent in SAS data, enhancing aspects of the data for relevant archaeological analysis and interpretation. We show in this paper how SAS technology represents significant improvements in our abilities to conduct high quality and high resolution seabed mapping. The adoption of this technology will both benefit archaeological research and provide knowledge for better decision making in underwater cultural heritage management.

• Hansen, Roy Edgar; Sæbø, Torstein Olsmo; Synnes, Stig Asle Vaksvik & Lorentzen, Ole J. (2018). Challenges in Coregistration of Repeated Passes in Synthetic Aperture Sonar, EUSAR 2018; 12th European Conference on Synthetic Aperture Radar. VDE Verlag GmbH. ISSN 9783800746361. p. 344–349. Full text in Research Archive Show summary

  Synthetic aperture sonar (SAS) is a technique for high resolution imaging, similar to synthetic aperture radar (SAR). In SAR, repeat pass interferometry, tomography, and coherent change detection are today established as standard methods. This is not true for SAS, where coregistration for coherent combination from repeated passes is a major challenge. Many factors may cause misregistration, including insufficient navigation accuracy, variability in the ocean environment, and nonideal tracks. In this paper we describe the challenges in coregistration, and evaluate a re-imaging scheme including corrections on real data collected by a HUGIN autonomous underwater vehicle carrying a HISAS interferometric SAS.

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• Geilhufe, Marc; Hansen, Roy Edgar; Olson, Derek R. & Synnes, Stig Asle Vaksvik (2026). Comparison of predicted and measured image quality in synthetic aperture sonar data. Full text in Research Archive
• Gereb, Gabor; Austeng, Andreas; Hansen, Roy Edgar & Lønmo, Tor Inge Birkenes (2024). Resolution of adaptive beamformers. Full text in Research Archive
• Geilhufe, Marc; Olson, Derek R.; Hansen, Roy Edgar & Synnes, Stig Asle Vaksvik (2024). Comparison of methods for detection of point scatterers in SAS images. Full text in Research Archive
• Polat, Seçkin; Austeng, Andreas; Näsholm, Sven Peter; Hansen, Roy Edgar & Blomberg, Ann Elisabeth Albright (2024). Marine Gas Seepage Detection using Hugin AUV. Full text in Research Archive
• Gereb, Gabor; Hansen, Roy Edgar; Lønmo, Tor Inge Birkenes; Näsholm, Sven Peter & Austeng, Andreas (2024). Case specific adaptive beamformer design for a multi beam echo sounder survey scenario. Full text in Research Archive
• Sæbø, Torstein Olsmo; Hansen, Roy Edgar & Lorentzen, Ole J. (2023). Long-range interferometric synthetic aperture sonar. Full text in Research Archive
• Gereb, Gabor; Austeng, Andreas; Hansen, Roy Edgar & Lønmo, Tor Inge Birkenes (2023). Performance analysis of adaptive beamformers in surveying pipelines by multi-beam echo sounders​. Full text in Research Archive
• Orescanin, Marco; Harrington, Brian; Olson, Derek R.; Geilhufe, Marc; Hansen, Roy Edgar & Warakagoda, Narada Dilp (2023). Uncertainty quantification with deep learning through variational inference with applications to synthetic aperture sonar. Full text in Research Archive
• Zhang, Xuebo; Heald, Gary; Lyons, Anthony P.; Hansen, Roy Edgar & Hunter, Alan J. (2023). Guest editorial: recent advances in synthetic aperture sonar technology. Electronics Letters. ISSN 0013-5194. 59(14). doi: 10.1049/ell2.12881. Full text in Research Archive
• Geilhufe, Marc; Olson, Derek R.; Hansen, Roy Edgar & Synnes, Stig Asle Vaksvik (2023). A wavelet shrinkage approach to detect candidate point scatterers in synthetic aperture sonar images for resolution estimation. Full text in Research Archive
• Orescanin, Marko; Harrington, Brian; Olson, Derek R.; Geilhufe, Marc; Hansen, Roy Edgar & Warakagoda, Narada Dilp (2023). A Study on the Effect of Commonly Used Data Augmentation Techniques on Sonar Image Artifact Detection Using Deep Neural Networks. Full text in Research Archive
• Polat, Seçkin; Austeng, Andreas; Näsholm, Sven Peter; Hansen, Roy Edgar & Blomberg, Ann Elisabeth Albright (2023). Algorithms for real-time detection of marine gas seeps using acoustic sensors mounted on Hugin AUV. Full text in Research Archive Show summary

  Carbon capture and storage (CCS) involves capturing CO2 from industrial and other sources, and permanently storing the gas in geological formations. Recent studies indicate that CCS is a promising, secure, and reliable long-term solution that can significantly contribute to reaching international climate goals and limit global warming (Flohr et al., 2021). There are several challenges in CCS, such as the high cost of monitoring large areas for a longer period of time, and to generate algorithms for real-time leakage detection based on large amounts of data (Dean et al., 2020). For automated real-time leakage detection, an accessible sonar image dataset would increase the progress of deep learning methods for detection, classification, and segmentation of sonar images (Steineger et al.,2022). This PhD project is part of a larger international research project (Smart AUVs) aimed at an efficient CCS monitoring using autonomous underwater vehicle (AUV) and inspection, maintenance, and repair vehicle (IMR). The PhD project is aimed at developing acoustics-based and sensor based approaches for detection and characterization of gas seepage from the seabed with Hugin AUV and Eulume IMR.

• Hansen, Roy Edgar (2023). Challenges in synthetic aperture sonar imaging of shipwrecks for change detection. Full text in Research Archive
• Geilhufe, Marc; Olson, Derek R.; Hansen, Roy Edgar & Synnes, Stig Asle Vaksvik (2023). A wavelet schrinkage approach to detect candidate point scatterers in synthetic aperture sonar images for resolution estimation. Full text in Research Archive Show summary

  Autonomous Underwater Vehicles (AUVs) that are equipped with side-looking synthetic aperturesonar (SAS) have become a popular asset for efficient high resolution imaging and mapping of theseafloor. There are situations where it can be challenging to produce SAS images of good quality. For instance, this depends on the ocean environment, vehicle stability, navigation accuracy orseafloor bathymetry. For running operations more autonomously, an AUV must be able to estimateits sonar performance through-the-sensor such that it can automatically adjust track orientation, processing techniques or other parameters to improve performance. Hence, it is imperative to developtechniques to assess image quality from the imagery itself. Given sufficient signal to noise ratio, werecognize image sharpness as the most important quality metric 1, preferably specified as a numberbetween [0, 1] expressing how close the achieved resolution is to the theoretical resolution of thesystem.SAS images are formed by coherently combining echoes gathered over an interval of acousticfrequencies and aspect angles. The standard approach for estimating resolution in an image is tocalculate the -3 dB width of candidate point scatterers, but there are various methods on how to findsuitable candidate scatterers 2–5. Point scatterers must be distinguished from larger targets as wellas speckle in order to be able to use them to correctly estimate the image resolution. Large targetsare undesirable because their width in space is much larger than the system resolution. Speckle isalso undesirable because the width of the peaks in speckle are always the same size, independentof the focus quality of the system.In this paper, for detecting candidate point scatterers in SAS images, we explore the suitability of awavelet shrinkage method6 using two non-overlapping looks obtained from a full resolution singlelook complex (SLC) image. We have applied this method in earlier work 3 and will here focus on acomplete description and explanation of how and why it works. The idea is that for point scatterers,the coherence between looks is high, while speckle and noise are uncorrelated. Focusing on therelevant scales and orientations, our approach can enhance point scatterers and de-emphasizetexture, thereby facilitating the detection of point scatterers for resolution estimation.The remainder of this paper is as follows. In Section 2 we give an introduction to the waveletshrinkage approach. Section 3 interprets the method in light of its application of detection of pointscatterers, followed by Section 4 highlighting details on our choices for the implementation. Resultsfor simulated and real SAS data are shown in Section 5. The paper concludes with a summary andoutlook over future work in Section 6.

• Sæbø, Torstein Olsmo; Hansen, Roy Edgar & Synnes, Stig Asle Vaksvik (2022). Phase ambiguities in Synthetic Aperture Sonar Interferometry. Full text in Research Archive
• Hansen, Roy Edgar (2022). Benefits and Limitations in MIMO Synthetic Aperture Sonar. Full text in Research Archive
• Hansen, Roy Edgar; Geilhufe, Marc; Synnes, Stig Asle Vaksvik; Sæbø, Torstein Olsmo & Thon, Stine Hverven (2022). Detection of Coherent Scatterers in Synthetic Aperture Sonar Using Multilook Coherence. Full text in Research Archive
• Lønmo, Tor Inge Birkenes; Austeng, Andreas & Hansen, Roy Edgar (2022). Adaptive Beamforming and Autocalibration for Swath Sonars. Full text in Research Archive
• Gereb, Gabor; Hansen, Roy Edgar; Lønmo, Tor Inge Birkenes; Näsholm, Sven Peter; Austeng, Andreas & Rindal, Ole Marius Hoel (2022). Improving Sonar Surveying of Subsea Cables and Pipelines with Adaptive Beamforming. Full text in Research Archive Show summary

  Background, Motivation and Objective Safe and efficient transportation of offshore energy requires well-maintained underwater infrastructure, no matter if the energy source is fossil, wind, sun, or wave. Subsea cables and pipelines can be efficiently surveyed using sonar systems mounted on surface or underwater vehicles. Adaptive array signal processing methods are increasingly explored to improve image quality and detection or estimation accuracy in specific survey scenarios. However, adaptive methods can be sensitive to the context they are used in. Therefore, choosing the right beamforming approach from the available candidates requires extensive testing in various scenarios, which is often not feasible out at sea. Statement of Contribution/Methods Realistic simulation of complex underwater scenarios has been previously demonstrated using the Field II Ultrasound Simulation Program in combination with the open access package UltraSound ToolBox (USTB). We use Field II to create digital twins of water column images from pipeline surveys, and then modify parameters such as the layout, depth, and SNR to extend the scope of testing. We add multiple realizations of random noise to the signal and then use USTB to beamform the data with different techniques. On the beamformed data we perform estimation of multiple parameters such as the position and size of the pipe and the seabed depth around it. Finally, using the known ground truth of the model, we compare the bias and variance of the estimations made using images reconstructed by different beamformers. Results/Discussion Following on the results of previous studies demonstrating the benefits of adaptive beamforming in sonar bathymetry, we present a benchmark study of adaptive beamforming techniques such as Low Complexity Adaptive (LCA) and Capon Minimum Variance (MVDR). Comparison with the Coherence Factor (CF) method is made as well. We use Delay-and-Sum (DAS) as a reference with various apodizations. We provide quantitative performance assessment based on the statistical distribution of the estimations with reference to the ground truth in different scenarios.

• Gereb, Gabor; Hansen, Roy Edgar; Lønmo, Tor Inge Birkenes; Näsholm, Sven Peter & Austeng, Andreas (2022). Underwater pipeline detection and localisation using multibeam echo sounder in a resolution limited case. Full text in Research Archive
• Gereb, Gabor; Hansen, Roy Edgar; Austeng, Andreas; Lønmo, Tor Inge Birkenes; Näsholm, Sven Peter & Rindal, Ole Marius Hoel (2022). Ultrasound Beamformer Benchmarking Pipeline. Full text in Research Archive
• Gereb, Gabor; Hansen, Roy Edgar; Lønmo, Tor Inge Birkenes & Austeng, Andreas (2022). PhD project: Adaptive beamforming in underwater detection and estimation problems. Full text in Research Archive
• Synnes, Stig Asle Vaksvik; Geilhufe, Marc; Hansen, Roy Edgar; Sæbø, Torstein Olsmo & Lorentzen, Ole J. (2022). Investigation of layover in interferometry. Full text in Research Archive
• Rindal, Ole Marius Hoel; Näsholm, Sven Peter; Hansen, Roy Edgar & Austeng, Andreas (2022). Image Enhancement Using Capon Minimum Variance Adaptive Beamforming in Synthetic Aperture Sonar. Full text in Research Archive
• Rindal, Ole Marius Hoel; Näsholm, Sven Peter; Hansen, Roy Edgar & Austeng, Andreas (2022). Compensating Dynamic Range Alterations from Adaptive Beamforming using Histogram Matching. Full text in Research Archive
• Austeng, Andreas; Näsholm, Sven Peter; Hansen, Roy Edgar & Rindal, Ole Marius Hoel (2022). Spatial Aperture Coherence with Synthetic Transmit Focusing in SAS. Full text in Research Archive
• Sæbø, Torstein Olsmo; Hansen, Roy Edgar & Lyons, Anthony P. (2022). Synthetic aperture sonar multi-aspect differential interferometry for detection of internal waves. Full text in Research Archive
• Crawford, Anna M; Hansen, Roy Edgar; Johnson, Sean; King, Jonathan L.; Midtgaard, Øivind & Sæbø, Torstein Olsmo [Show all 9 contributors for this article] (2022). Improvements to change detection tools for SAS images, OCEANS 2022 Hampton Roads. IEEE (Institute of Electrical and Electronics Engineers). ISSN 9781665468091. Full text in Research Archive
• Sæbø, Torstein Olsmo; Hansen, Roy Edgar & Synnes, Stig Asle Vaksvik (2022). Phase ambiguities in Synthetic Aperture Sonar Interferometry, OCEANS 2022 Hampton Roads. IEEE (Institute of Electrical and Electronics Engineers). ISSN 9781665468091. Full text in Research Archive
• Lorentzen, Ole J.; Hansen, Roy Edgar; Sæbø, Torstein Olsmo; Hunter, Alan Joseph; Synnes, Stig Asle Vaksvik & Geilhufe, Marc (2022). Hypothesis testing of 3D object shapes for synthetic aperture sonar interferometry. Full text in Research Archive
• Lønmo, Tor Inge Birkenes; Austeng, Andreas & Hansen, Roy Edgar (2022). Adaptive Beamforming and Autocalibration for Swath Sonars. Full text in Research Archive
• Crawford, Anna M; Hansen, Roy Edgar; Johnson, Sean; King, Jonathan L.; Midtgaard, Øivind & Sæbø, Torstein Olsmo [Show all 9 contributors for this article] (2022). Improvements to change detection tools for SAS images. Full text in Research Archive
• Manna, Nicolas La; Lyons, Anthony P. & Hansen, Roy Edgar (2021). The effect of internal waves on synthetic aperture sonar resolution. Full text in Research Archive
• Lorentzen, Ole J.; Hansen, Roy Edgar; Sæbø, Torstein Olsmo; Synnes, Stig Asle Vaksvik & Geilhufe, Marc (2021). 3D rendering of shipwrecks from synthetic aperture sonar. Full text in Research Archive Show summary

  Synthetic aperture sonar (SAS) bathymetric mapping provides centimeter-scale lateral and vertical resolution of the seafloor. Typically, SAS data has been visualized as 2D images or gridded 2.5D surfaces, but it can also be plotted as 3D point clouds. Even if the imaging system is not fully volumetric, the imaging geometry leaves the possibility of having data points that reside in the same lateral positions in a ground coordinate system. Visualizing these without geometric distortion or loss of data is challenging. In this paper we have investigated the use of full-fledged 3D rendering with artificial lighting and shading techniques using the open-source software Blender in order to convey more of the available information to a human observer. The motivation for this study is to produce better 3D visualization of shipwrecks from a chemical munitions dump site in the Skagerrak sea south of Norway. We provide an overview of which data products are available from state-of-the-art SAS processing, and explore existing visualization methods. We discuss the use of 2.5D surfaces, 3D surfaces and point clouds for 3D rendering, as well as various shading and rendering techniques. These include lighting, surface shading, volumetric shading, emission shading and camera techniques. Our goal is to display SAS bathymetry in a way that is both intuitive and appealing to a general audience, and can convey more supplemental information to the observer than other visualization techniques. Finally, we show image and video examples of rendering large shipwreck scenes using data collected by a HISAS1032 SAS carried by a HUGIN autonomous underwater vehicle.

• Lorentzen, Ole Jacob; Sæbø, Torstein Olsmo & Hansen, Roy Edgar (2020). Backscatter Features for Estimating Synthetic Aperture Sonar Bathymetry. OCEANS. ISSN 0197-7385. Full text in Research Archive Show summary

  Synthetic aperture sonar (SAS) bathymetric mapping has matured in recent years and is adopted by industry, government and academia for mapping and exploration of large areas in more detail than previously feasible. As with other types of bathymetric mapping, averaging is often a critical processing step in order to gain sufficient signal quality to ensure the reliability of the bathymetric product, i.e. the depth map. SAS stands out from many of the other mapping technologies by the availability of very high resolution, centimeter-scale backscatter images. In this paper, we investigate the use of backscatter features to enhance the resolution and accuracy of the bathymetric product. The aim is to allow averaging over larger spatial extent inside of the interferometric processing, while preserving sharp edges and object boundaries, as well as improving the similarity of the statistical properties for the averaged cells. We suggest a method for achieving this by using image segmentation on backscatter data, and demonstrate the method on simulated and real data sets. We achieved better edge preservation on protruding objects, more accurate bathymetry estimates, higher object signal to noise ratio and sharper SAS images after geometric corrections due to the improved bathymetry estimate.

• Otnes, Roald Wilhelm & Hansen, Roy Edgar (2020). Estimating the spatial coherence of narrowband underwater acoustic signals. OCEANS. ISSN 0197-7385. Full text in Research Archive
• Lorentzen, Ole Jacob; Sæbø, Torstein Olsmo & Hansen, Roy Edgar (2020). Backscatter Features for Estimating Synthetic Aperture Sonar Bathymetry. Full text in Research Archive
• Otnes, Roald Wilhelm & Hansen, Roy Edgar (2020). Estimating the spatial coherence of narrowband underwater acoustic signals. Full text in Research Archive
• Otnes, Roald Wilhelm & Hansen, Roy Edgar (2020). Estimating the spatial coherence of narrowband underwater acoustic signals. Full text in Research Archive
• Geilhufe, Marc; Hansen, Roy Edgar; Midtgaard, Øivind & Synnes, Stig Asle Vaksvik (2019). Through-the-sensor sharpness estimation for synthetic aperture sonar images. Full text in Research Archive
• Blachet, Antoine; Plets, Ruth; Sacchetti, Fabio; Austeng, Andreas; Hunter, Alan, J & Hansen, Roy Edgar (2019). MBES DATA SIMULATION: ASSESSMENT BY DIRECT COMPARISON WITH A HIGH-RESOLUTION MULTI-SETTINGS WRECK SURVEY. Full text in Research Archive
• Hansen, Roy Edgar (2019). Mapping the ocean floor in extreme resolution using interferometric synthetic aperture sonar. Proceedings of Meetings on Acoustics (POMA). 38(1), p. 1–10. doi: 10.1121/2.0001058. Full text in Research Archive
• Hansen, Roy Edgar (2019). Mapping the ocean floor in extreme resolution using interferometric synthetic aperture sonar. Full text in Research Archive
• Hansen, Roy Edgar; Geilhufe, Marc; Bakken, Erik Makino & Sæbø, Torstein Olsmo (2019). Comparison of synthetic aperture sonar images and optical images of UXOs from the Skagerrak chemical munitions dumpsite. Full text in Research Archive
• Pate, David J.; Cook, Daniel A.; Lyons, Anthony P. & Hansen, Roy Edgar (2019). Characterization of internal waves in SAS imagery via ray tracing. Full text in Research Archive
• Lorentzen, Ole J. & Hansen, Roy Edgar (2019). Fusion Digital Terrain Models for Synthetic Aperture Sonar Interferometry. Full text in Research Archive
• Blachet, Antoine; Austeng, Andreas; Prieur, Fabrice Jean Gabriel & Hansen, Roy Edgar (2018). Multibeam echosounder survey simulation. Full text in Research Archive Show summary

  - An efficient tool for simulating new sonar design and surveys. - Models sonar system for a given choice of array geometry and pulse shape. - Simulates the Doppler effect induced by the motion of each transducer. - Example of configurations and motion induced artifact are presented .

• Hansen, Roy Edgar (2018). Exploring the Ocean Floor with Synthetic Aperture Sonar. Full text in Research Archive
• Pate, David J.; Cook, Daniel A.; Lyons, Anthony P. & Hansen, Roy Edgar (2018). Characterization of internal waves in synthetic aperture sonar imagery via ray tracing. Full text in Research Archive
• Hansen, Roy Edgar (2018). High Resolution Imaging And Mapping Of The Seabed Using Synthetic Aperture Sonar. Full text in Research Archive
• Lyons, Anthony P.; Olson, Derek R. & Hansen, Roy Edgar (2018). Quantifying the effect of random roughness on synthetic aperture sonar image statistics. Full text in Research Archive
• Hansen, Roy Edgar; Lyons, Anthony P.; Cook, Daniel A. & Sæbø, Torstein Olsmo (2018). Quantifying the negative impact of breaking internal waves on interferometric synthetic aperture sonar. Full text in Research Archive
• Lorentzen, Ole J.; Synnes, Stig Asle Vaksvik; Hansen, Roy Edgar & Sæbø, Torstein Olsmo (2018). Investigating layover effects in cross-correlation based interferometry for synthetic aperture sonar. Full text in Research Archive
• Sternlicht, Daniel D.; Hayes, Michael P. & Hansen, Roy Edgar (2018). Historical development of seabed mapping synthetic aperture sonar. Full text in Research Archive
• Myers, Vincent; Sæbø, Torstein Olsmo & Hansen, Roy Edgar (2018). Comparison of co-registration techniques for synthetic aperture sonar images from repeated passes. Full text in Research Archive
• Hansen, Roy Edgar; Sæbø, Torstein Olsmo; Synnes, Stig Asle Vaksvik & Lorentzen, Ole J. (2018). Challenges in Coregistration of Repeated Passes in Synthetic Aperture Sonar. Full text in Research Archive
• Hansen, Roy Edgar; Lyons, Anthony P.; Cook, Daniel A. & Sæbø, Torstein Olsmo (2018). Quantifying the negative impact of breaking internal waves on interferometric synthetic aperture sonar. Full text in Research Archive
• Hansen, Roy Edgar; Sæbø, Torstein Olsmo; Lorentzen, Ole J. & Synnes, Stig Asle Vaksvik (2017). Mapping unexploded ordnance (UXO) using interferometric synthetic aperture sonar. Full text in Research Archive
• Lorentzen, Ole J.; Synnes, Stig Asle Vaksvik; Hansen, Roy Edgar; Sæbø, Torstein Olsmo & Geilhufe, Marc (2017). Local probability-based phase unwrapping for synthetic aperture sonar interferometry. Full text in Research Archive
• Lønmo, Tor Inge Birkenes; Austeng, Andreas & Hansen, Roy Edgar (2017). Phase Detections with Low Complexity Adaptive Beamforming on Swath Sonars. Full text in Research Archive
• Lyons, Anthony P.; Hansen, Roy Edgar; Prater, James; Connors, Warren A.; Rice, Glen & Pailhas, Yan (2017). Internal wave effects on seafloor imagery and bathymetry estimates. Full text in Research Archive
• Austeng, Andreas; Rodriguez-Molares, Alfonso & Hansen, Roy Edgar (2017). Performance of coherence based adaptive sonar beamformers. Full text in Research Archive
• Hansen, Roy Edgar; Sæbø, Torstein Olsmo; Synnes, Stig Asle Vaksvik & Lorentzen, Ole J. (2017). Synthetic aperture sonar interferometry for detailed seabed mapping: Performance considerations. Full text in Research Archive
• Sæbø, Torstein Olsmo; Hansen, Roy Edgar; Synnes, Stig Asle Vaksvik & Lorentzen, Ole J. (2017). Depth estimation of water column objects using interferometric synthetic aperture sonar. Full text in Research Archive
• Lønmo, Tor Inge Birkenes; Austeng, Andreas; Hansen, Roy Edgar & Tichy, Frank (2020). Adaptive Beamforming and Autocalibration for Swath Sonars. Universitetet i Oslo. Full text in Research Archive Show summary

  Accurate knowledge of the seabed is of vital importance for many human endeavors. Applications range from safe navigation to climate change models. Swath sonars are a key tool for efficient and high-resolution mapping of the seabed. This thesis aims to improve the quality of swath sonars by improving the beamformer, which is a key part of current signal processing. We explore two methods: Adaptive beamforming and autocalibration. Adaptive beamforming improves the beamforming process by adapting the beamforming to the received signal. We investigate how the adaptive Capon and Low Complexity Adaptive (LCA) beamformers can improve swath sonar beamforming on both simulated and field data, and their effect on the water column image and bathymetry. The Capon beamformer is well-tested and can give high performance, but has a high computational load and may have robustness issues. LCA is a recently developed and related adaptive beamformer which may be more robust and faster, with similar performance in many ways. We find that both beamformers improve resolution, edge definition and sidelobe level in the water column, and give more accurate amplitude detections. This leads to better defined features, better separation of features from the background, and sometimes detection of features not visible with the conventional delay and sum (DAS) beamformer. Capon has better resolution, somewhat better edge definition, and somewhat higher sidelobe level than LCA. We also find that an adaptive beamformer may improve interference rejection for phase detection, but generally reduces accuracy in the current configuration. This seems to be a side effect of the improved edge definition, and the effect can be reduced by reconfiguring the beamformers. Autocalibration estimates calibration errors without external reference sources. The errors, which particularly limit the sidelobe level, may then be compensated for. We model the errors by a complex factor per element and estimate them using data available during normal surveys. The method is based on the Generalized Interferometric Array Response. On simulated data, we are able to lower the sidelobe level below 50 dB. On field data, the sidelobe level is generally reduced, but the effect is much smaller. However, some sidelobes are unchanged and new sidelobes occasionally appear. We suggest that the reduced performance in the field is due to an insufficient calibration model.

• Ødegård, Øyvind; Sørensen, Asgeir Johan; Johnsen, Geir; Ludvigsen, Martin; Hansen, Roy Edgar & Maarleveld, Thijs (2018). Towards Autonomous Operations and Systems in Marine Archaeology. Norges teknisk-naturvitenskapelige universitet. ISSN 9788232631971. Full text in Research Archive Show summary

  A large part of our common cultural heritage, ranging from submerged prehistoric landscapes to modern steel wrecks, lies on the seabed. For centuries they have remained protected by depth and remoteness, but are now under pressure as our activities in the ocean space are increasing. Sound management of this underwater cultural heritage (UCH), especially in deeper waters, requires enabling methods and technologies for detection, investigation and monitoring. After what oceanographers have called “a century of undersampling”, the marine sciences are now benefiting from tremendous technological advances in sensors and sensor platforms. This thesis looks at how a selection of novel sensors and platforms can be used to improve and develop new methods for marine archaeological surveying. The potential for autonomous operations and systems for marine archaeology will in particular be adressed. Through a series of papers, the properties and capabilities of synthetic aperture sonar (SAS), underwater hyperspectral imager (UHI) and remotely operated vehicle (ROV) based stereo photogrammetry have been explored and applied to various UCH sites. Together they constitute a suite of sensors that can efficiently survey a large area with SAS, classify objects of interest based on spectral fingerprints with UHI, and finally perform detailed recording of archaeological sites with stereo cameras. Efficient exploration of the deep or remote marine environments depends on the use of underwater robotics, particularly untethered autonomous underwater vehicles (AUVs) that can be sent out on missions covering large areas and return with data from multiple sensors. However, for detailed investigations applying close range optical sensors, ROVs are preferable platforms. The resources and ROV pilot skills required for such operations still represent an obstacle for archaeological end users. The thesis discusses possible benefits, challenges and strategies for conducting surveys autonomously. New technology has led to new opportunities for a holistic environmental monitoring approach adjusted to purpose and object of interest. The proposed integrated environmental mapping and monitoring (IEMM) concept, presented in the thesis, describes the different steps in such an approach: from purpose of mission, to selection of parameters, sensors, sensor platforms, data collection, data storage, analysis and data interpretation for reliable decision making. The IEMM concept serves as a basis for a proposed framework for a fully autonomous marine archaeological survey, utilizing the sensors and platforms described in the papers. All sensors and platforms described in the thesis have been tested on actual archaeological materials and sites, both in laboratory and in the field. Experiences from interdisciplinary research cruises have been essential for developing frameworks for integrated operations, data processing and using collected data actively in mission planning and replanning. Theoretical issues concerning implications of new technologies and methods are considerable, and a thorough discussion of these issues is necessary for moving the field towards autonomous operations and systems in marine archaeology. The thesis is edited as a collection of papers.

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