OBJECT PERCEPTION IN UNDERWATER ENVIRONMENTS: A SURVEY ON SENSORS AND SENSING METHODOLOGIES

Underwater robots play a critical role in the marine industry. Object perception is the foundation for the automatic operations of submerged vehicles in dynamic aquatic environments. However, underwater perception encounters multiple environmental challenges, including rapid light attenuation, light refraction, or backscattering effect. These problems reduce the sensing devices’ signal-to-noise ratio (SNR), making underwater perception a complicated research topic. This paper describes the state-of-the-art sensing technologies and object perception techniques for underwater robots in different environmental conditions. Due to the current sensing modalities’ various constraints and characteristics, we divide the perception ranges into close-range, medium-range, and long-range. We survey and describe recent advances for each perception range and suggest some potential future research directions worthy of investigating in this field

Experiments on Surface Reconstruction for Partially Submerged Marine Structures

Over the past 10 years, significant scientific effort has been dedicated to the problem of three-dimensional (3-D) surface reconstruction for structural systems. However, the critical area of marine structures remains insufficiently studied. The research presented here focuses on the problem of 3-D surface reconstruction in the marine environment. This paper summarizes our hardware, software, and experimental contributions on surface reconstruction over the past few years (2008–2011). We propose the use of off-the-shelf sensors and a robotic platform to scan marine structures both above and below the waterline, and we develop a method and software system that uses the Ball Pivoting Algorithm (BPA) and the Poisson reconstruction algorithm to reconstruct 3-D surface models of marine structures from the scanned data. We have tested our hardware and software systems extensively in Singapore waters, including operating in rough waters, where water currents are around 1–2 m/s. We present results on construction of various 3-D models of marine structures, including slowly moving structures such as floating platforms, moving boats, and stationary jetties. Furthermore, the proposed surface reconstruction algorithm makes no use of any navigation sensor such as GPS, a Doppler velocity log, or an inertial navigation system.Singapore-MIT Alliance for Research and Technology. Center for Environmental Sensing and Modelin

A new 3-D modelling method to extract subtransect dimensions from underwater videos

Underwater video transects have become a common tool for quantitative analysis of the seafloor. However a major difficulty remains in the accurate determination of the area surveyed as underwater navigation can be unreliable and image scaling does not always compensate for distortions due to perspective and topography. Depending on the camera set-up and available instruments, different methods of surface measurement are applied, which make it difficult to compare data obtained by different vehicles. 3-D modelling of the seafloor based on 2-D video data and a reference scale can be used to compute subtransect dimensions. Focussing on the length of the subtransect, the data obtained from 3-D models created with the software PhotoModeler Scanner are compared with those determined from underwater acoustic positioning (ultra short baseline, USBL) and bottom tracking (Doppler velocity log, DVL). 3-D model building and scaling was successfully conducted on all three tested set-ups and the distortion of the reference scales due to substrate roughness was identified as the main source of imprecision. Acoustic positioning was generally inaccurate and bottom tracking unreliable on rough terrain. Subtransect lengths assessed with PhotoModeler were on average 20 % longer than those derived from acoustic positioning due to the higher spatial resolution and the inclusion of slope. On a high relief wall bottom tracking and 3-D modelling yielded similar results. At present, 3-D modelling is the most powerful, albeit the most time-consuming, method for accurate determination of video subtransect dimensions

Underwater iceberg profiling and motion estimation using autonomous underwater vehicles

Icebergs originating from high latitude glaciers have drawn much attention from scientists and offshore operators in the North Atlantic. Scientists are curious about the iceberg drift and deterioration, while the offshore industry is concerned about the potential risks and damages on offshore oil platforms and infrastructures. In order to provide information to improve the iceberg drift and deterioration model constructed by scientists, and to assess the threats posed by icebergs to offshore platforms, iceberg shapes need to be measured. For the above water portion, optical instruments such as a camera and a laser scanner/LIDAR can be used. However, measuring the underwater portion of an iceberg is more challenging due to navigational constraints and sensor limitations. One approach, commonly used, is to deploy a horizontal plane scanning sonar from a support vessel at several locations around the iceberg. There are many drawbacks to this method, including the cost, sensing trade-offs in resolution and coverage, as well as constraints because of weather conditions limiting safe operations. The technology of Autonomous Underwater Vehicles (AUVs) has been developing rapidly in the last two decades. AUVs are commonly chosen to carry scientific sensors for various oceanographic applications. Without human intervention, AUVs can accomplish pre-programmed missions autonomously and deliver scientific data upon the users’ request. With these advantages, AUVs are considered as potential candidates in underwater iceberg sensing operations because they can operate close to icebergs to measure shapes and collect environmental data of the surrounding water. Sonar is usually used for underwater mapping applications. Since AUVs are typically quieter acoustically than manned surface vessels, a low noise to signal ratio can be achieved on sonars carried by AUVs. In this research, a technology of AUV-based underwater iceberg-profiling is evaluated. An iceberg-profiling simulator is constructed to analyse underwater iceberg-profiling missions. With the simulator, the accuracy of AUV-based operation is compared with conventional methods of deploying sonar profilers around icebergs. Beyond the simulation, a guidance, navigation, and control (GNC) system is designed with an objective of guiding the vehicle traveling around the iceberg at a standoff distance. The GNC uses measurements from a mechanical scanning sonar to construct a vehicleattached occupancy map (VOM) that the probability of occupancy of the cells in the VOM is updated based on a dynamic inverse-sonar model. Using the occupancy information about the cells in the VOM, the line-of-sight (LOS) guidance law is used to compute the desired heading for the existing heading controller in the AUV. The GNC is first calibrated and validated in a simulated environment. Then, an AUV equipped with a forward side-looking mechanical scanning sonar is deployed in the field. The GNC guides the vehicle circumnavigated an iceberg autonomously, and underwater shape of the target iceberg is represented using the sonar samples. The point cloud may deviate from the original iceberg shape due to the iceberg movement. A motion estimation algorithm is developed to estimate the iceberg motion for converting the point cloud into an iceberg-centered coordinate system. Two point clouds measured at different times, inputs of the motion estimation algorithm, are presumed to be identical in the iceberg-centered coordinate system. Then, the algorithm iteratively updates the motion estimates based on the translational matrix and rotational matrix from an iterative closest point (ICP) algorithm to match the point clouds. The hypothesis that two point clouds are identical in the iceberg-centered coordinate system is valid when the motion estimates are converged in the updating process. Once the iceberg motion is resolved, the point cloud in the inertial coordinate can be converted in to the iceberg-centered coordinate to present the true iceberg shape. The algorithm for estimating iceberg motion is applied to data collected from the simulation environment and the field trials in Newfoundland

Limnoarchaeology Using Commercially Available Side Scan Sonar: The Locks of the Muscle Shoals Canal, Alabama

This research aims to introduce, utilize and analyze commercially available side-scan sonar units as a viable scientific research tool in the interest of historic preservation and archaeological inventory of submerged structures and landscapes using a case study of the Muscle Shoals Canal in Alabama. Opened in November 1890, the nine locks of the Muscle Shoals Canal allowed traffic on the Tennessee River to circumvent the treacherous Muscle Shoals , an achievement that replaced an abandoned attempt from over a half century earlier. The canal would continue operations until April 1918, when construction of Wilson Dam blocked river traffic through the canal. After completion of the dam in 1924, the canal system was flooded by the newly formed Wilson Reservoir. This study used commercial grade sonar equipment to image flooded remains of the canal system. Along with inexpensive commercial software, a single low-cost sonar unit was capable of producing 3D bathymetry, side-scan imagery of the canal system, and detailed down-scan imagery of individual features of remaining lock components. Sonar output was compared to historical photographs of the canal and lock structures in order to examine what level of detail is possible from this consumer grade technology. Results show that the consumer grade sonar used in this study is capable of providing data suitable for creation of resource inventories of historical sites in the interest of historic preservation. The low cost and efficiency of consumer grade sonar units have potential to open the doors to future research and exploration of underwater treasures

Development Of A Vision System For Ship Hull Inspection

Penyelidikan ini memperkenalkan strategi pengawalan untuk memperbaiki prestasi pemeriksaan visual badan kapal dengan menggunakan kenderaan dalam air. This work introduces a strategy to improve the performance of visual ship hull inspection using a Remotely-Operated Vehicle (ROV) as its underwater vehicle platform

Development Of A Vision System For Ship Hull Inspection [K4165. Z94 2007 f rb].

Penyelidikan ini memperkenalkan strategi pengawalan untuk memperbaiki prestasi pemeriksaan visual badan kapal dengan menggunakan kenderaan dalam air. Kaedah yang dicadangkan bertujuan untuk membangunkan sebuah sistem yang secara visualnya sentiasa kekal selari pada permukaan badan kapal. This work introduces a strategy to improve the performance of visual ship hull inspection using a Remotely-Operated Vehicle (ROV) as its underwater vehicle platform. The proposed method is aimed at developing a system that will maintain the camera viewing angle parallel to the ship hull surface