Seafloor Mapping Research Articles

CLASSIFICATION OF DATA FROM AIRBORNE LIDAR BATHYMETRY WITH RANDOM FOREST ALGORITHM BASED ON DIFFERENT FEATURE VECTORS

Abstract. Modern full-waveform laser bathymetric scanners offer the possibility of a practical application of airborne laser bathymetry (ALB) data algorithms as a valuable source of information in the study of the aquatic environment. The reliability of the obtained results and the efficiency of the classification depend on the applied features. The input data for the classifier should consist of variables that have the ability to discriminate within the data set, for the detection and classification of objects on the seabed. The automatic detection of underwater objects is based on machine learning solutions. In this paper, the ALB data were used to present a classification process based on the random forest algorithm. The classification was carried out using two independent approaches with two feature vectors. The quality of classifications based on the full-waveform features vector and the geometric features vector was compared. The efficiency of each classification was verified using a confusion matrix. The obtained efficiency of the point classification in both cases was about 100% for the water surface, 99.9% for the seabed and about 60% for underwater objects. Better results for the classification of objects were obtained for the features vector based on features obtained directly from full-waveform data than for the vector obtained from geometric relationships in the point cloud.

Degeneracy-Aware Imaging Sonar Simultaneous Localization and Mapping

High-frequency imaging sonar sensors have recently been applied to aid underwater vehicle localization, by providing frame-to-frame odometry measurements or loop closures over large timescales. Previous methods have often assumed a planar environment, thereby restricting the use of such algorithms mostly to seafloor mapping. We propose an algorithm to generate pose-to-pose constraints for pairs of sonar images, which may also be applied to larger sets of images, that makes no assumptions about the environmental geometry. The algorithm is sensitive to the inherent degeneracies of the imaging sonar sensor model, and may be tuned to tradeoff between providing more constraints on the sensor motion and not overfitting to noise in the measurements. For real-time localization, we fuse the resulting pair-wise sonar pose constraints with vehicle odometry in a pose graph optimization framework. We rigorously evaluate the proposed method and demonstrate improvement in accuracy over previously proposed formulations both in simulation and real-world experiments.

Compressive synthetic aperture sonar imaging with distributed optimization.

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

On the Information Advantage of Sidescan Sonar Three-Frequency Colour over Greyscale Imagery

A prototype three-frequency (114, 256, and 410 kHz) colour sidescan sonar system, built by Kongsberg Underwater Mapping Ltd. (Great Yarmouth, UK), was previously described, and preliminary results presented, in Tamsett, McIlvenny, and Watts. The prototype system has subsequently been modified, and in 2017, new data were acquired in a resurvey of the Inner Sound of the Pentland Firth, North Scotland. An image texture characterisation and image classification exercise demonstrates considerably greater discrimination between different seabed classes in a three-frequency colour sonar image of the seabed, than in a multi-frequency colour image reduced to greyscale display, or in a single-frequency greyscale image, with readily twice the number of classes of seabed discriminated between, in the colour image. The information advantage of colour acoustic imagery over greyscale acoustic imagery is analogous to the information advantage of colour television images over black-and-white television images. A three-frequency colour sonar image contains a theoretical maximum of a factor of 3 times the information in a corresponding greyscale image, for independent seabed responses at the three frequencies. Estimates of the average information per pixel (information entropy) in the colour image, and in corresponding greyscale images, reveal an actual information advantage of colour sonar imagery over greyscale, to be in practice approximately a factor of 2.5, empirically confirming the greater information based utility of three-frequency colour sonar over greyscale sonar. Reference: Tamsett, D.; McIlvenny, J.; Watts, A. J. Mar. Sci. Eng. 2016, 4(26).

Bathymetric properties of the Baltic Sea

Abstract. Baltic Sea bathymetric properties are analysed here using the newly released digital bathymetric model (DBM) by the European Marine Observation and Data Network (EMODnet). The analyses include hypsometry, volume, descriptive depth statistics, and kilometre-scale seafloor ruggedness, i.e. terrain heterogeneity, for the Baltic Sea as a whole as well as for 17 sub-basins defined by the Baltic Marine Environment Protection Commission (HELCOM). We compare the new EMODnet DBM with IOWTOPO the previously most widely used DBM of the Baltic Se aproduced by the Leibniz-Institut für Ostseeforschung Warnemünde (IOW), which has served as the primary gridded bathymetric resource in physical and environmental studies for nearly two decades. The area of deep water exchange between the Bothnian Sea and the Northern Baltic Proper across the Åland Sea is specifically analysed in terms of depths and locations of critical bathymetric sills. The EMODnet DBM provides a bathymetric sill depth of 88 m at the northern side of the Åland Sea and 60 m at the southern side, differing from previously identified sill depths of 100 and 70 m, respectively. High-resolution multibeam bathymetry acquired from this deep water exchange path, where vigorous bottom currents interacted with the seafloor, allows us to assess what presently available DBMs are missing in terms of physical characterization of the seafloor. Our study highlights the need for continued work towards complete high-resolution mapping of the Baltic Sea seafloor.

Direct linear and refraction-invariant pose estimation and calibration model for underwater imaging

Accuracy, detail, and limited time on site make photogrammetry a valuable means for underwater mapping. Imaging in such domains is subjected however to distortions which are caused by refraction of the incoming rays. As the literature shows, these distortions are depth-dependent, non-linear, and alter the standard single viewpoint geometry. To handle their effect, we derive in this paper a refraction-invariant representation and show that despite the pronounced distortions, such a model is attainable. We also show that its contribution is not only theoretical, as it also allows to estimate the pose parameters linearly and at a significantly improved accuracy. The paper then extends the model to calibrate the underwater-related system parameters and, again, demonstrates the ability to yield a linear model, to simplify the settings and requirements for calibration procedures, and most importantly to improve the accuracy of the system parameters by an order of magnitude or more. Experiments also show enhanced accuracy and stability of the model in the presence of high-level of noise. Thus, the paper provides an in-depth look into the geometrical modeling of underwater images and at the same time offers practical enhancement of the accuracies and requirements to reach them.

Seabed sediments of Søre Sunnmøre, Norway

ABSTRACT An increasing number of activities compete for space in the Norwegian coastal zone, making access to detailed seabed information particularly valuable. We present a suite of thematic marine base maps of the near-shore areas of five municipalities in west Norway (567 km2; at 62°N/5°E). This set of full-coverage seabed maps includes sediment grain size, seabed terrain (shaded relief), slope, sediment accumulation basins, anchoring conditions, and diggability (trenching properties). The sediment grain size map is a geological interpretation of multibeam echosounder data supported by video observations and physical samples of seabed sediments. The other maps in the suite are derived from this sediment map, and/or directly from the bathymetry data. All maps are at a scale of 1:20,000 and are freely accessible for download or online viewing. Marine base maps are intended for all end-users with a need for knowledge of seabed conditions and may be especially valuable for marine spatial planning.

Fight for the Arctic Ocean is a boon for science

Armed with seafloor maps, Russia, Denmark, and Canada seek control over the North Pole.

Generating higher resolution regional seafloor maps from crowd-sourced bathymetry.

Seafloor mapping can offer important insights for marine management, spatial planning, and research in marine geology, ecology, and oceanography. Here, we present a method for generating regional bathymetry and geomorphometry maps from crowd-sourced depth soundings (Olex AS) for a small fraction of the cost of multibeam data collection over the same area. Empirical Bayesian Kriging was used to generate a continuous bathymetric surface from incomplete and, in some areas, sparse Olex coverage on the Newfoundland and Labrador shelves of eastern Canada. The result is a 75m bathymetric grid that provides over 100x finer spatial resolution than previously available for the majority of the 672,900 km2 study area. The interpolated bathymetry was tested for accuracy against independent depth data provided by Fisheries and Oceans Canada (Spearman correlation = 0.99, p<0.001). Quantitative terrain attributes were generated to better understand seascape characteristics at multiple spatial scales, including slope, rugosity, aspect, and bathymetric position index. Landform classification was carried out using the geomorphons algorithm and a novel method for the identification of previously unmapped tributary canyons at the continental shelf edge are also presented to illustrate some of many potential benefits of crowd-sourced regional seafloor mapping.

Seafloor Mapping – The Challenge of a Truly Global Ocean Bathymetry

Detailed knowledge of the shape of the seafloor is crucial to humankind. Bathymetry data is critical for safety of navigation and is used for many other applications. In an era of ongoing environmental degradation worldwide, bathymetry data (and the knowledge derived from it) play a pivotal role in using and managing the world’s oceans in a way that is in accordance with the United Nations Sustainable Development Goal 14 - conserve and sustainably use the oceans, seas and marine resources for sustainable development. However, the vast majority of our oceans is still virtually unmapped, unobserved, and unexplored. Only a small fraction of the seafloor has been systematically mapped by direct measurement. The remaining bathymetry is predicted from satellite altimeter data, providing only an approximate estimation of the shape of the seafloor. Several global and regional initiatives are underway to change this situation. This paper presents a selection of these initiatives as best practice examples for bathymetry data collection, compilation and open data sharing as well as the Nippon Foundation-GEBCO (The General Bathymetric Chart of the Oceans) Seabed 2030 Project that complements and leverages these initiatives and promotes international collaboration and partnership. Several non-traditional data collection opportunities are looked at that are currently gaining momentum as well as new and innovative technologies that can increase the efficiency of collecting bathymetric data. Finally, recommendations are given towards a possible way forward into the future of seafloor mapping and towards achieving the goal of a truly global ocean bathymetry.

Improvement of Full Waveform Airborne Laser Bathymetry Data Processing based on Waves of Neighborhood Points

Measurements of the topography of the sea floor are one of the main tasks of hydrographic organizations worldwide. The occurrence of any disaster in maritime traffic can contaminate the environment for many years. Therefore, increasing attention is being paid to the development of effective methods for the detection and monitoring of possible obstacles on the transport route. Bathymetric laser scanners record the full waveform reflected from the object (target). Its transformation allows to obtain information about the water surface, water column, seabed, and the objects on it. However, it is not possible to identify subsequent returns among all waves, leading to a loss of information about the situation under the water. On the basis of the studies conducted, it was concluded that the use of a secondary analysis of a full waveform of the airborne laser bathymetry allowed for the identification of objects on the seabed. It allowed us to detect further points in the point cloud, which are necessary in the identification of objects on the seabed. The results of the experiment showed that, among the area of experiment where objects on the seabed were located, the number of points increased between 150 and 550% and the altitude accuracy of the seabed elevation model even by 50% to the level of 0.30 m with reference to sonar data depending of types of objects.

DEVELOPING THE CONSTELLATION UNMANNED SURFACE VEHICLE PROTOTYPE

Vessel navigation through the use of reliable nautical charts is vital to ensure safe passage. Having the ability to complement this with the ability to see accurately both what lies beneath the water and ahead of a vessel in real time provides an added dimension of safety and certainty. Combining the high value and deep draft of modern super and mega yachts with a penchant for exploration creates a tension which can test the availability and accuracy of navigation charts. The development of a 2 metre remote-operated and autonomous-capable surface vehicle with ultra-high-resolution echo-sounder capability provides a unique solution which can open up considerable exploration capability. The ability to produce ultra-high-resolution images and relay these back to the mother vessel in real time enables precise seabed mapping, providing safer navigation. Additionally, it has the capability to deploy personal water-safety devices in situations such as a man-overboard incident.

PRELIMINARY EXPERIMENTAL STUDY ON DESIGNING BALLAST SYSTEM FOR AUTONOMOUS UNDERWATER VEHICLE

ABSTRACT&#x0D; Unmanned submarine commonly called Autonomous Underwater Vehicle (AUV) is one type of underwater robots used for underwater mapping. AUV is an underwater vehicle capable of automatically moving in water, controlled by humans on vessel. To build AUV is not easy as many components play important roles in the operation of AUV, one of which is active ballast. Calculations on the making and benchmarks of active ballast systems are still very rare. Active ballast is a set of equipment used to fill its tanks with seawater and to empty sea water from the tanks on submarines. These tanks are intended to balance submarines and for active ballast systems on submarines so as to be able to dive and float as needed. In this paper an experimental study was carried out on a tube resembling AUV with both fresh water and sea water to obtain ballast volume in AUV.&#x0D; Keywords: AUV, Active ballast, Experimental study

SHALLOW WATER BATHYMETRY MAPPING FROM UAV IMAGERY BASED ON MACHINE LEARNING

Abstract. The determination of accurate bathymetric information is a key element for near offshore activities, hydrological studies such as coastal engineering applications, sedimentary processes, hydrographic surveying as well as archaeological mapping and biological research. UAV imagery processed with Structure from Motion (SfM) and Multi View Stereo (MVS) techniques can provide a low-cost alternative to established shallow seabed mapping techniques offering as well the important visual information. Nevertheless, water refraction poses significant challenges on depth determination. Till now, this problem has been addressed through customized image-based refraction correction algorithms or by modifying the collinearity equation. In this paper, in order to overcome the water refraction errors, we employ machine learning tools that are able to learn the systematic underestimation of the estimated depths. In the proposed approach, based on known depth observations from bathymetric LiDAR surveys, an SVR model was developed able to estimate more accurately the real depths of point clouds derived from SfM-MVS procedures. Experimental results over two test sites along with the performed quantitative validation indicated the high potential of the developed approach.

A Detailed Reconstruction of the Roman Landscape and the Submerged Archaeological Structure at “Castel dell’Ovo islet” (Naples, Southern Italy)

In this paper, we present the results of a multidisciplinary study aimed to reconstruct the Roman coastal landscape between Pizzofalcone hill and Megaris islet—the area of the ancient Parthenope, the first settlement along the Naples coast. This coastal sector was surveyed by a team of specialized divers (archaeologists and geomorphologists) and by using an Unmanned Surface Vessel (USV) equipped with acoustic and optical sensors. The indirect surveys provided a high-resolution dataset of morpho-acoustic and optical measurements, useful to obtain the geological, geomorphological and archaeological interpretations necessary to formulate hypotheses on the functionality of the complex submerged archaeological structure detected in the study area. In particular, the integration between the surveyed data, the high-resolution seafloor mapping and the previous knowledge deriving from the 1980s underwater research carried out by Centro Studi Subacquei, led us to interpret the submerged remains as a vivarium related to a 1st century BC Roman villa. Finally, by measuring the submersion of several channels and a well-preserved crepido, a relative sea level during the period of use at −2.2 m ± 0.2 m mean sea level (MSL) was deduced, in agreement with the previous geoarchaeological studies realized in the near coastal sectors.

Two-Dimensional Frontier-Based Viewpoint Generation for Exploring and Mapping Underwater Environments

To autonomously explore complex underwater environments, it is convenient to develop motion planning strategies that do not depend on prior information. In this publication, we present a robotic exploration algorithm for autonomous underwater vehicles (AUVs) that is able to guide the robot so that it explores an unknown 2-dimensional (2D) environment. The algorithm is built upon view planning (VP) and frontier-based (FB) strategies. Traditional robotic exploration algorithms seek full coverage of the scene with data from only one sensor. If data coverage is required for multiple sensors, multiple exploration missions are required. Our approach has been designed to sense the environment achieving full coverage with data from two sensors in a single exploration mission: occupancy data from the profiling sonar, from which the shape of the environment is perceived, and optical data from the camera, to capture the details of the environment. This saves time and mission costs. The algorithm has been designed to be computationally efficient, so that it can run online in the AUV’s onboard computer. In our approach, the environment is represented using a labeled quadtree occupancy map which, at the same time, is used to generate the viewpoints that guide the exploration. We have tested the algorithm in different environments through numerous experiments, which include sea operations using the Sparus II AUV and its sensor suite.

Cost-effective Framework for Rapid Underwater Mapping with Digital Camera and Color Correction Method

Geo-referenced mapping in an aquatic environment is challenging because it is hard to measure the location of objects and images underwater. In this paper, we propose the method and share the results: cost-effective framework to generate 2D and 3D underwater maps with digital imagery and a Structure from Motion (SfM) algorithm. The proposed method consists of data acquisition, image processing, and color correction. 292 and 437 images were acquired from each study site located in Spring Lake in San Marcos, Texas, U.S.A. Agisoft Photoscan Pro software was used to generate 3D point cloud data and orthomosaic images after feature matching and image alignment from geo-tagged imagery. The mosaic images with high resolution (< 0.2 cm per pixel) were generated with 2D underwater images. After color correction, the red reduction effect was recovered, and the bluer color was removed. The 3D underwater map was generated directly from 3D dense point clouds including geo-coordinates and RGB color information. As a result, the Very High Resolution (VHR) 2D and 3D maps were generated and the topographic surface of underwater structures was obtained in great detail. Although the RMSE were about 1 m, the proposed method provided more detailed surface of underwater features.

Design and evaluation of a full-wave surface and bottom-detection algorithm for LiDAR bathymetry of very shallow waters

Airborne Laser Bathymetry (ALB) is an attractive technology for the measurement of shallow water bodies because of the high acquisition rate and high point densities that can be achieved. Of special interest is the application of ALB in non-navigable areas where the only alternatives are conventional terrestrial surveying by wading with a pole, multi-media photogrammetry, or spectrally based depth retrieval. The challenge for laser based approaches in such very shallow waters (<2 m) is the difficulty of discriminating between echoes from the surface and the bottom. This work presents an algorithm for the detection of surface, volume, and bottom (SVB) designed to meet this challenge while requiring only a single wavelength (532nm) sensor. The accuracy of the algorithm is cross validated against reference measurements obtained from terrestrial survey with a total station and shows negligible bias and virtually no depth dependence for the experimental dataset.

Delineating Lake Bottom Structure by TEM on Water Surface

Recently the development of TEM (Transient Electromagnetic) provides higher resolution and greater exploration depth than ever before and it is time to consider whether this advanced TEM technique can be applied to under-water mapping. About 250 TEM soundings have been measured within a lake of about 120m by 60m in a muddy deposits environments in southern Taiwan. For the TEM measurements the terrTEM unit (Monex GeoScope) was used with an inloop configuration, for which the receiver spacing is about 5 m and the transmitter loop is about the same size as the lake. For imaging the vertical and horizontal resistivity distribution within the deposit and the surrounding area, all sounding results were interpreted using a smooth inverse procedure. Some auger drilling and sounding were available for correlation. This led to a final model. Underneath the water bottom is mud/shale inter-bed with a depth greater than 50 m and resistivity ranging from 1 to 5 ohm-m. The depth of the clayey layer predicted by TEM data is comparable to that of drilled wells nearby, and the thick mud to the southeast suggests the lake dipping to the southeast. The success of the method depends upon the appropriate design of survey parameters; this new application is open to further refinement, both in instruments and in survey techniques.

Sub-Antarctic Auckland Islands Seafloor Mapping Investigations Using Legacy Data

This paper demonstrates the richness of data collected for nautical charting and considers ways in which chart data can support scientific research, through a case study of two modern navigation surveys undertaken in the Auckland Islands. While legacy charts have coarser resolution, and may synthesize different epochs together into one final product, we examine how they may be used on their own and to complement more recent hydrographic surveys. We argue that the hydrographic and ancillary data, only a fraction of which appears on the final chart, also has scientific value and that the hydrographic surveying principles applied during data collection are equally relevant for all seabed mapping. While the benefits of full bottom coverage obtained by state of-the-art multibeam surveys are clear, there is much more to be discovered in legacy singlebeam datasets than what is displayed on the nautical chart alone.