The application of ERTS imagery to monitoring Arctic sea ice

The author has identified the following significant results. Because of the effect of sea ice on the heat balance of the Arctic and because of the expanding economic interest in arctic oil and minerals, extensive monitoring and further study of sea ice is required. The application of ERTS data for mapping ice is evaluated for several arctic areas, including the Bering Sea, the eastern Beaufort Sea, parts of the Canadian Archipelago, and the Greenland Sea. Interpretive techniques are discussed, and the scales and types of ice features that can be detected are described. For the Bering Sea, a sample of ERTS-1 imagery is compared with visual ice reports and aerial photography from the NASA CV-990 aircraft. The results of the investigation demonstrate that ERTS-1 imagery has substantial practical application for monitoring arctic sea ice. Ice features as small as 80-100 m in width can be detected, and the combined use of the visible and near-IR imagery is a powerful tool for identifying ice types. Sequential ERTS-1 observations at high latitudes enable ice deformations and movements to be mapped. Ice conditions in the Bering Sea during early March depicted in ERTS-1 images are in close agreement with aerial ice observations and photographs

Remote sensing of floe size distribution and surface topography

Floe size can be measured by several properties p- for instance, area or mean caliper diameter. Two definitions of floe size distribution seem particularly useful. F(p), the fraction of area covered by floes no smaller than p; and N(p), the number of floes per unit area no smaller than p. Several summertime distributions measured are a graph, their slopes range from -1.7 to -2.5. The variance of an estimate is also calculated

Sensitivity of radar altimeterwaveform to changes in sea ice type at resolution of synthetic aperture radar

Radar altimetry in the context of sea ice has mostly been exploited to retrieve basin-scale information about sea ice thickness. In this paper, we investigate the sensitivity of altimetric waveforms to small-scale changes (a few hundred meters to about 10 km) of the sea ice surface. Near-coincidental synthetic aperture radar (SAR) imagery and CryoSat-2 altimetric data in the Beaufort Sea are used to identify and study the spatial evolution of altimeter waveforms over these features. Open water and thin ice features are easily identified because of their high peak power waveforms. Thicker ice features such as ridges and multiyear ice floes of a few hundred meters cause a response in the waveform. However, these changes are not reflected in freeboard estimates. Retrieval of robust freeboard estimates requires homogeneous floes in the order of 10 km along-track and a few kilometers to both sides across-track. We conclude that the combination of SAR imagery and altimeter data could improve the local sea ice picture by extending spatially scarce freeboard estimates to regions of similar SAR signature

Sea ice thickness and iceberg distribution in the Southern Ocean

The sea ice thickness distribution, represented by the probability density function (PDF), is critical to Earth's climate system and knowledge of the distribution in the Antarctic is limited. A novel methodology, using an acoustic Doppler current profiler, was developed to measure sea ice draft based on measurements taken from Autosub (an autonomous underwater vehicle). The PDF has been derived for three missions undertaken in the eastern Amundsen Sea during March 2003. The PDFs for all missions were found to have a single mode although there is evidence for variability in mean thickness along the ice front, as the most western mission has a lower mean draft. Geostatistical analyses of the data have allowed the derivation of PDFs to account for the spatial sampling. A factor on the thickness of ice measured is the presence of icebergs within a study region. This thesis reports on work carried out to investigate whether a correction to the sea ice thickness PDF can be made to account for icebergs and over what scale(s) this correction is valid. To answer this question data from Autosub and satellite images were used to investigate whether icebergs were randomly distributed both in open ocean and within sea ice. In conclusion, it was found that icebergs do cluster on the scale of typical Autosub missions (~few km). However, there are differences between icebergs in sea ice compared with open water. Therefore, icebergs should be accounted for in the sea ice thickness PDF

Sea-ice habitat preference of the Pacific walrus (Odobenus rosmarus divergens) in the Bering Sea: a multiscaled approach

Thesis (M.S.) University of Alaska Fairbanks, 2015The goal of this thesis is to define specific parameters of mesoscale sea-ice seascapes for which walruses show preference during important periods of their natural history. This research thesis incorporates sea-ice geophysics, marine-mammal ecology, remote sensing, computer vision techniques, and traditional ecological knowledge of indigenous subsistence hunters in order to quantitatively study walrus preference of sea ice during the spring migration in the Bering Sea. Using an approach that applies seascape ecology, or landscape ecology to the marine environment, our goal is to define specific parameters of ice-patch descriptors and mesoscale seascapes in order to evaluate and describe potential walrus preference for such ice and the ecological services it provides during an important period of their life-cycle. The importance of specific sea-ice properties to walrus occupation motivates an investigation into how walruses use sea ice at multiple spatial scales when previous research suggests that walruses do not show preference for particular floes. Analysis of aerial imagery, using image processing techniques and digital geomorphometric measurements (floe size, shape, and arrangement), demonstrated that while a particular floe may not be preferred, at larger scales a collection of floes, specifically an ice-patch (< 4 km²), was preferred. This shows that walruses occupy ice patches with distinct ice features such as floe convexity, spatial density, and young ice and open water concentration. Ice patches that are occupied by adult and juvenile walruses show a small number of characteristics that vary from those ice patches that were visually unoccupied. Using synthetic aperture radar imagery, we analyzed co-located walrus observations and statistical texture analysis of radar imagery to quantify seascape preferences of walruses during the spring migration. At a coarse resolution of 100-9,000 km², seascape analysis shows that, for the years 2006-2008, walruses were preferentially occupying fragmented pack ice seascapes range 50-89% of the time, when, all throughout the Bering Sea, only range 41-46% of seascapes consisted of fragmented pack ice. Traditional knowledge of a walrus' use of sea ice is investigated through semi-directed interviews conducted with subsistence hunters and elders from Savoonga and Gambell, two Alaskan Native communities on St. Lawrence Island, Alaska. Informants were provided with a large nautical map of the land and ocean surrounding St. Lawrence Island and 45 printed largeformat aerial photographs of walruses on sea ice to stimulate discussion as questions were asked to direct the topics of conversation. Informants discussed change in sea ice conditions over time, walrus behaviors during the fall and spring subsistence hunts, and sea-ice characteristics that walruses typically occupy. These observations are compared with ice-patch preferences analyzed from aerial imagery. Floe size was found to agree with remotely-sensed ice-patch analysis results, while floe shape was not distinguishable to informants during the hunt. Ice-patch arrangement descriptors concentration and density generally agreed with ice-patch analysis results. Results include possible preference of ice-patch descriptors at the ice-patch scale and fragmented pack ice preference at the seascape scale. Traditional knowledge suggests large ice ridges are preferential sea-ice features at the ice-patch scale, which are rapidly becoming less common during the fall and spring migration of sea ice through the Bering Sea. Future work includes increased sophistication of the synthetic aperture radar classification algorithm, experimentation with various spatial scales to determine the optimal scale for walrus' life-cycle events, and incorporation of further traditional knowledge to investigate and interface crosscultural sea-ice observations, knowledge and science to determine sea ice importance to marine mammals in a changing Arctic

Sea Ice Field Analysis Using Machine Vision

Sea ice field analysis has motivation in various areas, such as environmental, logistics or ship maintenance. Among other methods, local ice field analysis from ship-based visual observations are currently done by human volunteers and therefore are liable to human errors and subjective interpretations. The goal of the thesis is to develop and implement a complete process for obtaining dimensions, distribution and concentration of sea-ice floes, which aims at assisting and improving part of the aforementioned visual observations. Such process involves numerous, organized steps which take advantage of techniques from image processing (lens calibration, vignetting removal and orthorectification), robotics (transformation frames) and machine vision (thresholding and texture analysis methods, and morphological operations). An experimental system setup for collecting the required information is provided as well, which includes a machine vision camera for image acquisition, an IMU device for determining the dynamic attitude of the cameras with respect to the world, two GPS sensors providing a redundant positioning and clock data, and a desktop computer used as the main logging platform for all the collected data. Through a number of experiments, the proposed system setup and image analysis methods have proved to provide promising results in pack ice and brash ice conditions, thus encouraging further research on the topic. Further improvements should target the accuracy of ice floes detection, and over and under-segmentation of the detected sea-ice floes

Image Processing for Ice Parameter Identification in Ice Management

Various types of remotely sensed data and imaging technology will aid the development of sea-ice observation to, for instance, support estimation of ice forces critical to Dynamic Positioning (DP) operations in Arctic waters. The use of cameras as sensors for offshore operations in ice-covered regions will be explored for measurements of ice statistics and ice properties, as part of a sea-ice monitoring system. This thesis focuses on the algorithms for image processing supporting an ice management system to provide useful ice information to dynamic ice estimators and for decision support. The ice information includes ice concentration, ice types, ice floe position and floe size distribution, and other important factors in the analysis of ice-structure interaction in an ice field. The Otsu thresholding and k-means clustering methods are employed to identify the ice from the water and to calculate ice concentration. Both methods are effective for model-ice images. However, the k-means method is more effective than the Otsu method for the sea-ice images with a large amounts of brash ice and slush. The derivative edge detection and morphology edge detection methods are used to try to find the boundaries of the ice floes. Because of the inability of both methods to separate connected ice floes in the images, the watershed transform and the gradient vector flow (GVF) snake algorithm are applied. In the watershed-based method, the grayscale sea-ice image is first converted into a binary image and the watershed algorithm is carried out to segment the image. A chain code is then used to check the concavities of floe boundaries. The segmented neighboring regions that have no concave corners between them are merged, and over-segmentation lines are removed automatically. This method is applicable to separate the seemingly connected floes whose junctions are invisible or lost in the images. In the GVF snake-based method, the seeds for each ice floe are first obtained by calculating the distance transform of the binarized image. Based on these seeds, the snake contours with proper locations and radii are initialized, and the GVF snakes are then evolved automatically to detect floe boundaries and separate the connected floes. Because some holes and smaller ice pieces may be contained inside larger floes, all the segmented ice floes are arranged in order of increasing size after segmentation. The morphological cleaning is then performed to the arranged ice floes in sequence to enhance their shapes, resulting in individual ice floes identification. This method is applicable to identify non-ridged ice floes, especially in the marginal ice zone and managed ice resulting from offshore operations in sea-ice. For ice engineering, both model-scale and full-scale ice will be discussed. In the model-scale, the ice floes in the model-ice images are modeled as square shapes with predefined side lengths. To adopt the GVF snake-based method for model-ice images, three criteria are proposed to check whether it is necessary to reinitialize the contours and segment a second time based on the size and shape of model-ice floe. In the full-scale, sea-ice images are shown to be more difficult than the model-ice images analyzed. In addition to non-uniform illumination, shadows and impurities, which are common issues in both sea-ice and model-ice image processing, various types of ice (e.g., slush, brash, etc.), irregular floe sizes and shapes, and geometric distortion are challenges in seaice image processing. For sea-ice image processing, the “light ice” and “dark ice” are first obtained by using the Otsu thresholding and k-means clustering methods. Then, the “light ice” and “dark ice” are segmented and enhanced by using the GVF snake-based method. Based on the identification result, different types of sea-ice are distinguished, and the image is divided into four layers: ice floes, brash pieces, slush, and water. This then makes it possible to present a color map of the ice floes and brash pieces based on sizes. It also makes it possible to present the corresponding ice floe size distribution histogram

Information fusion for estimation of summer MIZ ice concentration from SAR imagery

©1999 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.In this paper we define the concept of information fusion and show how we used it to estimate summer sea ice concentration in the marginal ice zone (MIZ) from single-channel SAR satellite imagery, We used data about melt stage, wind speed, and surface temperature to generate temporally-accumulated information, and fused this information with the SAR image, resulting in an interpretation of summer MIZ imagery, We also used the results of previous classifications of the same area to guide and correct future interpretations, thus fusing historical information with imagery and nonimagery data. We chose to study the summer MIZ since summer melt conditions cause classification based upon backscatter intensity to fail, as the backscatter of open water, thin ice, first-year ice, and multiyear ice overlap to a large degree. This makes it necessary to fuse various information and data to achieve proper segmentation and automated classification of the image. Our results were evaluated qualitatively and showed that our approach produces very good ice concentration estimates in the summer MIZ

Satellite radar altimetry of sea ice

The thesis concerns the analysis and interpretation of data from satellite borne radar altimeters over ice covered ocean surfaces. The applications of radar altimetry are described in detail and consider monitoring global climate change, the role that sea ice plays in the climate system, operational applications and the extension of high precision surface elevation measurements into areas of sea ice. The general nature of sea ice cover is discussed and a list of requirements for sea ice monitoring is provided and the capability of different satellite sensors to satisfy needs is examined. The operation of satellite borne altimeter over non-ocean surfaces is discussed in detail. Theories of radar backscatter over sea ice are described and are used to predict the radar altimeter response to different types of sea ice cover. Methods employed for analysis of altimeter data over sea ice are also described. Data from the Seasat altimeter is examined on a regional and global scale and compared with sea ice climatology. Data from the Geosat altimeter is compared with co-incident imagery from the Advanced Very High Resolution Radiometer and also from airborne Synthetic Aperture Radar. Correlations are observed between the altimeter data and imagery for the ice edge position, zones within the ice cover, new ice and leads, vast floes and the fast ice boundary. An analysis of data collected by the Geosat altimeter over a period of more than two years is used to derive seasonal and inter-annual variations in the total Antarctic sea ice extent. In addition the retrieval of high accuracy elevation measurements over sea ice areas is carried out. These data are used to produce improved maps of sea surface topography over ice- covered ocean and provide evidence of the ability of the altimeter to determine sea ice freeboard directly. In addition the changing freeboard of two giant Antarctic tabular icebergs, as measured by the Geosat altimeter, is presented. As a summary the achievements are reviewed and suggestions are made towards directions for further work on present data sets and for future data from the ERS-1 satellite

Observations aériennes de la fragmentation de la banquise par des vagues de navire

RÉSUMÉ: La zone marginale de glace (MIZ) est une région de la banquise caractérisée par la variabilité de la taille des morceaux de glace (floes) qui la composent et par son interaction avec les vagues. Qu'ils soient de nature mécanique ou thermodynamique, la majorité des processus qui se produisent dans cette étendue sont dépendant de la taille des floes. Les observations disponibles actuellement rendent un portrait global de la distribution spatiale de la taille des floes (FSD) qui ne permet pas de comprendre l'influence de chacun des processus sur sa forme. Les objectifs principaux de cette maîtrise sont d'observer l'influence de la fragmentation de la glace par les vagues sur la FSD et d'analyser l'évolution temporelle de ce phénomène. Ces observations serviront à approfondir les connaissances fondamentales de la dynamique de la MIZ et à améliorer la représentation de la taille des floes dans les modèles. On croit que ce processus génère une taille préférentielle mais il est à savoir si celle-ci est contrainte par la rigidité flexurale de la glace ou plutôt par la longueur d'onde de la vague principale causant les déflections. Pour vérifier cette hypothèse, deux expériences ont été effectuées: une dans le golfe du Saint-Laurent à l'hiver 2019 et l'autre dans la baie de Ban à l'été 2019, au cours desquelles des vagues ont été générées par le NGCC Amundsen afin de casser la glace. Le tout a été filmé et photographié par un drone afin de pouvoir en extraire la FSD et l'évolution temporelle. Lorsque représentée sous la forme d'une densité de probabilité, la FSD exhibe une forme modale et confirme donc l'hypothèse de la taille préférentielle. Une distance de fracture théorique dépendante de l'épaisseur et de l'élasticité de la glace a été comparée à la FSD. Sa corrélation avec la taille maximale indique que la banquise ne se fracture pas systématiquement à l'endroit où la déformation maximale se trouve comme ilest assumé dans les modèles couplés d'interaction vague-glace. La demie longueur d'onde des vagues se propageant dans la glace fut aussi comparée à la taille maximale observée mais n'y est pas corrélée. Ces comparaisons laissent croire que la distance entre les craques est déterminée par l'épaisseur et la rigidité de la glace et non pas par les vagues. -- Mot(s) clé(s) en français : Glace de mer, zone marginale, fragmentation, distribution de taille des floes. -- ABSTRACT: The marginal ice zone (MIZ) is a region of the ice pack characterized by its inner floesize variability and by its interaction with waves coming from the open ocean. Whether they are mechanical or thermodynamical in nature, the majority of processes taking place in theMIZ are dependent on floe size. Recent and dating observations of the spatial distributionof floe size (FSD) in the MIZ give a large scale portrait of its morphology which does notallow to understand the influence of each process on the shape of the FSD. The goals ofthis thesis are to observe the influence of wave-induced sea ice breakup on the shape of theFSD and to analyze the temporal evolution of this process. This will bring further the stateof fundamental knowledge on the MIZ dynamics and will help to better represent floe sizein models. It is thought that wave-induced breakup causes a preferential size but whether itis constrained by the flexural rigidity of sea ice or rather by the wavelength of the principalwave causing deflections in sea ice is still unknown from an observational point of view. Toverify this hypothesis, two experiments were carried out : one in the gulf of Saint-Lawrence atwinter 2019 and the other in Ban bay at summer 2019, during which waves were generatedby the CCGS Amundsen in order to generate break up in surrounding sea ice. The break upwas recorded by a drone for the extraction of the FSD and of temporal properties. Whenrepresented as a probability density function, the FSD exhibits a strong modal shape whichconfirms the preferential size hypothesis. A theoretical fracture distance dependent on seaice flexural rigidity (thickness, elasticity) was compared to the FSD. Its correlation with themaximum observed size indicates that sea ice does not systematically break up at the positionof maximum strain as is assumed in wave-ice interaction models. Half the wavelength of thewave propagating in ice was also compared to the maximum size but it is not correlated toit. These comparisons seem to indicate that the distance between cracks is dictated by sea icethickness and elasticity rather than by wave period or wavelength. -- Mot(s) clé(s) en anglais : Sea ice, Marginal Ice Zone, wave-induced sea ice breakup, floe size distribution