Image Processing for Ice Parameter Identification in Ice Management

摘要

Various types of remotely sensed data and imaging technology will aid thedevelopment of sea-ice observation to, for instance, support estimation of iceforces critical to Dynamic Positioning (DP) operations in Arctic waters. Theuse of cameras as sensors for offshore operations in ice-covered regions willbe explored for measurements of ice statistics and ice properties, as part of asea-ice monitoring system. This thesis focuses on the algorithms for imageprocessing supporting an ice management system to provide useful ice informationto dynamic ice estimators and for decision support. The ice informationincludes ice concentration, ice types, ice floe position and floe size distribution,and other important factors in the analysis of ice-structure interaction in an icefield.The Otsu thresholding and k-means clustering methods are employed to identifythe ice from the water and to calculate ice concentration. Both methodsare effective for model-ice images. However, the k-means method is more effectivethan the Otsu method for the sea-ice images with a large amounts ofbrash ice and slush.The derivative edge detection and morphology edge detection methods areused to try to find the boundaries of the ice floes. Because of the inabilityof both methods to separate connected ice floes in the images, the watershedtransform and the gradient vector flow (GVF) snake algorithm are applied.In the watershed-based method, the grayscale sea-ice image is first convertedinto a binary image and the watershed algorithm is carried out to segment theimage. A chain code is then used to check the concavities of floe boundaries.The segmented neighboring regions that have no concave corners betweenthem are merged, and over-segmentation lines are removed automatically.This method is applicable to separate the seemingly connected floeswhose junctions are invisible or lost in the images.In the GVF snake-based method, the seeds for each ice floe are first obtainedby calculating the distance transform of the binarized image. Based on theseseeds, the snake contours with proper locations and radii are initialized, andthe GVF snakes are then evolved automatically to detect floe boundaries andseparate the connected floes. Because some holes and smaller ice pieces maybe contained inside larger floes, all the segmented ice floes are arranged inorder of increasing size after segmentation. The morphological cleaning isthen performed to the arranged ice floes in sequence to enhance their shapes,resulting in individual ice floes identification. This method is applicable toidentify 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. Inthe model-scale, the ice floes in the model-ice images are modeled as squareshapes with predefined side lengths. To adopt the GVF snake-based method formodel-ice images, three criteria are proposed to check whether it is necessaryto reinitialize the contours and segment a second time based on the size andshape of model-ice floe. In the full-scale, sea-ice images are shown to bemore difficult than the model-ice images analyzed. In addition to non-uniformillumination, shadows and impurities, which are common issues in both sea-iceand model-ice image processing, various types of ice (e.g., slush, brash, etc.),irregular floe sizes and shapes, and geometric distortion are challenges in seaiceimage processing. For sea-ice image processing, the “light ice” and “darkice” are first obtained by using the Otsu thresholding and k-means clusteringmethods. Then, the “light ice” and “dark ice” are segmented and enhancedby using the GVF snake-based method. Based on the identification result,different types of sea-ice are distinguished, and the image is divided into fourlayers: ice floes, brash pieces, slush, and water. This then makes it possibleto present a color map of the ice floes and brash pieces based on sizes. Italso makes it possible to present the corresponding ice floe size distributionhistogram.