Image Processing for Ice Parameter Identification in Ice Management

• Main Author: Zhang, Qin
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: NTNU 2015
• Subjects: VDP::Technology: 500::Marine technology: 580; Arctic; Sea ice
• Online Access: http://hdl.handle.net/11250/2372997

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.