Habitat mapping of a cold-water coral reef off Norway: A comparison of visual and computer-assisted methods to interpret sidescan sonar data

The Sula Ridge Reef Complex, a large cold-water coral reef structure on the mid-Norwegian shelf built mainly by Lophelia pertusa, was mapped entirely using a high-resolution sidescan sonar. In addition, a dense echosounding grid, underwater video observations and dives using the manned research submersible Jago, provided precise high-quality ground-truthing, and allowed a detailed interpretation of the reef structure and its surrounding geological features. The result of this visual sidescan sonar interpretation is a facies map that delineates different potential habitats within the coral reef environment, e.g., live coral reef, dead coral structure and sediment-covered coral/rubble, etc. In an attempt to improve this interpretation, computer-assisted image analysis was applied to a representative section of the sonar data to try to reveal patterns 'invisible' to the human eye (using the TexAn software). Texture analysis uses Grey Level Co-occurrence Matrices (GLCMs) to calculate statistical indices quantifying the distribution of grey levels and their spatial relationship within the image. For example, regions of rough texture (coral mounds) can be distinguished from areas of smooth background sediment or zones of heterogeneous texture resultingfrom sediment-covered coral debris and dropstones colonized by sponges. The results of this computer-assisted approach were carefully compared with the earlier visual interpretation to identify the differences and to see where the interpretation could be improved. Overall, it is shown that texture analysis is a useful tool to make facies/habitat mapping from sidescan sonar easier and faster, revealing details overlooked during visual interpretation. However, validation of certain details by an experienced interpreter is still necessary, and therefore visual and computer-assisted interpretation should be used as complementary tools.