Application of high resolution bathymetric data in the analysis of scanning sonar images

Abstract

Stacjonarne wysokoczęstotliwościowe sonary skanujące są coraz częściej wykorzystywane do podwodnego obrazowania obszarów portowych, terenów z budowami hydrotechnicznymi czy miejsc charakterystycznych ze względów nawigacyjnych. Zazwyczaj dane sonarowe są uzupełnieniem danych batymetrycznych o informacje dotyczące charakteru dna oraz obiektów na nim występujących ze względu na swój potencjał interpretacyjny. Wysokorozdzielcze dane o głębokości pozwalają polepszyć analizę obrazu sonarowego poprzez uwzględnienie ich w procesie określania pozycji i przetwarzania obrazu.The use of rotary scanning sonar allows for more precise bottom examination than standard side scan sonar imaging. High frequency transducer, together with lack of disruption resulting from unit movement and its placement only little above the seabed, ensures very detailed representation of sea bed surface. However, the work characteristic of this device causes a number of issues not occurring in side scan and having a significant impact on the projection and further identification of underwater objects. The other typical data obtained from a surveyed area is its bathymetry. Nowadays, the information acquired by interferometric bathymetry system very precisely represents the sea floor. Usually, the sonar data is only an addition to bathymetric survey. Here, the application of the precise depth data allows to base the whole sonar image processing and to depend its analysis on this data. The basic information of the image is its location, which in this case depends directly on the position of sonar transducer during the signal registration. Due to its completely underwater stand-alone way of work, the exact position of the transducer is unknown. There is no rational possibility of receiving direct information from GPS-RTK receiver.The proposed method bases on approximate position, the bathymetric data, and synthetic sonar image simulator. Rough data of the transducers position may be obtained from the survey unit from which the scanning sonar is lowered to the bottom. On this basis and on the obtained bathymetric data, the synthetic polar sonar image is generated. By the conjunction method, as similarity function, real image is compared to synthetic one. In subsequent steps, new simulated images are generated and compared with the original ones until the best comparison is found. Knowing the seabed configuration, it is easy to tell if the objects found on the registered image lie in a local hollow area or on the hill. It has huge meaning for the way of object representation and identification, especially, when the registration process is taken 30 cm above the sea floor. The differentiation of sonar image processing, depending on a gradient map, allows for independent, local visibility improvement in objects and bottom fragments. In consequence, it allows for improvement in the image interpretational potential. High resolution, very few distortions and the possibility of taking into account additional information can contribute to automation of identification process in sonar imaging