Sidescan sonar principle and the case of gas hydrate survey
Speaker: Tsung-Yu Shih
Abstract
Sidescan sonar is a very useful instrument to investigate the seafloor. It can provide a high-resolution acoustic image like a optical photograph by appropriate data processing. We can use the image to interpret some features on the seafloor. Nevertheless, these are some acoustic concepts and principles which you must understand when using the sonar data.
The acoustic concepts and principles include: (1) fundamentals of sidescan sonar, (2) the nature of sonar targets, (3) resolution, (4) processing of the data, (5) image enhancement and processing. The first part is concerning with sidescan elements, sidescan sonar type and process of sonar transmitting and receiving. The second part is concerning with the signal response when sonar reflect different kinds of seafloors. The third part is concerning with the different kinds of sidescan resolution. The remained two parts are very important for producing a usable acoustic image. These parts correct the signal strength of raw data, adjust the contrast and apply some filters for creating a more obvious image. We can use the acoustic image to do geological interpretations after we understand the concepts, otherwise we’ll make some mistakes.
Sidescan sonar have been widely used for the gas hydrate survey because it can detect the seafloor property. If destabilized hydrate disrupt into gas and move along faults and fractures to seafloor, the gas will form authigenic carbonates when it contact seawater. Free gas also have the same phenomenon when it contact seawater along faults and other paths. When there are authigenic carbonate on seafloor, sidescan sonar can receive strong signal because their properties. In spite of sidescan is very useful, we also need other geophysical and geochemical constraints to make sure the result.
Reference
Johnson, H.P., Helferty, M., The geological interpretation of side-scan sonar. (1990) Rev. Geophys. 28, 357-380.
Johnson, J.E., C. Goldfinger, E., Suess, Geophysical constraints on the surface distribution of authigenic carbonates across the Hydrate Ridge region, Cascadia margin. (2003) Marine Geology 202, 79-120.