With the development of international communication and the advent of terrorism, safety and security of the underwater structures such as shipping dock, bridge pier and the bottom of ship hull become a key issue. Generally, in order to inspect the underwater structures, divers are sent down to make tactile examination, or with a camera to take photos or video tapes for further study and examination. However, in dark, turbid water, the quality of the optical images is practically not good enough to give much useful information of the underwater structures. And the working efficiency of this kind of inspection is relatively low. To overcome these problems, it is imperative that more efficient and effective inspection methods are developed. Recently, an acoustic lens camera called DIDSON (Dual-frequency Identification Sonar), which is invented by the Applied Physics Laboratory, University of Washington, can provide almost-video-quality images to identify objects even in turbid water. It has been proved to be very useful in underwater objects searching, especially in dark, turbid water. Although the quality of every single frame provided by DIDSON is quite good, its field of view is narrow, only 29 degrees in horizontal and 14 degrees in vertical, making it hard to comprehend the overall condition of the inspection area and guarantee the safety and security of underwater structures. In this paper, we present a new mosaicing method for the time-sequential images provided by DIDSON, which enables us to inspect the whole underwater structure in a computer screen. An experiment was carried out to inspect the underwater part of a shipping dork. Our experimental result has shown that the proposed method can successfully mosaic the images provided by DIDSON, and thereby expand the visual field of DIDON indirectly which enables it to be effectively used in underwater structures inspection.

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