Detailed Sidescan Imageries Obtained From the Southern Mariana Trough and the Suiyo Seamount of the Izu-Bonin Arc.

Side-scan-sonar (SSS) is a tool to create 2-dimensional imagery of the seafloor, and sub-bottom-profiler (SBP) is for a cross sectional (vertical) structure beneath the seafloor. These 2 instruments were combined into a compact (portable) package (off-line system) to accommodate into various vehicles such as ROVs and deep-tow frame. We named the system _eDAI-PACK: Deep-sea Acoustic Imaging Package_f. During the cruise TN167 at the back-arc spreading area in the southern Mariana, ROPOS-dive#777 was assigned to do the mapping by DAI-PACK. 2 box survey lines were planned at Fryer site and Yamanaka site on the axial hydrothermal area. Mesotech 971 forward-scanning sonar was used and its record was saved digitally. We were able to process the Mesotech 971 data and the images were plotted on maps. 2 boxes and transit between them were successfully processed using smoothed data of LBL positions. The cruise NT03-14 was carried out at the Suiyo seamount using ROV Hyper-Dolphin -dive#262,#265,#266, with SSBL acoustic positioning system. The ROPOS and the Hyper-Dolphin could have been maintained at the height of 3-5m from the sea-bottom. From the records of ROPOS_fs 7.5 hours, 4 and a half survey lines (East-West direction) were successfully recovered from Box-1. Each survey line was separated by 50m and the half swath range of the SSS was set to 30m to make full coverage of the box 1 and 2. We can recognize the rocks of 1m in size from the mosaic imagery, but the preliminary image is skewed and distorted because the vehicle_fs attitude correction is not quite right yet. Although the SSS transmits 10 pulses per second, attitude sensors of the ROPOS were recorded only one per second, which sometimes makes it difficult to relocate every pulse of the SSS to its true position. At the pre-cruise meeting it was announced that ROPOS would be installed with new navigational/scanning tools, i.e. a doppler sonar and a pencil-beam sonar, which could make the post processing of the SSS data easier. Both of them, however, could not be used during our cruise because of insufficient preparation. The video camera imagery is now being used to correct the vehicle_fs position/attitude data to make better mosaic.