3-1. Seabeam �@[Y. Harada and J. Robinson]

The SEA BEAM 2100 multibeam sonar seafloor mapping system was run every night from August 16 to September 24, except while in port. Dedicated survey took place every night for 12 to 14 hours, roughly 1700 to 0500 or 0700 depending of the piston core or dive site location. We experience two stoppages during Leg 1a totaling only four hours. The bathymetry data represents 120 data points per sonar ping, while the sidescan data contains 2000 pixels per ping. The swath was fixed at 60 degrees, so a typical swath width for these depths (3000-5000m) was 10 km. Both the bathymetry and sidescan data are included in the same binary SEA BEAM file. Typical survey speeds ranged from 6-16 knots, depending on sea conditions and other survey requirements. The R/V technical staff preformed the initial processing and gridding of the SEABEAM data.

Products from the SEA BEAM system include standard contour maps, artificially illuminated bathymetry showing texture, beam amplitude, and sidescan data. The sidescan data is better at distinguishing between bare rock (recent lava flows) and sedimented areas, as well as highlighting small blocks, structural trends, fault scarps, and other steep slopes. These products were used daily for the planning of Piston Core, Dredge, and ROV Kaiko dives.

During the KR01-12 cruise, the SEA BEAM system added roughly 29,000 km2 to the data collected on the previous cruises, KR98 and KR99 and served in a support role collecting bathymetry data on the sampling locations. The data collected confirmed the positions of many important and interesting features surrounding the Hawaiian Islands. On August 20, we imaged an interesting feature at the intersection the southernmost strand of the Molokai facture zone and the moat north east of the island of Hawaii. During transit from PC-11 to PC-12 on August 22, we were able to collect more data along a series of north and south trending horst and grabens that had been recognized in previous cruises. Along our southernmost transit, we crossed the southernmost extent of a northwest trending fault that is 100 km south of South Point. On August 24, we were able to image part of the Waianae Landslide south of Oahu that had previously only had scattered bathymetry data. On the last day of Leg 1a, the bathymetry and sidescan sonar data clarified the shield like character of the southwest volcanic system, possibly a shield volcano.

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