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Seafloor Mapping

Launch of Pisces V

A SeaBeam 3012 Phase 1 hybrid multibeam sonar bathymetric mapping system is installed aboard R/V Ka‘imikai-o-Kanaloa. It was designed and manufactured by L-3 Communications/Klein Associates of Salem, New Hampshire in collaboration with L-3 ELAC/Nautik of Kiel, Germany. The system is capable of acoustically charting the seafloor peaks and valleys with complete high-resolution coverage to depths of 11,000 meters (nearly 7 miles deep). The system projects and receives a pitch, roll, and heave compensated swath of sound (a ping), comprised of more than 150 acoustically formed 2° x 2° beams, across the vessel’s path. The sound waves reflect off the sea bottom and are then sensed by underwater microphones (hydrophones) attached to the hull of the ship. Knowing the angle of the sound projection, the speed of sound in water, and the time to receive the projected signal, an onboard computer records and calculates the depth to the seafloor at each point. More than 250 individual across-track depth measurements can be made per ping cycle (typically 151), which typically occurs every five to ten seconds in moderately deep water. A swath width of 150° at 1000 meters (90° at 11,000 m) water depth can be ensonified during each pass and SeaBeam can typically acquire good data at ship speeds of 6-9 knots on R/V Ka‘imikai-o-Kanaloa. Average accuracy across the swath is better than 0.5% of the water depth. Sidescan backscatter data with 12-bit resolution to a maximum of 2000 pixels is coincidentally collected.

Numerous parallel track lines with slight overlap of the swaths are laid out on top of the targeted survey area. The logistics of the field operation are analogous to “mowing a lawn.” A near real-time bathymetric plot of the current survey is produced, and the digital data are recorded for later post-processing on the shipboard workstation computers. These procedures include editing of errant data, navigation merging, gridding of all swaths, combination with other data sets, and more complex color shaded relief plots and interactive 3D “virtual reality” fly-thrus using specialized software and peripherals. The main use of this system for HURL is to map potential submersible or ROV dive sites, which increases safety, operational efficiency, and scientific return. Other applications of this technology include hydrographic charting for hazards to navigation, search and recovery operations, submersible support, marine resource exploration, scientific research, and surveying of fisheries habitat.

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