Seafloor Mapping
A SeaBeam 3012 Phase 1 hybrid multibeam sonar
bathymetric mapping system is installed aboard R/V
Kaʻimikai-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 Kaʻimikai-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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