Seafloor Habitat Mapping
Groundtruthing
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A benthic grab sampler draws sediment samples from the seafloor. |
Remotely sensed data from sonar must be verified, or groundtruthed, using some form of optical (i.e.
video or cameras) or physical (sediment grabs) sampling technique to allow scientists to examine
small sections of the seafloor at a fine scale and apply those findings to the information provided
by the sonar. Groundtruthing provides the largest spatial scale (1:10 – 1:1000) data and the most
specific information about seafloor sediments.
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A towed, lighted video sled allows scientists to visualize seafloor sediments and benthos. |
One of the simplest ways to groundtruth the seabed is using a
benthic grab sampler, a claw that drops to the seabed off a vessel and brings up a sample of the
sediment. OCNMS uses a Smith-MacIntyre benthic grab. Sediment samples from the grab can be
collocated with various sonar reflections to verify the sediment type in the sonar imagery. The
strengths of the benthic grab are that it is affordable, easily deployed off any vessel, and it
provides a physical sample to be studied by scientists. The weaknesses are that it does not provide
spatial context around each grab and that the exact location of a grab may not be exact since the
sampler may be pulled by underwater currents during deployment.
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A remotely operated vehicle provides scientists with cameras, lights and tools for benthic exploration. |
The sanctuary also groundtruths with a lighted video sled
that drifts along the seafloor behind a slow-moving vessel. The sled is connected to computers and
video recorders on the boat which capture images of seafloor sediment types. The sled is especially
useful for recording sediment transition zones – that is, changes between mud and sand or rock. The
lighted video sled is a useful groundtruthing tool when the sea conditions are calm and flat and
underwater currents are mild.
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Seafloor sampling using tools from an ROV. |
Groundtruthing in the sanctuary is best conducted using a sophisticated
Remotely Operated
Vehicle (ROV) a tethered, unoccupied, underwater vehicle that is
operated by a technician aboard a surface support vessel. The tether carries electrical power to video, lights,
lasers and sometimes oceanographic instruments, sonars, lasers, or manipulators such as cutting arms
or grabs samplers. ROV groundtruthing provides data that verifies sonar reflectance and may add
significantly to the information scientists collect about the seafloor. They have been used for many
years to gather information on the deep sea coral and sponge communities near the Juan de Fuca
Canyon.
Contact for page content: Nancy Wright
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