Olympic Coast National Marine Sanctuary Science section includes Seafloor Mapping, Oceanography, Deep Sea Coral and Sponges, Wildlife Research, Coastal Habitats, Citizen Science, Ecosystem Processes, Research Surveys, and Research Assets
Olympic Coast National Marine Sanctuary Science section includes Seafloor Mapping, Oceanography, Deep Sea Coral and Sponges, Wildlife Research, Coastal Habitats, Citizen Science, Ecosystem Processes, Research Surveys, and Research Assets
Olympic Coast National Marine Sanctuary Science section includes Seafloor Mapping, Oceanography, Deep Sea Coral and Sponges, Wildlife Research, Coastal Habitats, Citizen Science, Ecosystem Processes, Research Surveys, and Research Assets Olympic Coast National Marine Sanctuary Science section includes Seafloor Mapping, Oceanography, Deep Sea Coral and Sponges, Wildlife Research, Coastal Habitats, Citizen Science, Ecosystem Processes, Research Surveys, and Research Assets

Seafloor Habitat Mapping
Groundtruthing

Picture of benthic grab sampler suspended in air by cable on vessel
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.

Photo of video sled made from welded aluminum with attached lights and camera
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.

Photo of remotely operated vehicle hanging over the ocean, connected to vessel by an electronic tether
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.

Picture of some of the remotely operated vehicle tools gathering rock samples from the ocean bottom
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.



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