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Fieldwork

Ground-Truthing Data for Mapping of Seafloor Habitat and Geology along the California Coast



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Brian Edwards, Peter Harkins, and Pete Dartnell.
Above: USGS camera sled being deployed off the stern of the Channel Islands National Marine Sanctuary research vessel Shearwater. Crew members included (left to right) Brian Edwards, Peter Harkins, and Pete Dartnell. [larger version]

U.S. Geological Survey (USGS) scientists recently conducted two research cruises along the California coast as part of the California Seafloor Mapping Program. They collected seafloor video, still photographs, real-time observations, sediment samples, and seismic-reflection profiles. The California Seafloor Mapping Program is a State and Federally funded program to create a series of geologic and seafloor-habitat basemaps for all of California's State waters (from the shore out 3 nautical miles). This work is being performed in support of the Marine Life Protection Act and is a cooperative effort between Federal and State agencies, universities, and industry.

The California Seafloor Mapping Program has five main components:

  • Bathymetry and acoustic-backscatter data: ship-based collection of high-resolution sonar data for all parts of the coast that are currently unmapped. (Bathymetry is water depth, calculated from the time it takes a pulse of sound to travel to the seafloor and back; acoustic backscatter is the strength of the sound energy reflected back to the sonar system, which yields information about seafloor materials.)
  • Data ground-truthing: video, still photography, and (or) physical sampling of the seafloor.
  • Subbottom profiling: high-resolution seismic-reflection profiling to determine the thickness and distribution of sediments and other geologic units, locations of active faults and folds, and other features.
  • Geographic-information-system (GIS) data and map production: creation of GIS databases and multisheet map sets (1:24,000 scale) that will include bathymetric, geologic, and habitat maps spanning the entire California land-sea margin.
  • Data management and dissemination: creation of an online data repository for public access to all digital data and map products covering the California State waters.

The first cruise took place in northern California from July 13 to August 2, 2009, aboard the Humboldt State University research vessel Coral Sea (USGS cruise ID C-1-09-SC). Work focused on ground-truthing data from the California-Oregon border to Eureka, a distance of about 145 km. Participants on the cruise included Guy Cochrane, Eleyne Phillips, Lisa Krigsman (National Marine Fisheries Service [NMFS]), Pete Dartnell, Andy Ritchie, Hank Chezar, Tim Elfers, Dave Gonzales, Pete Dal Ferro, Jackson Currie, Peter Harkins, and Nadine Golden. Although seafloor visibility was poor in water depths shallower than about 30 m, it was better in the deeper regions and allowed for good imaging of seafloor environments. In areas of poor visibility, ground-truthing focused on sediment sampling and seismic-reflection profiling.

Northern California, showing extent of ground-truthing effort during July 13-August 2 cruise. Southern California, showing the three nearshore regions ground-truthed during the August 10-21 cruise.
Above left: Northern California, showing extent (solid red) of ground-truthing effort during July 13-August 2 cruise. Open squares are additional areas that have been or will be mapped as part of the California Seafloor Mapping Program. [larger version]

Above right: Southern California, showing the three nearshore regions (solid red) ground-truthed during the August 10-21 cruise. Open squares are additional areas that have been or will be mapped as part of the California Seafloor Mapping Program. [larger version]

The second cruise took place one week later in southern California, from August 10 to 21, aboard the Channel Islands National Marine Sanctuary (CINMS) research vessel Shearwater (USGS Cruise ID S-1C-09-SC). On this cruise, work focused on three main areas: Point Conception, Point Mugu, and Santa Catalina Island. Participants on this cruise included Pete Dartnell, Brian Edwards, Eleyne Phillips, Lisa Krigsman (NMFS), Jackson Currie, Peter Harkins, Tim Elfers, Hank Chezar, Nadine Golden, and Lisa Gilbane (Minerals Management Service). Good weather and seafloor visibility allowed for 70 camera-sled transects that imaged many seafloor environments, including tar mounds, fields of brachiopods, and rock outcrops.

Seafloor photograph of a boulder, cobbles, and shell debris taken during the northern California ground-truthing cruise. Seafloor photograph of a rock outcrop covered with strawberry anemones
Above left: Seafloor photograph of a boulder, cobbles, and shell debris taken during the northern California ground-truthing cruise. Image is about 2 m (7 ft) across. [larger version]

Above right: Seafloor photograph of a rock outcrop covered with strawberry anemones, taken near Reading Rock in northern California. (See map of northern California, above, for location.) Green laser dots (near bottom of photograph, just left of center) are 9 in. apart. [larger version]

Seafloor photograph of a tar mound covered with a thin veneer of sediment Seafloor photograph of a field of brachiopods
Above left: Seafloor photograph of a tar mound covered with a thin veneer of sediment, taken near Point Conception, California. Green laser dots are 9 in. apart. [larger version]

Above right: Seafloor photograph of a field of brachiopods covering seafloor sediment on the northern side of Santa Catalina Island, California. Green laser dots are 9 in. apart. [larger version]

Comparison of seafloor photographs with shaded-relief bathymetry near Reading Rock in northern California
Above: Comparison of seafloor photographs with shaded-relief bathymetry near Reading Rock in northern California (see map of northern California, near top of page, for location). Top left photograph confirms that the rougher seafloor (see enlargement in lower right) is composed of rock outcrop, while the top right photograph confirms that the intervening smooth seafloor is composed of sediment. [larger version]

On both cruises, the video footage and photographs were taken from a USGS camera sled towed about 1 to 2 m above the seafloor at speeds of less than 1 knot. The sled housed three regular video cameras, one high-definition video camera, an 8-megapixel still camera, lights, and paired lasers (to provide scale in images). As the video and photographs were recorded to tape and disk, real-time observations of both geologic characteristics (such as rock or sediment type, local slope, complexity) and biologic characteristics (such as species and percent cover) were recorded to a GIS using a programmable keypad. These data will be incorporated with the bathymetric and acoustic-backscatter maps to create 1:24,000-scale seafloor character and habitat maps.


Related Web Sites
California Seafloor Mapping Program
cooperative program

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in this issue:

Fieldwork
cover story:
Surprises from the Samoa Tsunami

Scientists Cruise Deep into Coral Ecosystems

Mapping Seafloor Habitat and Geology along the California Coast

How Mangrove Islands Kept Up With Sea-Level Rise

Research Tsunamis Generated from Outer-Rise Earthquakes

South Carolina Beaches a Model for Preservation

Outreach Flat Isabel Goes on a Research Cruise

Awards USGS Employees in Louisiana Garner Awards

Staff New Director of USGS Woods Hole Science Center

Publications December 2009 Publications List


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