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Fieldwork

Mapping the Sea Floor on the San Pedro Shelf, Southern California


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Shaded-relief bathymetry of the San Pedro shelf, showing the extent of the study area
Above: Shaded-relief bathymetry of the San Pedro shelf, showing the extent of the study area (outlined in red). Water depths within the study area range from about 10 m near the shore to about 100 m at the shelf break. Blue lines show sea-floor video/photography transects; yellow triangles show locations of sea-floor samples recently collected from the research vessel Early Bird II. [larger version]

Closeup of a preliminary sea-floor-facies map of part of the San Pedro shelf, overlaid by realtime sea-floor observations recorded during the video/photography cruise.
Above: Closeup of a preliminary sea-floor-facies map of part of the San Pedro shelf, overlaid by realtime sea-floor observations recorded during the video/photography cruise. The facies map was generated from multibeam and other data before both the video/photography and sediment-sampling cruises. Rock observations are regions where both the major (more than 50 percent) and minor (less than 20 percent) sea-floor attributes include exposed rock. Track/trail observations include regions where benthic organisms left relatively long impressions in the finer-grained sediment. [larger version]

Orange County Sanitation District personnel George Robertson (left) and Mike McCarthy recover their "box/grab" sampler onboard the research vessel Early Bird II.
Above: Orange County Sanitation District personnel George Robertson (left) and Mike McCarthy recover their "box/grab" sampler onboard the research vessel Early Bird II. Photograph by Brian Edwards.

U.S. Geological Survey (USGS) scientists with the Western Coastal and Marine Geology Team (WCMG) are leading an interdisciplinary study to map sea-floor composition and habitat on the San Pedro shelf, a part of the continental shelf off southern California. Offshore of the populous Los Angeles metropolitan area, the San Pedro shelf is affected by recreational and commercial fisheries and is at risk from numerous human impacts, such as sanitation outfalls, shipping, anchor dragging, sand mining for beach replenishment, and waste disposal. This tectonically active area is also cut by numerous faults, whose earthquake potential could pose both shaking and tsunami hazards to onshore communities.

Brian Edwards, Pete Dartnell, and contractor Eleyne Phillips will use multibeam-sonar data collected on the shelf during the late 1990s as a detailed base map from which to

  • understand and quantify controls on multibeam-backscatter intensity (a measure of sound energy reflected from the sea floor that contains clues about sea-floor composition),
  • classify and map surface-sediment facies (for example, rock, sand, or mud) at 8-m/pixel resolution, and
  • use the facies data to quantitatively map the complexity of marine communities and benthic habitats of the San Pedro shelf.

The study is a cooperative effort between USGS scientists and personnel from the Los Angeles County and Orange County Sanitation Districts (LACSD and OCSD). The USGS is providing expertise in geology and marine mapping, and the sanitation districts are providing funding, biological expertise, and ship time. Project scientists are also working closely with the California Geological Survey to identify possible sources of offshore sand for beach replenishment.

The San Pedro shelf is one of the broadest mainland continental shelf segments between Monterey, CA, and the United States-Mexican border. Approximately 75 to 80 percent of this shelf segment is composed of low-relief, sediment-covered sea floor, and the remaining 20 to 25 percent is composed of rock outcrop interspersed with boulders and cobbles.

The multibeam base map used in the study was developed from Kongsberg Simrad EM300 and EM3000 multibeam bathymetric and acoustic-backscatter data collected in the 1990s (see Los Angeles Margin, California). The science team conducted two cruises in 2004 to refine and ground-truth the base map and to help identify and map the distribution of benthic fauna (living on the sea floor) and demersal fish (living on and just above the sea floor). The first cruise (October 7-21) collected sea-floor video along more than 365 km of trackline and shot more than 13,000 high-quality still photographs, using WCMG's midsize camera sled (designed by Hank Chezar) and LACSD's 63-ft-long research vessel Ocean Sentinel. The WCMG science team included Brian Edwards, Pete Dartnell, Eleyne Phillips, Hank Chezar, and Kevin O'Toole, with mobilization help from John Gann and Gerry Hatcher. Eleven biologists, database specialists, and managers from LACSD provided additional support during the 12-day cruise. Real-time sea-floor observations were recorded every minute, using programmable keypads and Gerry Hatcher's GNAV software. More than 120 potential geologic and biologic attributes (such as sea-floor composition, abiotic complexity, biotic complexity, biologic modifiers of the sea floor, and number of benthic fauna and demersal fish) were keyed during each 20- to 30-second observation window. These attributes were quickly incorporated into a geographic information system (GIS) for comparison with the multibeam bathymetry and backscatter and associated facies map. The second cruise (November 29 - December 10) was dedicated to sea-floor sampling conducted aboard the 42-ft research vessel Early Bird II, contracted by the OCSD. The WCMG scientists on that cruise were Brian Edwards and Pete Dartnell. Five OCSD scientists and one California Geological Survey scientist provided additional support. During the 10-day cruise, the team collected sea-floor samples from 181 sites, using a grab sampler designed by MEC Analytical Systems, Inc.

To date, the science team has produced a preliminary facies map, using an empirical technique developed in central Santa Monica Bay by Pete Dartnell and Jim Gardner (U.S. Geological Survey Open-File Report 2004-1081; see Predicted Seafloor Facies of Central Santa Monica Bay, California). The map, based on the multibeam bathymetry and acoustic-backscatter data, as well as historical textural data from usSEABED and hardground observations, shows the distribution of exposed hardground (including rock outcrops and artificial reefs) and sedimented regions composed of sand and mud.

Work is progressing on two fronts. First, the surficial-facies maps will be refined and ground-truthed, using sea-floor video, photography, realtime observations, and sediment texture. Second, WCMG geologists and LACSD biologists are collaborating to identify and map the distribution of benthic organisms, using sea-floor video and photography. Once the maps of surficial facies and benthic organisms are completed, sea-floor geology and benthic faunal relationships will be integrated to develop species-specific suitability maps and more general habitat maps designed to support California Fish and Game Marine Protected Area (MPA) work and demersal-fisheries management. Final products from this project can also be used to study shelf-sediment processes, offshore hazards, marine resources, and anthropogenic impacts on the sea floor.

Sea-Floor Videos
Underwater video of sand waves Underwater video of rock region of San Pedrp Shelf
Sand Waves: Sea-floor video (approx 15 sec) showing sand waves of various sizes on the San Pedro shelf, southern California. The larger sand waves (60- to 70-cm wavelength) at the beginning of the clip (0-10 sec) abruptly change to smaller sand waves (less than 16-cm wavelength) toward the end of the clip (10-15 sec).

Choose a version to watch:

Rocky Region with Eel Grass: Sea-floor video (approx 10 sec) of a rocky region on the San Pedro shelf, southern California. Note the wolf eel 2 to 3 seconds into the video. Red lasers shining through the water column and onto the sea floor are approximately 15 cm apart, and green lasers are approximately 23 cm apart.

Choose a version to watch:

NOTE - QuickTime(TM) Required: You will need to have the free QuickTime(TM) Player installed on your computer to watch the videos.

Kevin O’Toole (top) and Hank Chezar (wearing hat) recover the camera sled onboard the research vessel Ocean Sentinel Sea-floor photograph taken during the October 2004 video/photography cruise onboard the research vessel Ocean Sentinel, showing a rocky region with coral, erect gorgonia, drift weed, a sea star, white anenomes, and a wolf eel.
Above Left: WCMG personnel Kevin O’Toole (top) and Hank Chezar (wearing hat) recover the camera sled onboard the research vessel Ocean Sentinel. Attached to the camera sled were two video cameras, one oriented obliquely and the other vertically, and one 6-megapixel digital camera. The sled was navigated by using a shipboard differential geographic positioning system (GPS) and LACSD's trackpoint system. Photograph by Tom Parker (LACSD). [larger version]

Above Right: Sea-floor photograph taken during the October 2004 video/photography cruise onboard the research vessel Ocean Sentinel, showing a rocky region with coral, erect gorgonia, drift weed, a sea star, white anenomes, and a wolf eel. Such photographs are used to refine and ground-truth the surficial-facies map and to identify and map the distribution of benthic fauna and demersal fish. [larger version]


Related Web Sites
Western Coastal and Marine Geology
U.S. Geological Survey (USGS), Santa Cruz & Menlo Park, CA
Los Angeles Margin, California
U.S. Geological Survey (USGS)
Predicted Seafloor Facies of Central Santa Monica Bay, California - USGS Open File Report 2004-1081
U.S. Geological Survey (USGS)

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in this issue: Fieldwork cover story:
Tsunami Deposits

Mapping the San Pedro Shelf

Research Nearshore Impacts of Dam Removal

Pulley Ridge Deepest Known US Coral Reef

Raising Crane

Climate Features Influenced 2004 Hurricane Landfall Count

Outreach USGS Employees Donate Toys

Staff & Center News Carl Goodwin Honored at National Ecosystem Conference

Oceanographer Joins Western Coastal & Marine Geology Team

Blood Donor

Netherlands Students Contribute to USGS Studies

Florida Integrated Science Center, Gainesville, FL

NMFS Employees Visit CCWS

Publications February Publications List


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