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FieldworkCover Story

Offshore Mapping Captures Tar Seeps in the Santa Barbara Channel, California


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Research vessel Zephyr
Above: Research vessel Zephyr, used by USGS scientists for recent work in the Santa Barbara Channel. During mapping, an interferometric sonar sits about 2 m below the water surface, attached to the pole to the right of the word "Survey." [larger version]

U.S. Geological Survey (USGS) scientists completed a 20-day mapping and ground-truthing cruise on the mainland shelf of the Santa Barbara Channel, California, revealing rock outcrops, possible sediment-transport pathways, and recent tar seeps. The fieldwork was part of a multiyear, multidisciplinary mapping project that the USGS is conducting in cooperation with the Department of the Interior's Minerals Management Service (MMS); California State University, Monterey Bay (CSUMB); the University of California, Santa Barbara (UCSB); and local agencies. The mapping supports numerous studies in the region, including investigations of offshore oil and gas seeps, coastal sediment transport, and benthic habitats.

Over the past 3 years, scientists from the USGS Western Coastal and Marine Geology team (WCMG) and CSUMB scientists have mapped the mainland shelf from the coast to the 3-mi State limit, from Naples southeastward to Point Mugu (more than 100 km of coastline), using interferometric and multibeam sonars. This past June and July, WCMG collected bathymetry (depth to the sea floor) and acoustic backscatter (strength of sound returned from the sea floor) in two areas, using a 117-kHz interferometric sonar. Simultaneous with the interferometric mapping, a 434-kHz chirp seismic profiler was used to locate gas bubbles in the water column in order to map the distribution of natural-gas seeps. A camera sled with one forward-looking video camera, a vertical video camera, and a vertical high-definition video camera was used to ground-truth the mapped regions, as well as to ground-truth multibeam data collected in 2006 offshore of Ventura and Oxnard by CSUMB.

Diane Minasian map of California, showing placenames mentioned in text
Above left: Diane Minasian on watch as a marine-mammal observer. If marine mammals approach within a certain distance of the vessel, acoustic equipment must be shut down to prevent adverse effects on the animals. Only one such shutdown, lasting half an hour, was necessary during last summer's cruise in the Santa Barbara Channel. [larger version]

Above right: California, showing placenames mentioned in text. [larger version]

This summer's mapping consisted of three legs. During the first leg, David Finlayson, Mike Boyle, Larry Kooker, Thomas Reiss, Peter Triezenberg, and Pete Dartnell collected bathymetric data, acoustic-backscatter data, and seismic-reflection profiles offshore of Gaviota State Park. During the second leg, Jamie Conrad, David Finlayson, Mike Boyle, Larry Kooker, Thomas Reiss, and Diane Minasian collected the same types of data offshore of Naples, adjacent to an area surveyed by the USGS in 2006 off Coal Oil Point (see "Mapping the Sea Floor Off Santa Barbara, California," Sound Waves, September 2006). The third leg was used to collect seabed video footage to ground-truth the data collected offshore of Gaviota and Naples, as well as the multibeam data collected by CSUMB in 2006; participants in this leg included Brian Edwards, Eleyne Phillips, Gerry Hatcher, Merit McCrea (UCSB), and Pete Dartnell.

The participants in leg 2 were joined by Tom Lorenson (WCMG), Mary Elaine Helix (MMS), and Ira Leifer (UCSB), who hosted a team of correspondents from the Houston Chronicle writing about natural oil seepage in the Santa Barbara Channel. The reporters joined the USGS team aboard the research vessel Zephyr for a day of real-time operations, photography, videography, and interviews. The reporters spent additional days with Tom Lorenson, visiting onland oil and tar seeps similar to those mapped in the marine study area, and with Ira Leifer, tracking the genesis of tar from oil seep to tarball deposition on local beaches. The finished article and video should be available on the Houston Chronicle Web site in September 2007.

Shaded-relief bathymetry offshore of Gaviota State Park
Above: Shaded-relief bathymetry offshore of Gaviota State Park in the northwestern Santa Barbara Channel, California. Red-dotted lines show video tracks during camera-sled ground-truthing operations. Labels "Gaviota1.mov" and "Gaviota2.mov" show approximate locations of tar-seep video clips. [larger version]

Tar-Seep Video Clips
still image from sea-floor video showing a recent tar seep at 48 meters water depth
Watch the video (21 MB)
still image from sea-floor video showing a recent tar seep at 40 meters water depth
Watch the video (27 MB)

Sea-floor videos showing recent tar seeps offshore of Gaviota State Park, Santa Barbara Channel, California. See map of bathymetry offshore of Gaviota State Park for approximate locations of video clips; see text for discussion. Image at left is from 48-m water depth; image at right is from 40-m water depth. Beams of green laser light are 16 cm apart.

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

The bathymetric data collected offshore of Gaviota State Park show a narrow inner shelf (0- to 30-m water depth) with linear depressions perpendicular to the shoreline. Approximately parallel to the coastline, the shelf steepens abruptly along the 30-m isobath. Lobes of sediment extend as flows from the base of this feature onto the adjacent outer shelf. A 2.3-km-long linear arrangement of 2- to 3-m-high mounds was mapped crossing a 300-m-wide gully. Seabed video collected in this area shows evidence of recent tar seeps. For example, video clip "Gaviota1.mov," captured at 48-m water depth, shows a hemispherical mound pushing up the surrounding sediment; the absence of sediment on its top indicates that the mound is relatively young. The second video clip, "Gaviota2.mov," captured at 40-m water depth, also shows what appears to be a recent tar seep, with a shiny surface and no sediment cover. Unlike the mound in the first video clip, the tar appears to be flowing over the seabed sediment. Seabed oil and gas seeps are known to be common in the area, but this is the first time the USGS has obtained images with a towed video camera. The map of bathymetry offshore of Gaviota State Park shows the locations of these video clips.

Shaded-relief bathymetry offshore of Naples and Coal Oil Point
Above: Shaded-relief bathymetry offshore of Naples and Coal Oil Point in the northwestern Santa Barbara Channel, California. USGS scientists mapped area outlined in dashed green in 2006 and area outlined in orange in 2007. Red lines show video tracks during camera-sled ground-truthing operations in 2007. [larger version]

The bathymetric data collected offshore of the Naples area show a relatively uniform sediment-covered shelf. Video footage from an area of rougher sea floor in the southwest corner of the survey area revealed that the seabed here is sediment covered and exhibits both small recent tar seeps and older seeps dusted with sediment. Also, about 1.5 km south-southeast of Naples Point is an area of 3- to 6-m-high outcrops that the video reveals to be differentially eroded bedrock with high concentrations of rockfish, starfish, and purple sea urchins.

Related Sound Waves Stories
Tar Balls Washed Onto Central California Beaches by Storms
May 2007
Mapping the Sea Floor Off Santa Barbara, California
September 2006
Sonar Survey of Sea-Floor Habitats Southeast of Santa Barbara, California
October 2005
Mapping Benthic Habitat Around Oil Platforms in the Santa Barbara Channel, California
November 2004
Beginning the Search for Offshore Oil Seeps Near Point Conception, California
September 2001

Related Web Sites
Multibeam Bathymetry and Backscatter Data: Northeastern Channel Islands Region, Southern California - Open-File Report 2005-1153
USGS (U.S. Geological Survey)
Houston Chronicle
newspaper

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Fieldwork
cover story:
Tar Seeps in the Santa Barbara Channel

California Sea Otter Count Reaches New High

Outreach Upcoming! "A Tale of Two Kelp Forests" Public Lecture

MIT Students Tour USGS Woods Hole Science Center

Coastal-Hazards Research Featured in First USGS Podcast

Meetings Scientists Meet Managers at Coastal Zone 2007

Airborne-Lidar Technology and Applications Workshop

Awards USGS Hydrologist Honored for Outstanding Community Outreach

Staff Upcoming! New Vice President of Pacific Section SEPM Helps Plan Fall Field Trips

Publications

September Publications List


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