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Offshore Mapping Captures Tar Seeps in the Santa Barbara Channel, California
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.
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.
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.
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.
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in this issue:
California Sea Otter Count Reaches New High
MIT Students Tour USGS Woods Hole Science Center Coastal-Hazards Research Featured in First USGS Podcast
Airborne-Lidar Technology and Applications Workshop
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U. S. Department of the Interior | U.S. Geological Survey
