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Survey of Offshore Hazards in Southern California

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Larry Kooker and Mike Fisher monitor seismic-reflection data
Larry Kooker (left) and cruise chief scientist Mike Fisher monitor seismic-reflection data on the Auriga.
On June 14, the contract vessel Auriga left the USGS' Marine Facility in Redwood City, CA, bound for Southern California. Its mission was to conduct a two-week survey of offshore geologic hazards in the Santa Barbara Channel, a large east-west waterway between the coastal city of Santa Barbara on the north and several of the Channel Islands on the south. This waterway is prized equally, but with continued controversy, for its wildlife and Mediterranean climate and for its rich offshore oil reserves. Over a long time scale, the area's tranquillity has been disrupted by offshore earthquakes and submarine landslides, both of which can trigger tsunamis. Each of these issues was addressed by the recent cruise, whose main goals were to

  • collect seismic-reflection data to investigate offshore geological hazards, such as earthquakes, landslides, and tsunamis

  • conduct subsidiary research funded by the Department of Interior's Minerals Management Service (MMS), which is keenly interested in locating natural submarine seeps of tar and heavy oil in its efforts to determine whether oil on the shoreline comes from spills or seeps

The USGS scientists conducting this survey benefited from early, close consultation with cooperating scientists at the University of California, Santa Barbara, and at Moss Landing Marine Laboratories. Data collected during the survey will be interpreted in collaboration with academic colleagues to support several National Science Foundation proposals.

map of study area in southern California, ranging from Point Conception in the west, eastward through the Santa Barbara Channel and on to Point Dume, west of Santa Monica
map of study area in southern California, ranging from Point Conception in the west, eastward through the Santa Barbara Channel and on to Point Dume, west of Santa Monica
Cruise map: Study area in Southern California. Zigzag lines offshore show track of the Auriga as it collected seismic-reflection data to investigate offshore geologic hazards, such as earthquakes, landslides, and tsunamis.

The Auriga and its crew enjoyed splendid weather during the early-morning departure past the glittering cityscape of downtown San Francisco, then ran headlong into full-gale conditions in southern California. On the way south, the 150-ft-long ship was crowded with crew and equipment. The tools of the marine geologist's trade jammed the rear deck: four bright-yellow "fish" housed the sidescan-sonar, the Huntec, the chirp profiler, and the 12-kHz bathymetric-profiler systems. Also on deck were the multichannel seismic streamer, the air compressor, and various winches. Much of the deck space was occupied by four large shipping containers, which housed facilities for data recording and archiving, and tools and repair space for all USGS equipment.

The vessel was staffed by six members of the ship's crew, six USGS geologists, three technicians from the USGS' Marine Facility in Redwood City, CA, one technician under contract, and five biologists from Cascadia Research, a nonprofit research organization based in Olympia, WA. These 21 people shared 14 first names, as well as tight quarters. "Mike" was the go-to guy, as there were five of them.

map of study area in southern California, ranging from Point Conception in the west, eastward through the Santa Barbara Channel and on to Point Dume, west of Santa Monica
Seismic-reflection data: Dual monitors display seismic-reflection data from the chirp subbottom profiler. The image displayed across the two monitors represents data collected along a seismic line about 2 km long. Approximate thickness of the sedimentary deposits (resting on bedrock) is labeled in two spots on the right-hand screen. An unconformity within the sedimentary deposits is clearly evident on the same screen.

The first part of the survey involved 4 days of collecting sidescan-sonar data near Point Conception for the MMS. Sidescan-sonar data provide a photograph-like view of the sea-floor surface. These data will help the MMS update its map of natural oil and gas seeps in this area. The main issue is that long stretches of beach are befouled by tarballs, and iridescent oil sheens are evident offshore, much to the dismay of the local populace. Scientists and managers in MMS are working with USGS scientists to assess the extent of natural sea-floor seepage of tar and heavy oils. USGS geochemists are also "fingerprinting" the natural oils, which will help MMS distinguish them from produced oils spilled during human activities. Previous surveys showed numerous sea-floor pockmarks and both acoustic and methane anomalies in the water column. Fortunately, this work in the typically windiest area was completed before the gales struck.

Ray Sliter and Mike Boyle
Ray Sliter (left) and Mike Boyle maneuver the 12-kHz fish back into its cradle after two weeks' steady duty.
The main and later part of the survey focused on collection of seismic-reflection data to support studies of geologic hazards, such as earthquakes, landslides, and tsunamis. Seismic-reflection data reveal information about sediment layers and bedrock features below the sea floor. Such data were collected to image some of the more ominous offshore features, such as the Oak Ridge thrust fault and its along-strike extension, the Mid-Channel thrust fault, which might be capable of generating damaging earthquakes. The scientists surveyed the area where the Oak Ridge fault intersects the coast, to fill gaps between maps of onshore and offshore fault segments. Also, they surveyed numerous fault strands that offset rocks very close to shore near the city of Santa Barbara. This survey of possible earthquake faults sets the stage for important future research goals, which include obtaining rock and sediment samples from opposite sides of some of the main faults to determine deformation rates and earthquake-recurrence intervals.

The scientists also investigated the Goleta submarine landslide, which is famous among marine researchers, in part because of its clear expression in bathymetric maps created from multibeam-sonar data. To foster research into the mechanics of slide generation and emplacement, the USGS scientists obtained both high-resolution and airgun seismic-reflection data over this feature. The high-resolution data provide a detailed look at sediment layers as much as 75 m below the sea floor. The airgun data provide less detail but give scientists a look at features as much as 1 km below the sea floor.

Surveying had to be suspended whenever marine mammals got too close to the vessel, which occurred several times each day and occasionally lasted for more than half an hour. Starting in the mid-1990s, marine researchers planning to use sound energy to probe beneath the sea floor were required to obtain permits specifying the conditions under which they could operate seismic sound sources, such as airguns. The permit requirement was established to protect marine mammals that use sound themselves for various purposes, including communication, navigation, and locating prey. Once a bureaucratic quagmire, acquiring permits to conduct seismic surveys near marine mammals has become routine. A new twist is that endangered marine turtles have been added to the list of protected species. For last June's Auriga cruise, the USGS scientists obtained permits from the California Coastal Commission, the California State Lands Commission, and the National Marine Fisheries Service. The Channel Islands National Marine Sanctuary also advised them.

A common dolphin swims near the Auriga
Dolphin: A common dolphin (Delphinus sp.) swims near the Auriga. It was one of hundreds of dolphins surrounding the ship, all moving so swiftly in and out of the water that this is the sole photo!
The biologists aboard the Auriga identified and observed the behavior of marine mammals, such as blue and humpback whales; common, Pacific white-sided, and Risso's dolphins; and California sea lions, harbor seals, and sea otters. Surveying operations were suspended while the animals were in the designated safety zones, which differed by species and sound source, as specified in the permit. One night, operations were suspended for 3 hours while dolphins followed the Auriga despite the crew's efforts to evade them.

Other cooperative work was conducted with Cascadia Research biologists who are investigating the effects of airgun signals on large whales. Their ship, the research vessel Robert Gordon Sproul from the Scripps Institution of Oceanography, and the Auriga converged at Santa Rosa Island, where, by chance, 20 to 30 large whales loitered. Biologists on both ships and in an inflatable boat observed whale behavior as the Auriga fired its airgun along several tracklines. Initial reports from the biologists are that the whales first made room as the Auriga approached them and then returned to the original area once the vessel departed. Apparently, the airgun caused neither hurried dispersal nor long-term interruption of whale activities. Eventually, such investigations will provide answers to outstanding controversies about whether sound sources used for marine research actually harm ocean animals.

Commonly, seismic data are collected with the ship headed parallel to the troughs of offshore waves. This direction is good for data but stressful for people. To atone for long seasick hours, we celebrated the end of the cruise with a swim call. Many participants, faces in frozen shock, spent as much as 30 seconds in the 52°F water.

Related Web Sites
USGS Marine Facility
U.S. Geological Survey (USGS), Redwood City, CA
Minerals Management Service (MMS)
U.S. Department of Interior
Cascadia Research
non-profit research organization

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in this issue: Fieldwork cover story:
Southern California Offshore Hazards Survey

Mapping Georges Bank

Research Ground Water Diesel Fuel Contamination

Parasites as Indicators of Coastal-Ecosystem Health

Outreach Florida's Hillsborough River

Climate-Change Effects Lecture

Meetings Coral-Reef Meeting

usSEABED - Seabed Characteristics

Awards Lake Mead Poster

von Huene Receives Prestigious Award

Staff & Center News Hapke's Thesis Defense

New WHFC Employees

WHFC Summer Interns

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