Shipwreck

The SS Montebello was sunk by a Japanese submarine in 1941, seven miles off California's Central Coast during World War II. Its tanks were full of three million gallons of Santa Maria crude oil. If it hasn't already leaked out, the California Department of Fish and Game is trying to keep it contained.

Monterey Bay Aquarium Research Institute sent its robotic submarine down to the wreck, 900 feet down, last week to take sonar images. Researchers and F&G scientists hope the images will help them determine what condition those old tanks are in, and how to keep the oil from getting into the water. In this picture, MBARI's mapping AUV is prepared for launch from the research vessel Zephyr by Senior Research Specialist Dave Caress. Image credit: Duane Thompson © 2005 MBARI

“Although it is still uncertain whether or not the 3 million gallons of oil loaded onto the vessel before its departure on Dec. 23, 1941 remain in its tanks, we aren’t taking any chances,” said Steve Edinger, Administrator for the Department of Fish and Game’s Office of Spill Prevention and Response (OSPR) in a press release. “We are taking proactive steps to determine if there is a pollution threat and, if so, to prevent an oil release that could impact California’s coastal areas.”

The Montebello Assessment Task Force commissioned the sonar survey to determine if the Montebello poses a pollution risk to California marine waters and coastal habitat. The task force was convened in 2008 at the request of Assemblyman Sam Blakeslee and includes representatives from OSPR, the U.S. Coast Guard, the National Oceanic and Atmospheric Administration, California Department of Transportation (Caltrans), the
Monterey Bay National Marine Sanctuary, Monterey Bay Aquarium Research
Institute (MBARI) and Assemblyman Sam Blakeslee’s (R-San Luis Obispo) Office. OSPR has provided $100,000 in funding from its Oil Spill Prevention and Administration Fund for this and other research on the Montebello. The task force is working with federal agencies to secure additional funding for the project.

The sonar images will help the scientists understand the sea floor conditions on which the wreck rests. How stable it is, what it's like down there, will be used to plan two more remotely-operated vehicle dives to the wreck, planned for summer and fall 2011. The ROV on those dives will record video of the wreck, which will be compared to videos taken in 1996 and 2003, to see how the sunken ship is deteriorating. It will also take samples from the cargo tanks, to see whether there's any oil left in them.

The upper portion of this image shows a vertical sonar image of the wreck of the S. S. Montebello, along with lines indicating the path that MBARI's seafloor mapping AUV followed while surveying the wreck. The lower portion shows a "cross section of the stern of the S. S. Montebello, with each dot indicating a single sonar "ping." Image credit: © 2010 MBARI

MBARI's autonomous underwater vehicles were sent down on three days for the sonar images. They are programmed at the surface, then sent below to scan the area with sonar and record the results. Back at the surface, researchers apply the specially designed software to create the images.

MBARI’s seafloor-mapping AUV carries three different types of sonar, which send brief pulses of sound toward the seafloor, then measure how quickly and how intensely these sound waves are reflected back to the vehicle.

Sidescan sonar yields a black-and-white image of the ocean bottom that shows how strongly sound is reflected. Sidescan sonar can show areas of hard and soft seafloor, as well as hard objects such as the wreck.

Multi-beam sonar can be used to create a detailed bathymetric map or a three-dimensional image of the seafloor (and hopefully the wreck as well).

Sub-bottom profiling sonar can indicate the density of layers of rock or sediment beneath the seafloor.