Response to a Freighter Grounding Incident in the Bering Sea

by Melanie Engram, ASF User Services

After it lost power and efforts to tow it to safety failed, the freighter Selendang Ayu went aground on December 8, 2004, and broke apart off the shore of Unalaska Island near Skan Bay, Alaska.

The ship was carrying approximately 424-thousand gallons of intermediate fuel oil, 18-thousand gallons of marine diesel, and 132-million pounds of soybeans, according to the NOAA Office of Response and Restoration (NOAA/OR&R). Fortunately, only about 40-thousand gallons of oil spilled when the ship broke in two.

Responding to the disaster, the Alaska Satellite Facility immediately submitted RADARSAT-1 acquisition requests for the National Ice Center (NIC) and NOAA’s National Environmental Satellite Data and Information System (NOAA/NESDIS). In turn, the NIC supplied the U.S. Coast Guard (USCG) with the RADARSAT-1 data, and Bill Pichel from NOAA/NESDIS interpreted the SAR images for the NOAA/OR&R and for the USCG.

When system incompatibilities prevented the coast guard from ingesting images in the NIC’s GeoTIFF format into their ArcGIS software application, ASF bridged the gap by sending the USCG a GeoTIFF product created with ASF’s new conversion tool.

When conditions are ideal, an oil slick on the ocean surface will appear as a very dark area on a SAR image—not because the oil is black, but because the
oil smooths the ocean surface. This smoothed surface fails to reflect SAR microwave pulses back to the satellite and returns a very low backscatter value which shows as black on a SAR image. Usually, the open ocean is slightly rough due to the presence of small waves, called Bragg waves, which will refl ect some of the radar pulses back to the satellite. Therefore, open ocean normally shows as a variety of grey values in a processed SAR image.

Since conditions other than oil slicks may cause low backscatter from ocean water, a black splotch on a SAR image does not necessarily indicate an oil spill. Sea water on the verge of freezing shows as flat, dark black due to splinter ice on the surface dampening the small Bragg waves.

The ocean surface will also be smooth if a nearby mountain onshore is sheltering the ocean surface from wind. Conversely, an oil slick can be hard to detect with SAR data if the ocean waves are too large.

“The best that can be done for coastal spills is to use the SAR data along with in situ data to provide a synoptic overview of potential areas of oil extent and movement,” said Pichel. “SAR observations must be confirmed by observer reports, or by subsequent SAR images that show consistency.”

An additional complicating factor in the Selendang Ayu incident was that spilled oil mixed with the spilled soy beans, creating a foam on the ocean surface instead of a classic smooth oil slick.

All factors considered, the first image of the grounded ship showed low backscatter in the area of the spilled oil.

To increase imaging opportunities, ASF extended the RADARSAT-1 acquisition plan and added imaging from ERS-2 in January 2005 to provide SAR imagery of the incident site. Using this data, NOAA/NESDIS and the NIC continue to monitor the position of the two pieces of the stranded vessel for the USCG and the Unified Command, as well as watch for additional oil spilling.

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