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Searching for a Shipwreck

May 25, 2013

One of the goals of the second leg of this cruise is to explore several shipwrecks that are of historical significance with the ROV Jason. In-depth exploration of wreckages can provide vital archaeological information that may improve our current understanding of an important historical event or artifact as well as promote efforts to preserve shipwreck locations. However, before valuable time can be spent diving on a wreck, we need to have a really clear idea of where it is located. To pinpoint the suspected location of a wreck we rely on multibeam sonar.

After successfully exploring and mosaicing several wrecks in shallow waters ranging from approximately 100 to 200 m we moved to waters exceeding 2000 m in depth. We had an idea of the general location of two shipwrecks but did not have a determined diving site. To narrow our search, multibeam profiling was conducted in the areas of interest. The R/V Ronald Brown is equipped with a Kongsberg Echosounder Multibeam 122. This system emits an acoustic signal at 12 kHz and provides great bottom resolution at depths up to 10,000 m. The area of coverage, or swath, for this system is between five and five and one half times the water depth. So at depths of around 2,000 m, with each transmittance, we are able to cover an area of approximately 10,000 m. By constantly emitting and receiving the acoustic signal we get a pretty clear image of what the landscape looks like underneath and lateral to the ship.

Discussing Sonar

Dr. Rod Mather (bottom) discusses the multibeam data with engineer Tony Dahlheim while others watch patiently. Image courtesy of Art Howard, Deepwater Canyons 2013 Expedition, NOAA-OER/BOEM/USGS.

Early Thursday morning we were closing in on possible targets. A raw image of the multibeam survey can be tracked in real-time in the scientific computer lab. What you see is a black screen with an icon showing the orientation of the ship. Slowly, a topographic map grows with each scan, displaying hues from dark blue (deeper topography) to red (shallower topography). If you’ve ever tried to watch an HD video on YouTube with a slow Internet connection speed, this is how I felt waiting for the next swath to appear. I wanted it now. But, the excitement when something positive does appear on the screen is worth the wait. A small red feature appeared at the edge of a swath! Could it be the shipwreck we are looking for? Change the ship’s coordinates!

When a potential target is identified, a message must be relayed from the computer lab to the bridge via phone to move the ship over the area in question. After the relay of coordinates we sit and wait, and then suddenly the orientation of the ship begins to change. This change in declination was visible as a nice radial pattern of swaths on the map until we were headed straight toward the site. Closing in on the position is another waiting game filled with anticipation. While there were several exciting moments like this, unfortunately, with his keen eye for shipwrecks and knowledge of hydrogeography, Dr. Rod Mather, chief scientist for the second leg of the cruise, declared these features to be natural geologic formations and not wreck sites.

Raw data from the multibeam sonar system. Image courtesy of Art Howard, Deepwater Canyons 2013 Expedition, NOAA-OER/BOEM/USGS.

Raw data from the multibeam sonar system. Image courtesy of Art Howard, Deepwater Canyons 2013 Expedition, NOAA-OER/BOEM/USGS.

The search continues! We were able to conduct multibeam surveys over a large area, and these data can be further processed in the lab and reanalyzed for possible future locations of exploration. Also, being resourceful as one must be when conducting research at sea, with the extra time we were able to complete a higher resolution photo mosaic of a ship we had previously dived on.

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