The Military Sealift Command oceanographic ship USNS Mary Sears was to leave Sasebo Naval Base, Japan, on Thursday and join the USNS John McDonnell in the Indian Ocean to conduct hydrographic surveys of the ocean floor in areas affected by the 9.0-magnitude earthquake and tsunamis. (Greg Tyler / S&S)
The Military Sealift Command oceanographic ship USNS Mary Sears was to leave Sasebo Naval Base, Japan, on Thursday and join the USNS John McDonnell in the Indian Ocean to conduct hydrographic surveys of the ocean floor in areas affected by the 9.0-magnitude earthquake and tsunamis. (Greg Tyler / S&S)
Eric Griffith, left, senior Naval Oceanographic Office representative of the USNS Mary Sears, and Data Manager David Brazier explain how the ship's computer center records data and creates images of the ocean floor using a hydrographic sonar system. (Greg Tyler / S&S)
High-tech sonar equipment on the USNS Mary Sears produces images of wrecks and debris on the bottom of the ocean, as well as exact depths on and around the wrecks. The information tells mariners if there is enough room to pass through. The two ships shown in the image were sunk off the Mississippi coast to create breeding grounds for fish. (Greg Tyler / S&S)
SASEBO NAVAL BASE, Japan — Two Military Sealift Command ships ordered to the Indian Ocean are set to help overcome a major obstacle for other ships trying to position themselves close to areas most devastated by South Asia’s recent 9.0-magnitude earthquake and killer tsunamis.
The high-tech oceanographic survey ships — USNS Mary Sears and USNS John McDonnell — have hydrographic sonar systems and side-scan sonar used to determine the draft (depth clearance) in various channels where ships helping with humanitarian assistance may need to travel.
The Sears was to leave Sasebo Naval Base late Thursday afternoon and eventually meet up with the McDonnell, which left Monday. Both ships arrived in Sasebo on Dec. 22 for routine maintenance.
The channels, harbors and inlets near the coasts of the nations pummeled by tsunami waves on Dec. 26 have bottoms littered with sunken ships and fishing boats, as well as other debris swallowed by the crashing waves.
“The tsunamis caused lots of ships, smaller vessels and debris to sink in the navigable channels, so we are going to check for these obstructions,” said senior Naval Oceanographic Office representative Eric Griffith.
“We’ll report what we find to those doing the dredging, who can then clear a channel, making it safe for some of the vessels assisting with relief,” he added.
The two ships are particularly suited for the work.
The McDonnell is designed in part to operate sonar to chart the depth of shallow seas, or littoral depths nearest coastlines, up to 328 feet deep, and to help chart depths up to 3,300 feet.
The Sears can do the same, and more. The ship’s hydrographic sonar is accurate for measuring the draft in waters up to 6.8 miles deep.
“Both ships use hydrographic survey launches, or HSLs. On this ship we have two of those … with two, we could survey a channel every day on this mission,” Griffith said. The HSLs are 30 feet long.
David Brazier, the Sears’ data manager, maintains the computer systems and their gigantic storage units.
The HSLs collect data that are relayed to the Sears computer control center, where terabytes of information are analyzed. (A terabyte is 1,048,576 megabytes.)
“The hydrographic sonar system sends sonar to the bottom of the ocean directly under the ship and fanning out on either side,” Brazier said.
“To illustrate,” he said, while drawing a diagram on a dry-erase board, “if we were in water that was about 1,000 meters deep (about 1,100 yards), with the range of the hydrographic sonar fanning out on either side, we would be collecting accurate data covering 2.5 kilometers (a little more than 1.5 miles).”
The ship also uses side-scan sonar, an unmanned device the ship drags through the water, recording data translated into high-resolution sea-floor images.
Side-scan sonar images will show exactly where wrecks and other large hunks of debris on the bottom are positioned but won’t reveal detailed depth information. That’s furnished by the hydrographic sonar.
When the ship moves directly over the epicenter of the 9.0-magnitude earthquake that spawned the tsunamis, what ocean-floor changes likely will be observed?
“Probably not very much,” Brazier said. “It’s not like an earthquake occurs and then settles back in the way it was before the event. In whatever way it changes the bottom, that’s how it remains.”
“But our mission,” Griffth said, “is to chart the bottom in those navigable channels, work which could create further options for ships getting in there with relief cargo for tsunami victims.”
At present, most of the largest loads of water, food, medicine and other relief items must be delivered by helicopter.
“So,” he added, “our orders are definitely being termed as ‘humanitarian assistance mission.’”
