Inside The NOAA Ship Thomas Jefferson

When Hurricane Irene approached the Connecticut coast last month, the survey vessel Thomas Jefferson of the National Oceanic and Atmospheric Administration . We visited the ship recently at State Pier, where she was docked while resupplying.

The 208-foot vessel is currently conducting a survey of the floor of Block Island Sound using multibeam and side scan sonar. The purpose of the mission is to provide information to update nautical charts for marine traffic.

Larry Krepp, the commanding officer and chief scientist on the Thomas Jefferson, said the crew aims to finish surveying the Block Island area this year and return next year to do work in Long Island Sound. The ship has been making stops in New London through the summer, and will be in the area until about Nov. 16 before returning to its home port in Norfolk, Va.

Krepp said large portions of the sound have not been surveyed since before World War II. These surveys were also less precise, using techniques such as narrow sound beams or lead lines. In the latter practice, a ship would make periodic stops and lower a piece of lead on a line, marking the depth at that point.

“You had no idea what was happening between those points,” said Krepp.

Mike Davidson, operations officer on the Thomas Jefferson, says the multibeam sonar sends out 512 individual beams with each ping. With 14 pings each minute, the number of data points rapidly increases, especially as the ship is able to operate 24 hours a day. This information can translate into a color-coded map of the seafloor, which in turn can be used to update nautical charts.

The side scan sonar provides black and white imagery of the seafloor and is typically used in shallow areas. Davidson said the multibeam sonar is more efficient in any depth over 100 feet. Side scan imagery can portray obstacles such as rocks, which appear in black, and their acoustic shadows, which appear in white.

“A lot of academics and geologists are interested in this kind of data,” said Davidson.

The sonar systems are also essential in identifying anomalies such as shipwrecks. Krepp said divers are available on board to examine any wrecks, and the crew works with state historic preservation officers to determine any protective measures needed.

The Thomas Jefferson is one of three NOAA survey vessels and does work on the East Coast and in the Gulf of Mexico. The other two ships work along the West Coast and Alaska. Davidson said the work is split approximately 50-50 between NOAA, which is part of the Department of Commerce, and private contractors.

Krepp said 31 people are working on the ship now, slightly less than its full complement of 34. The length of operations varies with federal funding, with this season lasting about 140 days.