Mowing the lawn: NOAA ship and crew go to great lengths to create accurate charts


On a brilliant fall day, the white hull of the oceanographic survey ship Thomas Jefferson gleamed in the sun as it moved slowly away from the pier in New London, Conn.

On the bridge arrayed with Halloween decorations, Ensign Anthony Klemm had the conn, while Ensign Lindsey Norman was at the helm. As Klemm used the main engine, bow thrusters and spring line to move the ship away from the dock, Norman issued a sécurité call over the VHF radio to alert other vessels that NOAA Ship Thomas Jefferson, part of the survey and research fleet of the National Oceanic and Atmospheric Administration, was backing out into the channel.

John Doroba, an assistant hydrographic survey technician, retrieves the moving vessel profiler, commonly known as the fish. This device allows the surveyors to compute the speed of sound through the water at various depths. That information is crucial for correctly interpreting depth soundings collected by the sonar gear. (John Gormley photos)

With the 208-foot ship's port side to the pier, Klemm reversed the engines, while using the thruster to keep the bow from hitting the dock. At this slow speed, the right-handed propeller moving in reverse produced a paddlewheel effect that tended to walk the stern back toward the dock. So Klemm alternated backing a bit and then coasting, to let the wind off the port beam and a flooding 3-knot tide keep the stern moving away from the dock.

Once the bow cleared the dock, "I used a lot of bow thruster to move us in the right direction," Klemm explained. With the bow pointed up into the wind and the current, the ship was soon moving south through the channel toward the open waters of Block Island Sound, where Thomas Jefferson would be conducting its hydrographic surveys.

The ship normally carries a crew of about 36, but has enough berthing space to accommodate 38. On this voyage, the crew consisted of eight officers belonging to the NOAA Commissioned Officer Corps, 24 merchant mariners, scientific staff of two, and one electronic technician. While the NOAA bridge crew is navigating the ship and the scientific staff is collecting and massaging survey data, the NOAA wage mariners, as they are classified, are responsible for a range of other tasks.
 

Lt. Michael Davidson, the ship's field operations officer, oversees the collection of the sonar data used to create and update nautical charts.

For example, First Assistant Engineer (1AE) Armando Mangaya oversees the ship’s propulsion plant; a 2,556-hp 12 cylinder EMD main engine, backed up by a 250-hp auxiliary engine. The ship also has a bow thruster and three Detroit Diesel generators. A native of the Philippines, he served in the U.S. Navy for 24 years, before joining the NOAA crew 15 years ago.

The work life of Mangaya and his crewmates is something of a hybrid. For several months in the year, their lives are like those of shore-based workers who commute to work during the morning but get to go home at night, eat dinner with their families and sleep in their own beds.

From late November until March, Thomas Jefferson normally sits at the dock in its home port of Norfolk, Va. Many of the crew, like Mangaya, live in the Norfolk area. During the winter months, they get to work what he described as "bankers' hours — 7:30 to 4."

During the surveying season, which generally begins in March and ends in November, the crew operates more like that of a bluewater vessel, with long stretches away from home. The ship can end up conducting survey work anyplace from the Gulf of Mexico to New England.
 

NOAA Ship Thomas Jefferson at the pier in New London, Conn., the ship's base as it conducted surveys in the waters off Block Island, R.I., and Long Island, N.Y. The ship normally carries a crew of about 36. Its eight officers belong to the NOAA Commissioned Officer Corps. The rest of the crew on this voyage included 24 merchant mariners, a scientific staff of two and one electronic technician.

For example, in June 2010, the ship was in the Gulf of Mexico checking on the presence and distribution of subsurface oil following the Deepwater Horizon/BP oil spill. In the spring of 2011, it was surveying the approaches to Chespeake Bay. By the fall, the ship was off the coast of Rhode Island and Connecticut conducting surveys to update charts for the very busy waterways in the vicinity of Block Island and Long Island. While performing this kind of survey work, the ship typically spends 12 to 18 days underway, then puts into a nearby port for three days to take on food, fuel and water.

"We're permanent crew. Our season ends one week before Thanksgiving," Mangaya said. "I've been married 36 years — My wife told me I spend more time in the bed on the ship" than in his own bed at home.

Central to Jefferson's surveying mission is its three types of sonar gear: single beam, multi-beam and side scan. All of them work on the same basic principle. The gear emits sounds — or pings — and then measures precisely the time it takes for an echo to return. That data is then used to create a profile of the seabed. If you know the speed of sound through the water and the time it takes an echo to return, then you can compute with great precision the distance to the seabed. And with enough of these data points, you can create a chart.

However, as Lt. Michael Davidson, the ship's field operations officer, explained, the speed of sound in water is not constant. It varies according to temperature and salinity. For every bit of data collected, the survey technicians and scientists need to know the speed at which the ping was moving through the water column.
 

First Assistant Engineer Armando Mangaya, left, in the engine control room with Otis Tate, engineer utility. The ship is propelled by a 2,556-hp 12-cylinder EMD diesel, a 250-hp auxiliary engine and a bow thruster.

It's Davidson's job to make sure they get that information. He does it using the ship's moving vessel profiler or MVP. Generally referred to as the fish, this device looks like a bomb on a tether. Attached by a wire to a small crane, the fish is released, or cast, from the stern and allowed to free fall until the wire stops it at a prescribed depth close to the bottom.

"You want to get as low as possible," Davidson said.

The MVP has a sensor with a small transducer, a reflector and a pressure place to measure depth. As the MVP descends, the transducer emits a sound that hits the reflector plate and bounces back. The time the sound takes to travel the known distance to and from the reflector is measured. The depth sensor assigns a depth to each of these time measurements.

"We know the time and distance, so we can figure the speed," he explained.
 

Cmdr. Lawrence T. Krepp, the ship's commanding officer and lead scientist. As a member of the NOAA Corps, he belongs to a small group with less than 350 members.

The result is a profile of he entire water column showing the speed of sound at every depth.

With that knowledge, the ship's computers can assign a precise speed of sound to every sonar ping and echo generated by the sonar equipment surveying the bottom.

"We initiate a cast (of the MVP) about every 30 minutes," he said. That way the crew can detect any changes in the speed of sound through the water as the ship moves along.

And water temperature and salinity can change quite significantly, especially in an area like the one where Thomas Jefferson was surveying. Here in Block Island Sound, the cold, saline water of the open Atlantic mixes with the warmer, less salty water coming out of the inland sea that is Long Island Sound.
 

The side scan sonar on one of the ship's two launches. These boats have essentially the same surveying capacity as the ship and are used to survey waters that are too shallow or confined for the ship to operate in safely.

The entrance to Long Island Sound is a narrow strait that runs past the North Fork of Long Island to the south, and Fishers Island to the north. Aptly named The Race, it squeezes the tidal flows to create currents of up to 6 knots.

"There's so much current it mixes the water up quite a bit," Davidson said.

Because of the MVP, Thomas Jefferson's crew can quickly detect any changes in sound propagation without having to stop or slow the ship to take water samples.

"We never slow down," Davidson said proudly. "That device alone is a 25-percent efficiency saver."

In addition to the speed of sound, the surveyors also have to account for the precise movements of the ship in relation to the bottom — for example, as it rises and falls on the swell. To solve that problem, the ship is equipped with an inertial measurement unit (IMU) that employs multiple gyros to record the ship's heave, pitch, roll and yaw. Then there is the position orientation system that uses differential GPS and IMU data to precisely determine the ship's position and heading. All the sonar systems are coupled to a master clock.

Even then, there are a few other things that need to be accounted for or adjusted: the ship's trim, its draft and even the amount of squat.

"Everything is dialed in really tight," Davidson said. The integration of this information from all these sources creates what he described as "a tightly coupled solution."

And that's what it takes to convert the raw soundings data from the sonar systems into precise information that can be used to create accurate charts.

The hydrographic team is also dependent on the skill of the ship's navigation crew in order to conduct the surveys efficiently. When the sonar systems are engaged, the ship runs back and forth over the survey area trying to collect data on every square inch of bottom.

The navigation crew calls this "mowing the lawn."

In both kinds of mowing, each row should slightly overlap the previous one, so that the mower makes the minimum number of sweeps without leaving any "uncut" patches. But just like a shore-side mower, sometimes a survey vessel misses a spot here and there. These unmown patches are called "holidays." And the ship has to return at some point to survey them. Clearly holidays need to be kept to a minimum.

Several years ago, Thomas Jefferson got a new autopilot — affectionately known as Iron Mike — that helps the crew keep the ship on the survey line.

With the help of Iron Mike, there has been a significant reduction in the amount of time spent surveying the holidays, according to Ensign Joe Carrier, perhaps as much as a couple of days of surveying per project. And those efficiency gains are expanding, he said, as the crews become increasingly skilled in applying the autopilot technology.

Cmdr. Lawrence Krepp is the skipper aboard Thomas Jefferson. As commanding officer and lead scientist, he is responsible for both the nautical and scientific operations. He brings a strong sense of humor to the job that seems to have infected the crew. His influence could perhaps be detected in the Halloween decorations on the bridge (including a small paper gravestone inscribed "CO").

And also in the chatter on the bridge. As the ship was leaving the dock, Ensign Norman, whose duties include those of morale officer, was asked what she did to keep up the spirits of her crewmates. Her tart reply: "I pretend to care about how they feel."

That retort, issued just a few feet from where Krepp was standing on the bridge, prompted him to observe the she deserved another title, "sarcasm officer."

Later during an interview in his office, Krepp explained that the informality evident on the ship was in some ways the result of the village-like nature of life as a NOAA mariner. It's a small group, with fewer than 350 people in the NOAA Commissioned Officer Corps.

(One of the seven uniformed services of the United States, the NOAA Commissioned Officer Corps is by far the smallest. The largest and most well known services are the Army, Marines, Navy, Air Force and Coast Guard. The Public Health Service Commissioned Corps, with the U.S. surgeon general as it head, is the sixth. While it too might stump trivia contestants asked to name the seven uniform services, with over 6,500 officers, the Public Health Service Commissioned Corps is 20 times the size of the NOAA Corps.)

As a consequence, its members tend to be thrown back together through a variety of assignments throughout their careers and often come to know each other well.

For example, a year after he graduated from college with a degree in marine biology and education, Krepp was working for NOAA's National Undersea Research Center as a dive technician and mate on the center's research vessel. During that time he attended a job fair at the University of North Carolina at Wilmington and stopped at the NOAA booth where he talked to a recruiter.

Demonstrating just how small a world the NOAA Corps is, that recruiter was Denise Gruccio, who currently holds the rank of lieutenant commander and serves aboard Thomas Jefferson as its executive officer, or No. 2 in the chain of command after Krepp.

(Krepp now ranks ahead of Gruccio because she left the NOAA Corps in 1999 to start a family and returned to the Corps in 2009.)

In the end of course, commanding an oceangoing vessel with a complex scientific mission is a serious and challenging undertaking, with little margin for error. Skeletons, bats and gravestones notwithstanding, Krepp has to run a tight ship.

"It's a very stable crew. All of them know their job — deck, engine, survey," he said. "I'm happy to be here. It's a great ship."

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