Bay Journal

Getting to the bottom of the Bay’s depths

NOAA vessel provides immediate detailed images of underwater terrain, offering up-to-date details for navigation charts

  • By Lara Lutz on October 01, 2009
  • Comments are closed for this article.
NOAA's Bay Hydro II serves the Bay from Solomons, MD.  (Dave Harp) A detail of a shipwreck, above, near the mouth of the Patuxent River is visible on the multibeam screen.  (Dave Harp) Physical science technician Rob Mowery launches the side-scan sonar Michael Davidson deploys the CTD, a device that measures conductivity, temperature and depth. Data collected on the CTD is downloaded to the boat's computers, which use the measurements to calculate changes in the speed of sound at various depths in the water column that refract, or bend, the sonar pulse. Sonar readings must be corrected for refraction to accurately measure the depth of the sea floor. (Dave Harp) Mike Davidson, left, and Rob Mowery monitor data terminals in the Bay Hydro II's main cabin. Physical scientist Vitad Pradith, center, steers the boat for the mapping exercise. (Dave Harp) The size of ships has changed dramatically since the Bay was first charted, making charts of the bottom increasingly important. According to Howard Danley, chief of navigation services for NOAA's Office of Coast Survey, most 18th- and 19th-century cargo ships drew roughly 15 feet. In the 1960s, they drew approximately 30 feet. Today, container vessels traveling the Bay often draw 48 feet.  (NOAA) The Bay Hydro II uses sonar to map the Bay's floor. The multibeam sonar device releases pulses of sound from a fixed point underneath the boat, then receives and registers the returning echo. The side-scan sonar device is towed behind the boat and sends sonar pulses out to either side.  (NOAA) Rob Mowery, a physical science tech aboard Hydro II, monitors telemetry from the side-scan and multibeam sonar devices.  (Dave Harp)

Lt. Michael Davidson took a broad stance near the stern of the Bay Hydro II as the deck opened up at his feet.

"This is the part when some people say, 'Wow, it's like James Bond.'"

There are no super spies aboard the Bay Hydro II. But slick technology does take a star-studded role.

The Bay Hydro II is a new vessel with the National Oceanic and Atmospheric Administration, serving the Chesapeake Bay from its home port in Solomons, MD.

From the upper deck, a small, T-shaped device descends at the base of what looks like a giant motorized rudder. The entire unit plunges through the open hatch into the water below.

Sound waves emit from its core, hit the river bottom and echo back to the receiver.

Within moments, the device will deliver what centuries of laborious efforts could not: an immediate and detailed image of underwater terrain.

In this case, it's a shipwreck. Inside Bay Hydro II's cabin, the form of the wreck emerges in a stream of colorful pixels that flow across one of nine flat-screen monitors. On another, a crisp 3-D aerial view slides down the screen in gray-toned image called a "waterfall."

The gunwales of the wreck are in various states of decay. Sediment is gouged away from one side of the hull.

Davidson has seen it before. The wreck is one of many underwater features near the mouth of the Patuxent River that provide consistent test sites for sonar technology.

While the image itself is intriguing, less visible work is also under way.

Shipboard computers are logging an enormous amount of data about the movement of the boat, its position and heading, the depth of the wreck, water temperature, conductivity in the water column, tide and the speed of the vessel.

When combined, the data produces a highly accurate rendering of the river bottom-and, eventually, an equally accurate navigational chart.

"It's pretty impressive," Davidson said.

This particular device is a multi-beam echo sounder. It's among the newest and best technology for revealing the shifting depths of the Chesapeake Bay.

"The theory behind multibeam echo sounders has been around since the advent of sonar in the 1940s, but we are now just truly getting into the realm of multi-beam, in part because computers are finally powerful enough to process it," Davidson said.

Sonar (short for Sound Navigation and Ranging) measures distance. A pulse of sound-beyond human hearing-reaches a target. The echo returns. Calculating the elapsed time and the speed of the pulse through the water reveals how far the pulse traveled before hitting its target.

For centuries, surveyors measured water depth by tying lead weights to ropes and dropping them overboard. The farther the rope dropped, the deeper the water. These single "soundings" were repeated at intervals throughout the survey area.

In the early days of sonar, a series of "pings" mimicked the lead lines by taking readings at different, disconnected points in the survey area.

Now, multibeam sonar sends an entire line of pings across a continuous swath of the Bay's floor, a curtain of acoustic pulses at least three times wider than the depth of the water.

"You can collect thousands of soundings per second, and millions of soundings per survey," Davidson said.

So many pings, tightly aligned, trace underwater elevation and submerged obstructions with amazing accuracy.

Side-scan sonar is a related technique. A torpedo-shaped "tow fish" is dragged behind the boat, and the sound beams down to either side.

The Bay Hydro II makes use of both multibeam and side-scan sonar when responding to survey requests throughout the Chesapeake Bay.

The Bay Hydro II also tests emerging technology, including the sonar-equipped Autonomous Underwater Vehicle-a torpedo-shaped robot programmed to guide itself through the water and collect data. NOAA first used the AUV for navigational surveys during the 2008 hurricane season.

Such technology is transforming NOAA's charts of the Chesapeake, and for the nation's coastal waters as a whole.

The Chesapeake Bay is characterized as a relatively shallow body of water, with a deep, north-south channel at the center. But the Bay's bottom does not stand still. It suffers from widespread shoaling-the buildup and movement of sediment that washes off the land. Channels shift.

The water is also filled with man-made obstructions, including ship and plane wrecks, debris from piers and other marine structures that become dislodged, and objects that fall from boats.

Habitat restoration features, like designated fish havens and man-made oyster reefs, are also a concern.

"If the permitting people don't talk to the navigation people, it can get interesting," said Howard Danley, chief of navigation services for NOAA's Office of Coast Survey.

Commercial shipping has always driven the production of good maritime charts, dating back to Thomas Jefferson's request for the first national survey of the coastal waters in 1807. In recent decades, the growing size of cargo ships has made precise charts of the Bay even more important.

"In the 18th and 19th centuries, ships drew about 15 feet, and in the 1960s they drew about 30 feet," Danley said. "Currently, the average container vessel at Baltimore or Norfolk draws 48 feet."

The main approach to the port of Baltimore is a 50-foot channel. That's not much wiggle room. And because ships sit deeper in the water, they need more time to change course. The fewer surprises, the better.

Electronic navigation charts, available from NOAA since 2001, feed vital, current information to captains and pilots.

Safely guiding such large vessels to Baltimore and Norfolk is critical to the region's economy. It also prevents environmental disaster.

"If the captain of the Exxon Valdez had electronic navigational charts, he would have had six or seven warnings," Danley said.

The Exxon Valdez struck a reef off the coast of Alaska in 1989, spilling more than 10 million gallons of crude oil along 11,000 square-miles of coastal waters.

Updating charts for middle and southern sections of the Bay are among NOAA's top survey priorities.

About $4.3 million in federal stimulus funds from the American Recovery and Reinvestment Act will update charts for 125 square nautical miles offshore Virginia between Tangier Island and Stingray Point. The area swarms with recreational and commercial boat traffic, including tankers, barges and container ships, but some sections have not been surveyed since 1939.

David Evans and Associates, Inc., a private company, is conducting the Virginia surveys. By law, NOAA must contract out at least 51 percent of its survey work to the private sector.

Jon Dasler, director of marine services for David Evans and Associates, said two boats operating from Reedville, VA, are gathering data primarily with multibeam and side-scan sonar.

"The crew is working 12 hours a day, seven days a week, weather permitting," Dasler said. "They've been going since midsummer and will finish this fall."

The effort has turned up several large uncharted shipwrecks and other undocumented obstructions. NOAA has already alerted navigators to seven of the finds through updates through on-line charts and local notices to mariners.

Technology will put these and other charts in the hands of mariners faster than ever before.

"It used to be that computers would process the data for hours and hours, with about 25 hours of processing for every hour of data collection in the field," said NOAA's Davidson. "With recent advances in processing technology, a one-to-one ratio may be just around the corner."

Charts that once took years to complete can be published in months. And the amount of information available has never been greater-not just for navigation, but for research, engineering and environmental purposes, too.

Davidson said it's an efficient way to work. "By acquiring data and maintaining it in the archive at the highest resolution, it can be used for more than just nautical charting."

Resource managers, biologists, academic researchers and engineers make use of the survey data to better understand the Bay's dynamics and support decision-making. For example, information about the Bay's floor helps to classify habitat zones. Computer models also depend on such data to predict currents and water levels, which in turn produce forecasts of coastal flooding, harmful algae blooms, and even the presence of sea nettles.

On board the Bay Hydro II and other survey vessels, though, crew members are focused on raw data. They specialize in technology, not biology-armed with specialized knowledge of computer systems, GIS and spatial analysis.

"The technology has really matured, but there's a lot more out there," Davidson said. "This field is always pushing the envelope."

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About Lara Lutz

Lara Lutz is a writer and editor who specializes in the environment, heritage, and outdoors enjoyment of the Chesapeake region. .(JavaScript must be enabled to view this email address).

Read more articles by Lara Lutz

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