Archive for the ‘Hydrographic surveys’ Category
by Ensign Kaitlyn Seberger, onboard NOAA Ship Thomas Jefferson
Nautical charts are an important tool in navigating safely in coastal waters, and Coast Survey’s mission is to keep these charts up to date. However, maintaining accurate charts can be a challenge in locations where sandy shoals may shift seasonally and present a danger to navigation. These areas differ from the current nautical charts, and bottom contours change so rapidly that it may seem an impossible task to keep up using the traditional survey methods. Office of Coast Survey and NOAA Ship Thomas Jefferson are seeking a solution to this ongoing problem and may have an answer with satellite-derived bathymetry.
Satellite-derived bathymetry (SDB) begins with using multi-spectral satellite imagery, obtained by satellites such as Landsat and WorldView2, which compares green and blue color bands.
Multi-spectral satellite imagery of Mutton Shoal in Nantucket Sound, overlaid on the chart.
Green color bands are attenuated by the water faster than blue bands and help to infer relative depths of the water (blue areas being deeper than green). These images are then transformed into a color range scale applicable to the color scale used when surveying with a multibeam echo sounder. With the color range applied, reds on the image represent an area that may be shoal whereas blues and greens represent deeper water.
Satellite-derived bathymetry of Mutton Shoal with a color range scale that is correlated with the color scale used for multibeam processing.
Since the images are based on attenuation of color bands, depth can only be inferred, so survey equipment (such as vertical beam and multibeam sonars) is necessary to acquire true depth.
This fall, NOAA Ship Thomas Jefferson investigated the use of satellite-derived bathymetry imagery as a new survey tool. Survey technicians will calibrate the application of this imagery through bathymetry studies for Nantucket Sound and Chincoteague Island. NOAA Lt. Anthony Klemm, who is leading the studies, chose these project areas because they both had relatively clear shallow water and were in a highly changeable area. At these locations, he chose specific shoals for exploration based on vessel traffic density.
In October, Thomas Jefferson spent two days in Nantucket Sound researching shifting shoals using the satellite-derived imagery overlain on the most recent chart. Ensign Marybeth Head developed line plans to acquire data over the potential location of shoals as seen with the satellite images, as well as their charted locations. Survey launches acquired multibeam data in water deeper than six feet, and Z-Boats were sent in to acquire vertical beam data in areas too shoal for the launches to safely operate.
The video shows Z-boat surveying alongside the launch in shoals too shallow for the launches to operate safely. (Video credit: ENS Head)
Satellite-derived bathymetry of Mutton Shoal with multibeam data from the investigation overlaid. This picture demonstrates how accurate the location of the shifted shoal was compared to the SDB imagery.
During routine conductivity, temperature, and depth casts for sound speed velocity, Ensign Head and Ensign Kaitlyn Seberger used a Secchi disk to determine the attenuation coefficient at each cast location for later comparisons.
The satellite imagery was a vital tool in project planning, as well as determining safe navigation of the ship and the survey launches. Below is a picture of the chart location where Thomas Jefferson intended to anchor. The adjacent image is the satellite-derived bathymetry imagery indicating the anchorage would have been within a shoal area and unsafe for anchoring.
Side-by-side picture of the chart and SDB imagery for the intended anchorage location in Nantucket Sound. SDB imagery indicated a shoal that covered half of the anchorage safety circle. A Z-boat verified the indicated shoal was almost 30 ft shoaler than charted and without this useful imagery, the ship and launches could have run aground.
Ensign Head determined safe passage routes for the survey launches, using the satellite-derived bathymetry imagery overlaid on a chart of the area, as the charted soundings were not reliable. For example, a safe passage route between the study areas and the ship was located between two shoals that had shifted considerably from the chart of the area. Sections of the passage are currently charted at 20 feet or more of water, but the fathometer on the launch displayed depths of less than 10 feet.
Boat sheet for the launches indicating a potential safe passage route from the project area to the ship.
After processing the multibeam data, Ensign Head determined that more than half of the charted shoals in the project area had shifted and the red zones depicted in the satellite-derived bathymetry imagery were significantly shoaler than charted depths for the surrounding area. Results from the investigation showed that the satellite-derived bathymetry for Nantucket Sound was exceptionally accurate and aided in the identification of current navigational dangers.
However, more research is needed regarding the use of satellite-derived bathymetry as a contemporary survey method. Limitations on use of the imagery can include variables such as cloud cover, turbidity, Chlorophyll a, and other water quality properties that may affect attenuation. Despite these challenges, satellite-derived bathymetry is a new tool that could support survey efforts by reducing the amount of time and area necessary to survey and by increasing the effectiveness of NOAA’s efforts to efficiently provide safe navigation to the local mariner.
By Starla Robinson, project manager in Coast Survey’s Hydrographic Surveys Division
Two hundred years after Otto von Kotzebue and the crew of the Ruiric explored what would later be named Kotzebue Sound, NOAA ships Fairweather and Rainer follow in the same tradition. Two centuries ago they were searching for the Northwest Passage in support of trade. Today, we explore to improve the science and safety of navigation in support of commerce, environmental protection, and local communities. Our bathymetric data and observations will also be used to better inform coastal decision-making.
Original chart of Kotzebue Sound (left). 1973 chart of Kotzebue Sound (right). Today’s chart of the project area is not significantly different from that of 1973.
Many things have changed since the crew of the Ruiric braved these waters. However, operations in the Arctic are still challenging. For much of the year Kotzebue Sound is frozen over. The remote location makes arriving and maintaining basic needs of the ships and crew difficult–just being here is a success.
Technology has made navigation safer and surveying more efficient. For example, rather than the discrete lead lines that were once used to obtain depth measurement data in this project area (which is about the size of Delaware), multibeam echo sounders acquire the same amount of data in just one square meter. For multibeam surveys, the speed of sound must be measured in the water column and the motion of the vessel must be recorded and corrected in the data. We use side scan sonar to produce imagery of the sea floor. GPS is used to triangulate our position rather than sailors taking bearings on shore stations. To better refine our precision, we construct horizontal and vertical control stations that must be operational before bathymetry data can even be collected.
It takes teamwork on and off the ship and NOAA has brought together many resources. Contractors are used to establish vertical control stations recording water levels. The Center for Operational Oceanographic Products and Services (CO-OPS) monitors the data and creates tide models. Subject matter experts in side scan sonar assist with the surveying effort. Teams on land plan and support the expedition and continue to process the data for the chart after the ships have left. Many things have to align to make our charting efforts a success.
On the ship, our exposed location limits survey activities. The small boats for survey can only be deployed when the sea state is safe. Teams must brave the surf to maintain the control stations. The crews of the Rainier and Fairweather work hard to take advantage of windows of good weather. They work long hours, in rough conditions, away from convenience and family, in pursuit of the chart. We are today’s explorers seeing the full picture of the seafloor for the first time.
NOAA survey progress map highlighting hydrographic survey coverage by NOAA ships Fairweather and Rainier as of August 17, 2015.
Alaska’s nautical charts need to be updated — we all know that. The diagram below shows the vintage of survey data currently used for today’s charts in Alaska. The graphic includes all surveys done by NOAA’s Office of Coast Survey (and its predecessors), and some limited data acquired by other agencies, i.e., the U.S. Coast Guard. Areas that are not colored in have never been surveyed or have data acquired by another source — from Russia or Japan, for instance — before the U.S. was responsible for charting in that area.
What are the differences between data collected in 1900, 1940, or 1960? Let’s take a look at a…
Brief Historical Sketch of Survey Technologies
Nautical charts have a lot of information, but mariners especially are concerned with two major components: water depths (known as “soundings”) and obstructions (like underwater seamounts or wrecks).
Different eras used different technologies to find, measure, and determine the position of the two components. Note that adoption of new systems does not happen abruptly; rather, new technologies are phased in as techniques and equipment improves.
Measuring Water Depth (Soundings)
3.7 million years ago to present day: sounding pole
It isn’t inconceivable that the earliest humanoid, Australopithecus afarensis, used sticks to gauge water depths before crossing streams and rivers. People still do it today.
Note the ancient Egyptian on the far right, using a sounding pole.
~ 2000 B.C. to 1930s: lead line
As good as they were for their eras, 19th and 20th century surveyors faced technological challenges. The first challenge was accounting for gaps between depth measurements. The second was the inability to be totally accurate in noting the position of the measurement. (In other words, a specific location out in the ocean may be 50 feet deep, but a surveyor must also accurately note the position of that specific location.)
This surveyor is casting a lead line.
Early Coast Survey hydrographers measured depths by lead lines — ropes with lead on one end — that were lowered into the water and read manually. Even though soundings were generally accurate, coverage between single soundings was lacking. And we need to remember that this was before the age of GPS. While sextants gave accurate positions when a hydrographer could fix on a shoreline feature, the further offshore the survey, the less accurate the position.
(Interesting fact: Hydrographers still use lead lines occasionally, in some circumstances — but not for a complete survey.)
There have been variations on lead lines through the centuries. From 1492 to the late 1870s, for instance, mariners used hemp rope for deep-sea soundings.
(Interesting fact: Christopher Columbus and Ferdinand Magellan each tried to measure mid-ocean depths with about 1,200 feet of hemp rope. Neither one of them found the sea bottom.)
In 1872, the hemp was replaced by small diameter piano wire (again, primarily for deep-sea work), and the weight of the lead was increased. Later, hydrographers added a motorized drum to wind and unwind the line, with a dial to record the length of the line.
(Interesting fact: In 1950, the British ship Challenger used piano wire in the first sounding that established Mariana Trench as the deepest place on earth.)
20th century to the present: echo sounders
Compare the bottom coverage achieved by the different survey methods.
1918 to 1990s: single beam echo sounder
Sonar came into its own in 1913. The first echo sounders (also known as “fathometers”) had single beams that measured the distance of the sea floor directly below a vessel. The echo sounders were able to take many more depth measurements than was possible with the lead line, but the technique still resulted in gaps between the lines where the beam measured the water depth.
The U.S. Coast and Geodetic Survey (a NOAA predecessor agency) adopted this acoustic sounding technique in 1923, installing it on USCGS Ship Guide. But full-fledged change didn’t happen right away. These early sounding systems were too large to install on survey launches used in harbor and inshore work, so from 1924 until the early 1940s many surveys were still conducted with a lead line, and many were totally acoustic — and some were hybrid, using soundings from both methods, depending on coverage area and seafloor configuration.
→ 1940: U.S. Coast and Geodetic Survey fully adopts single beam echo sounding technology
The development of smaller “portable” fathometers for shallow waters, about 1940, was a primary impetus in the obsolescence of lead line as survey technology and the adoption of acoustic systems. The development of World War II electronic navigation systems for bombing purposes led to the development in 1945 of the first survey-quality electronic navigation systems, which allowed for more accuracy in positioning.
1964 to current day: multibeam echo sounder
By mid-century, scientists were increasing the beams projected by the echo sounder, to get a broader swath of measurements. The multibeam echo sounder was developed for the Navy in 1964, but it remained secret until the late 1970s when commercially available systems were developed.
Coast Survey first used a MBES technique, called the “Bathymetric Swath Survey System,” in 1977 on NOAA Ship Davidson, for depths ranging from 160 to 2,000 feet. In 1980, NOAA Ship Surveyor installed a deep-water MBES system called “Sea Beam,” for depths from 1,600 to 33,000 feet.
About 1986, Coast Survey began using GPS to calibrate medium-frequency navigation systems while operating in the far reaches of the United States Exclusive Economic Zone. By the mid-1990s, GPS was the primary control for accurate positioning.
→ 2000: Coast Survey fully adopts multibeam surveying
By 2000, Coast Survey was performing full-coverage multibeam hydrographic surveys for charting purposes. NOAA survey ships now use multibeam echo sounders that measure navigable coastal depths from 45 to 1,000 feet. For shallower and more constricted waters, the ships deploy hydrographic survey launches with multibeam echo sounders that efficiently and safely survey areas from 12 to 200 feet deep. These systems make it possible to acquire 100% sea floor coverage in the survey grounds (excluding ultra-shallow, near-shore, or obstruction areas).
Finding Underwater Obstructions
1880s to early 1990s: wire drag
Surveyors used wire drag, not as a sounding system but as a way to look between the sounding lines to find obstructions to navigation and establish safe navigational channels. The first documented wire drag was conducted in the 1880s, in French Indochina, Gulf of Tonkin area, attaching the wire to buoys at each end and letting it drift with tidal currents.
Around 1900, the U. S. Lake Survey developed the technique of using a ¼-mile wire drag between two boats. In 1903, Coast Survey began using the technique, and within a few years was using it extensively in Alaskan waters as they looked for pinnacle rocks. Coast Survey’s Alaska wire drags were up to 3.5 miles long. (Initially, “least depths” over discovered obstructions were determined by lead line, then acoustic means and, ultimately, by divers with depth/pressure gauges.)
Survey vessels conduct wire drag operations.
1960 to present day: side scan sonar
Side scan sonar is essentially the sonar equivalent of an aerial photograph. It improves the ability to identify submerged wrecks and obstructions. Evolving from submarine detection sonars of World War I and World War II, side scan sonar was fairly well developed by 1960, when the United Kingdom Hydrographic Office started using it regularly with their surveys.
→ 1990: Coast Survey fully adopts side scan sonar for East Coast and Gulf Coast surveys
NOAA Ship Whiting used the technology in 1984-1985 for approaches to New York. U.S. Coast Survey fully adopted side scan sonar (in place of wire drag) in the early 1990s.
Side scan sonar operations use “towfish” like this one, lowered into the water and towed from the back of the vessel.
Side scan sonar captures images of objects, which improves the ability to identify submerged objects.
Today’s Charts Reflect Different Tech Eras
Each of NOAA’s 1000-plus nautical charts, even today, can contain information collected by any or all of these sounding and positioning techniques.
Most nautical charts are an amalgamation of geospatial information collected using different techniques at different times. For example, one area of a specific current-day nautical chart might be based on a lead line and sextant survey conducted in 1910, and another area on the same chart might be based on a multibeam and GPS survey conducted in 2010. If we dig deep enough, we will probably find a sounding or two from the 18th century British explorer, Captain James Cook.
NOAA cartographers mold this disparate information so that it fits together as a coherent representation of the geographic area.
So when was the data acquired for the chart you’re using? NOAA cartographers add a “source diagram” to large-scale charts. (See the diagram on the current chart 16240, pictured below.) Check yours. That will give you the years of the surveys… and now you have a better idea on the technology used by the surveyor.
This is the source diagram on nautical chart 16240.
In 2015, NOAA survey ships Thomas Jefferson and Ferdinand R. Hassler are scheduled to survey nearly 1,800 square nautical miles in the U.S. coastal waters of the lower 48 states, collecting data that will update nautical charts for navigation and other uses. In Alaska, NOAA ships Fairweather and Rainier will increase their Arctic operations, planning to acquire 12,000 nautical miles of “trackline” depth measurements of the U.S. Coast Guard’s proposed shipping route. (See this NOAA article.) The ships will also conduct several “full bottom” hydrographic survey projects, acquiring data from over 2,800 square nautical miles in survey areas along the Alaskan coastline.
We are also planning several projects for our contractual private sector survey partners, and those projects will be announced after work orders are finalized.
The Office of Coast Survey will manage the surveys that measure water depths and collect ocean floor data for charting, identifying navigational hazards, informing wind farm decisions, mapping fish habitats, and assisting with coastal resilience. Check the useful story map, 2015 Hydrographic Survey projects, for the survey outlines and more information. Coast Survey will update the map as weather and operational constraints dictate.
See the story map for all 2015 in-house projects.
Briefly, this year’s NOAA survey projects include:
1. Gulf of Maine, where chart soundings in heavily trafficked and fished areas are decades old and need updating for navigational safety
2. Buzzards Bay (Massachusetts and Rhode Island), where increased use of deeper-draft double-hull barges – and possible installation of marine transmission cable routes and wind energy development — requires updated soundings
3. Rhode Island Sound, where the Bureau of Ocean Energy Management has identified a wind energy lease area
4. Approaches to Chesapeake (North Carolina), where charts of critical navigational areas need updating for navigation and to assist the Bureau of Ocean Energy Management manage windfarm activity.
5. Approaches to Charleston (South Carolina), where updated soundings will provide the correct under-keel clearance information for the expected transit of larger and deeper-draft ships
6. Approaches to Savannah (Georgia), where the Savannah Harbor Expansion Project will increase the authorized depth of the harbor from 42 to 47 feet and updated soundings will provide the correct under-keel clearance information for the expected transit of larger and deeper-draft ships
7. Chatham Strait (Alaska), where charts need to be updated for cruise liners, ferries, Coast Guard cutters, Navy vessels, tugs, and barges that use this waterway on a regular basis or when avoiding storms in the Gulf of Alaska
8. Approaches to Kotzebue (Alaska), where deep-draft vessels have their cargo lightered to shore by shallow draft barges
9. Point Hope (Alaska), where shipping traffic is increasing due to receding ice but charted soundings are sparse and date back to the 1960s
10. West Prince of Wales Island (Alaska), where updated charts are needed by smaller vessels that use Televak Narrows as an alternate passage during foul weather
11. Shumagin Islands (Alaska), where Coast Survey needs data to create a new, larger scale, nautical chart
12. Port Clarence (Alaska), where Coast Survey needs data to create a new, larger scale, nautical chart
13. South Arctic Reconnaissance Route, where trackline data will assist consideration of the U.S. Coast Guard’s proposed Bering Strait Port Access Route Study
14. North Coast of Kodiak Island (Alaska), where we need to update charts for Kodiak’s large fishing fleet and increasing levels of passenger vessel traffic
With over 3.4 million square nautical miles of U.S. waters to chart, NOAA’s Office of Coast Survey is constantly evaluating long-term hydrographic survey priorities. Now, for the first time, Coast Survey is posting its three-year survey plans and making them publicly available at the Planned NOAA Hydrographic Survey Areas (2015-2017) in ArcGIS Online. In addition to seeing the outlines of planned survey areas for the next three years, users can obtain additional metadata (project name, calendar year, and area in square nautical miles) for each survey by simply clicking on the outlines. Other features display the survey area information in a tabular format, and can filter the information using metadata fields.
The Hydrographic Survey Division is Coast Survey’s primary data acquisition arm. They plan and manage the large survey ships’ hydrographic operations. (The Navigation Services Division manages the smaller survey boats used by the navigation response teams. Their survey plans will soon be added to this webmap.)
Because of the enormousness of our area of responsibility and limited resources, Coast Survey develops long-term survey priorities using a number of parameters, including navigational significance, survey vintage (when the area was last surveyed), vessel usage, and potential for unknown dangers to navigation. Coast Survey then culls the long-term priorities for annual survey plans using other factors such as urgent needs (recent grounding, accidents, etc.), compelling requests from the maritime industry and U.S. Coast Guard, traffic volume, and identified chart discrepancies.
While Coast Survey tries to consider operational constraints, ice coverage, and weather patterns while making plans, sometimes the unexpected does occur. We have to emphasize that these are plans, subject to reevaluations, operational constraints, weather, and resource allocation. Because plans often change, people should bookmark the site and check back often. This is an operational site, and we will update plans as they change.
For more information about specific survey areas or to request a survey, please submit an inquiry through NOAA’s Nautical Inquiry & Comment System or contact the regional navigation manager for your area.
The Planned NOAA Hydrographic Survey Areas webmap is powered by Esri’s ArcGIS Online technology.
Under beautiful blue skies yesterday in Gulfport, Mississippi, David Evans and Associates, Inc. commissioned its new 82-foot hydrographic survey and scientific vessel Blake. Rear Admiral Gerd Glang, Coast Survey’s director, welcomed the addition to the nation’s hydrographic surveying assets. DEA is under contract to NOAA to provide critical hydrographic data for updating the nation’s nautical charts.
Speaking at the Blake‘s commissioning ceremony were (left to right) Mayor William Gardner Hewes, U.S. Senator Thad Cochran, Rear Admiral Gerd Glang, and Jon Dasler, director of Marine Services, David Evans and Associates, Inc. Photo by Tim Osborn, NOAA.
Glang pointed out that, for the past 15 years, NOAA has fulfilled its charting mission through a successful partnership with private sector surveying firms.
“David Evans and Associates, who owns and will operate the Blake, has been an important partner in that effort,” Glang said. “They conducted their first survey for Coast Survey in 1999. In the last 15 years, they have completed 72 hydrographic surveys – nearly 1200 square nautical miles – in the coastal waters and bays of seven different states.”
“David Evans and Associates consistently produces outstanding hydrographic surveys for NOAA. And, they are pioneers in applying new surveying technologies and methods.”
U.S. Senator Thad Cochran, who did the honor of breaking a Champagne bottle across the Blake‘s bow, spoke of the vessel’s value to the nation.
“The survey vessel Blake is an example of the skilled workmanship of our Gulf Coast shipbuilding industry. This vessel will provide an important national seagoing capability to perform valuable research for our state and nation,” Cochran said. “I commend David Evans and Associates, Inc. for its hard work in constructing a world-class research vessel that can serve our nation for decades to come.”
The custom-built Blake is an aluminum catamaran. The vessel is designed to be a stable, efficient and cost-effective survey platform with wave-piercing bows, tier-3 diesel engines, twin 50-kilowatt generators, and a full suite of state-of-the-art survey instrumentation. Built by Geo Shipyard, Inc. in New Iberia, Louisiana, the new vessel will complement the firm’s national operations and expand DEA’s hydrographic and geophysical survey and marine science capabilities in the Gulf of Mexico and beyond.
The new vessel is named after the 19th century U.S. Coast and Geodetic Survey steamer George S. Blake, an oceanographic and hydrographic vessel renowned for testing innovative technologies such as the Pillsbury current meter, which was the first deep-sea current meter, and the Sigbee deep-sea sounding machine. The federal Blake was commissioned in 1874, operated in the Gulf of Maine, the Gulf of Mexico, and the Caribbean Sea. The Blake’s accomplishments are documented in Three Cruises of the BLAKE, by Alexander Agassiz. The vessel had numerous seafloor features named after her, including the Blake Abyssal Plain, Blake Plateau, Blake Canyon and Blake Ridge, all of which are off the southeastern coast of the U.S.
Noting the advanced technologies onboard the S/V Blake, that were not even imagined 140 years ago when the USC&GS steamer Blake did it’s innovative work, Glang congratulated the firm on its significant contribution to the survey mission.
“I look forward to the legacy of accomplishment and innovation that the S/V Blake begins today,” Glang said, “and to a sustained partnership between David Evans and Associates and NOAA.”
For more, see this news report from WLOX TV.