The NOAA Ship Rainier is one of five NOAA vessels equipped primarily for conducting hydrographic surveys in support of nautical charting. Operated by NOAA Marine and Aviation Operations, Rainier performs survey activities off the Pacific Coast and Alaskan coastal waters. The Rainier and her aluminum survey launches are outfitted with multibeam swath and single beam echo sounders, hydrographic data acquisition systems, Differential Global Positioning Systems, sound velocity profilers, and a side scan sonar unit.
The Office of Coast Survey (OCS), part of NOAA's National Ocean Service (NOS), conducts hydrographic surveys to measure the depth and bottom configuration of water bodies, to produce the nation’s nautical charts and ensure safe navigation in the U.S. Exclusive Economic Zone, an area of 3.4 million square nautical miles that extends 200 nautical miles offshore from the coastline. The surveys also identify sea-floor materials (important for anchoring, dredging, and pipeline and cable routing), dredging areas, cables, pipelines, wrecks and obstructions, and fish habitats. They support a variety of activities such as port and harbor maintenance (dredging), coastal engineering (beach erosion and replenishment studies), coastal zone management, and offshore resource development. Hydrographic surveys are conducted primarily by ships using side scan and multibeam sonar. Using this technology, NOAA played a crucial role in finding the wreckage of: TWA flight 800, John F. Kennedy, Jr.'s plane and EgyptAir flight 990. OCS has conducted over 10,600 hydrographic surveys since it began in the early 19th century.
OCS conducts hydrographic surveys primarily with side scan and multibeam sonar. SONAR (Sound Navigation and Ranging) uses sound waves to find and identify objects in the water and determine water depth.
The shipwreck Herbert D. Maxwell depicted by side scan sonar (top) and Digital Terrain Model (DTM) generated from multibeam sonar data (bottom). The Maxwell is a four-masted schooner built in 1905 that sunk on May 16, 1910, east of Annapolis, Maryland, after colliding with the SS Gloucester. The NOAA S/V Bay Hydrographer acquired side scan sonar and multibeam sonar data while using the Maxwell to test and demonstrate various sonar equipment.
Side scan sonar consists of three basic components: a towfish, a transmission cable, and the topside processing unit. The towfish is dragged behind a ship near the sea floor, where it transmits sound energy in the shape of a fan and receives echoes from the surrounding sea bed. The strength of the return echo is recorded, creating a “picture” of the sea floor where objects protruding create a dark image (return) and shadows from these objects are light areas (little or no return). Side scan sonar is most useful to locate sea-floor features and possible obstructions, but does not provide depth information.
Multibeam sonar systems emit sound waves from directly beneath a ship's hull to produce fan-shaped coverage of the sea floor. These systems measure and record the time elapsed between the emission of the signal from the transducers to the sea floor or object and back again. Multibeam sonars produce a “swath” of soundings (i.e., depths) to ensure full coverage of an area.
Raw soundings are digitally acquired for both side scan and multibeam sonar. These soundings are then adjusted to a standard or absolute water level (datum) using predictions from long-term tidal observations at long-term measuring stations and, where available, from active, near-real-time water level stations. In addition, short-term water level monitoring stations may be installed in the immediate survey area to measure relative water levels more accurately. Horizontal positions (latitude and longitude) are recorded using Differential Global Positioning System equipment and need no further adjustment. The coastline is compared extensively to recent coastal survey maps to identify new features and modifications. Eventually, the sounding data are re-processed to produce the final soundings. The final soundings are compiled with a digital representation of the shoreline to update nautical charts.
NOS field parties and outside contractors complete 40 to 50 hydrographic surveys each year. To collect up-to-date hydrographic survey data and to serve the maritime community, OCS has developed a survey priority plan to identify those areas most in need of surveying. These areas are determined by an assessment of traffic volume and patterns, adequacy of current charts or surveys, potentially insufficient underkeel clearance, and input from the maritime community.
The mandate to create nautical charts of the nation's coasts dates back to 1807, when President Thomas Jefferson ordered a survey of the young nation's coast. The Organic Act of 1807 authorized the newly formed coastal survey agency, then called the U.S. Coast Survey, to construct and maintain the nation's nautical charts. The Coast Survey is the oldest scientific organization in the United States and has been a part of NOAA since 1970.
Hydrographic surveying techniques and procedures have changed over time with evolving technology. Multibeam sonar technology makes it possible to collect 100 percent bottom coverage of an area and creates large volumes of data. The OCS Hydrographic Surveys Division continues to perform research and evaluation of emerging technologies. For example, interferometric sonar systems are capable of providing large swath bathymetry in shallow-water areas. This technology has the potential to greatly improve NOAA's productivity in shallow, near-shore areas.
Early hydrographic surveys consisted of depths measured by sounding pole and hand lead line, with positions determined by three-point sextant fixes to mapped reference points. Lead lines were ropes, or lines, with depth-markings and lead-weights attached that were lowered and read manually, a labor-intensive and time-consuming process. While the initial depth soundings may have been accurate, they were limited in number, and thus, coverage between single soundings was lacking.
In the early 1930s, the development of single-beam echo sounders enabled measurement of a single water depth per sonar ping. This greatly speeded the survey process and allowed more data points to be collected. This process was advanced further with the development of electronic navigation systems in the 1940s. In the 1970s, modern multibeam swath mapping systems were developed, improving quality and coverage with multiple sounding sensors.