From 1984 to 1991, the USGS conducted the first systematic mapping of an Exclusive Economic Zone (EEZ). For those surveys the USGS used the GLORIA II system, a shallow-towed digital system that was at that time a state-of-the-art deep-ocean sidescan sonar. At that time, the immense size of the U.S. EEZ precluded using any of the other existing deep-ocean mapping systems. In addition, none of the sea-floor mapping systems available in the 1980's were efficient enough in coverage to warrant mapping in water depths shallower than ~200 m; consequently, the US continental shelves were left unmapped: a total area exceeding 1,470,000 square km!
In the mid 1990s a new generation of digital sea-floor
mapping systems was developed that falls under the heading of high-resolution
multibeam mapping systems. These latest generation systems provide efficient
collection of georeferenced bathymetry and backscatter data.
Current Mapping Activities
In 1996 the USGS began to systematically map areas of the US continental shelves using the latest-generation multibeam systems in cooperation with the Ocean Mapping Group (OMG) of the University of New Brunswick, Canada. Although the area of the US continental shelves is large, the new high-resolution multibeam systems have wide-enough swath widths and can be used at fast ship speeds so that it begins to become economical and efficient to map the continental shelves. The criteria the USGS uses to determine where and how much of a shelf to map include:
High-resolution multibeam mapping systems typically are hull-mounted sonar arrays that collect bathymetric soundings with a series of electronically formed receive beams precisely pointed at a swath of angles away from the vertical. Some of these systems also can record the absolute amount of sound reflected back to the array providing a backscatter map. The precise system used depends on the anticipated water depths; lower-frequency systems are used for deep water, higher-frequency systems are used for shallow water. One of the key components of the system is an integrated vehicle motion sensor that determined the pitch, roll, and yaw of the boat 100 times per second with an accuracy of 0.01°. The vehicle motion sensor provides the electronic stabilazation required for accurate bottom location of each beam. In additon, very accurate measurements of the water properties are required throughout the survey so that compensations can be made for sound refraction, caused by density variations in the water. One great advantage of a hull-mounted array is that the survey can be run at relatively high speeds, making the surveys very cost effective.
The ship's position is determined by both an inertial navigation system and dual differential GPS. Because the transducer arrays are hull mounted rather than towed in a vehicle somewhere behind the ship, the data are all accurately georeferenced to ±1 m. Accurately georeferenced base maps become a requirement when attempting relocation of a sampling station or change detection between multiple investigations of features found on the base maps, etc.
The USGS is using high-end graphic workstations with visualization software developed at the University of New Brunswick to analyze the multibeam datasets. Entire datasets can be visualized at full resolution as 3-D, color-coded, shaded-relief surfaces, backscatter can be draped on a bathymetric surface, and the data can be investigated by interactive flythroughs. An pixel on the image can be interrogated for latitude, longitude, depth, and slope and interactive profiles can be drawn across any area. Viewing data with these techniques provides an instant grasp of what the data actually depict.
Data processing is at the heart of any digital marine mapping operation. Typically a team of USGS and OMG scientists processes the multibeam data while at sea. Final data editing, compilation, mosaicking, and plotting are completed within a day of the end of a cruise. Data processing includes: editing navigation, removing spurious bathymetry and backscatter data, correcting water-refraction errors, gridding and mosaicking the imagery, and compiling the final map. The final maps of bathymetry and backscatter are readily transferred to a GIS for additional analyses and compilations.
For more in-depth information about Multibeam-mapping Technology please see USGS Open File Report 98-509, The Bathymetry of Lake Tahoe, California-Nevada