128 * Marine Minerals: Exploring Our New Ocean Frontier 



used in making bathymetric charts (except for 

 charts of very small areas), and generating charts 

 with a 1- meter contour interval is impractical. Sea 

 Beam, unlike SASS, may be installed on small 

 ships. In order to build a Sea Beam with a 1 ° beam 

 width, an acoustic array 2.5 times longer than cur- 

 rent models would be required. To accommodate 

 such an array, one must either tow it or use a larger 

 ship. The Navy has found that the current Sea 

 Beam system is capable of producing contour charts 

 of sufficient quality for most of its needs and is cur- 

 rently considering deploying Sea Beam systems for 

 several of its smaller ships. 



It is important to match resolution requirements 

 with the purpose of the survey. Use of additional 

 exploration technologies in conjunction with Sea 

 Beam data may provide better geological interpre- 

 tations than improving the resolution of the Sea 

 Beam system alone. For instance, combined ba- 

 thymetry and side-looking sonar data may reveal 

 more features on the seafloor. 



Improving swath coverage is probably more im- 

 portant than improving resolution for reconnais- 

 sance surveys. Wider swath coverage, for exam- 

 ple, could increase the survey rate and reduce the 

 time and cost of reconnaissance surveys. Sea 

 Beam's swath angle is narrow compared to that of 

 GLORIA or SeaMARC (figure 4-6); thus the area 

 that can be surveyed is smaller in the same time 

 period. It may be possible to extend Sea Beam ca- 

 pability from the current 0.8 times water depth to 

 as much as 4 times water depth without losing hy- 

 drographic quality. ^^ The current limit is imposed 

 by the original design; hence, a small amount of 

 development may produce a large gain in survey 

 coverage without giving up data quality. 



Another factor that affects the survey rate is the 

 availability of the Global Positioning System (GPS) 

 for navigation and vessel speed. Currently, NOAA 

 uses GPS when it can; however, it is not yet fully 

 operational. When GPS is unaccessible, NOAA 

 survey vessels periodically must approach land to 

 maintain navigational fixes accurate enough for 

 charting purposes. This reduces the time available 

 for surveying. Ship speed is also a factor, but in- 



creases in speed would not result in as great im- 

 provements in the survey rate as increases in swath 

 width. Operating costs for some typical bathymet- 

 ric systems are shown in figure 4-7. 



Shallow-Water Systems 



Several shallow-water bathymetric systems are 

 available from manufacturers in the United States, 

 Norway, West Germany, and Japan. NOAA uses 

 Hydrochart, commonly known as the Bathymet- 

 ric Swath Survey System (BS^), for charting in 

 coastal waters less than 600 meters (1,970 feet) 

 deep. One of the principle advantages BS' has over 

 Sea Beam is the wider angular coverage available, 

 105° versus 42.7°, enabling a wider swath to be 

 charted. This angular coverage converts to about 

 260 percent of water depth, in contrast to 80 per- 

 cent of depth for Sea Beam. Data acquisition is 

 more rapid for BS^ because the swath width is wider 

 and transmission time in shallow water is reduced. ^^ 

 Hence, signal processing and plotting requirements 

 for BS' are different than those for Sea Beam. GI 

 has recently introduced Hydrochart II, an im- 

 proved version of Hydrochart. The principal differ- 

 ence is a maximum depth capability of 1,000 

 meters. With its 17 beams, Hydrochart II offers 

 much greater resolution and accuracy than older 

 single-beam sonars. 



Along the narrow continental shelf bordering the 

 Pacific Coast, bathymetry in very shallow water is 

 fairly well known. Thus, NOAA has set an inshore 

 limit of 150 meters for its BS^ surveys (except for 

 specicd applications), even though BS^ is designed 

 to be used in water as shallow as 3 meters. In re- 

 gions where there are broad expanses of relatively 

 shallow water and where the bathymetry is less well 

 known, as off Alaska and along the Atlantic Coast, 

 BS^ may be used in water less than 150 meters deep. 



Various bathymetric charting systems are cur- 

 rently under development which may enable sys- 

 tematic surveying of very shallow waters, limited 

 only by the draft of the vessel. One such system, 

 for use in waters less than 30 meters deep, is the 

 airborne laser. Laser systems are under develop- 

 ment, by the U.S. Navy, the Canadians, Aus- 

 tralians, and others. NOAA's work in this field 



'^R. Tyce, Director, Sea Beam Users Group, OTA Workshop on 

 Technologies for Surveying and Exploring the Exclusive Economic 

 Zone, Washington, DC, June 10, 1986. 



'Farr, "Multibeam Bathymetric Sonar," p. 91. 



