124 • Marine Minerals: Exploring Our New Ocean Frontier 



Photo credit: International Submarine Technology, Ltd. 



Sea MARC II towfish 



essing, in materials used in transducers, and in 

 graphic recording techniques.'^ 



Short-Range Side-Looking Sonar 



Short-range side-looking sonar systems are used 

 for acquiring acoustic images of small areas. They 

 are not used for regional reconnaissance work, but 

 they may be used for detailed imaging of seafloor 

 features in areas previously surveyed with 

 GLORIA or SeaMARC I or IL Operating fre- 

 quencies of short-range sonars are commonly be- 

 tween 100 and 500 kilohertz, enabling very high 

 resolution. Like midrange systems, they are towed 

 close to the ocean bottom. Deep Tow, developed 

 by Scripps Institution of Oceanography, has been 

 used to study morphology of sediment bedforms 

 and processes of crustal accretion at the Mid- 

 Atlantic Ridge. ^^ SAR (Systeme Acoustique Re- 

 morque) is a similar French system, reportedly ca- 

 pable of distinguishing objects as small as 30 by 76 

 centimeters (12 by 30 inches). It is towed about 60 

 meters off the seafloor and produces a swath of 

 about 1,000 meters. Both of these deep-water sys- 

 tems have been used in the search for the Titanic. '^^ 



SeaMARC CL is a short-range deep-towed inter- 

 ferometric system which is under development (fig- 

 ure 4-3). One model has been built for use in the 

 Gulf of Mexico; another has been configured by 

 Sea Floor Surveys International for use by the pri- 

 vate sector and is available for hire. Shallow water, 

 high-resolution, side-looking sonar systems devel- 

 oped by EG&G and Klein are used for such activ- 

 ities as harbor clearance, mine sweeping, and 

 detailed mapping of oil and gas lease blocks. 



Bathytnetric Systems 



Bathymetry is the measurement of water depths. 

 Modern bathymetric technologies are used to de- 

 termine water depth simultaneously at many loca- 

 tions. Very accurate bathymetric charts showing 

 the topography of the seafloor can be constructed 

 if sufficient data are collected with precise naviga- 

 tional positioning (figure 4-4). These charts are im- 

 portant tools for geological and engineering inves- 

 tigations of the seafloor, as well as aids to navigation 

 and fishing. If bathymetric and side- looking so- 

 nar data are integrated and used jointly, the prod- 

 uct is even more valuable. 



Most existing charts are based on data acquired 

 using single beam echo-sounding technology. This 

 technology has now been surpassed by narrow, 

 multi-beam technology that enables the collection 

 of larger amounts of more accurate data. The older 

 data were obtained without the aid of precise posi- 

 tioning systems. Moreover, existing data in the off- 

 shore regions of the EEZ generally consist of sound- 

 ings along lines 5 to 10 miles apart with positional 

 uncertainties of several kilometers. ^^ Charts in the 

 existing National Oceanic and Atmospheric Ad- 

 ministration/National Ocean Service (NOAA/ 

 NOS) series are usually compiled from less than 

 10,000 data points. In contrast, similar charts using 

 the newer multi-beam technology are compiled 

 from about 400,000 data points, and this quantity 

 constitutes a subset of only about 2 percent of the 

 observed data. Hence, much more information is 

 available for constructing very detailed charts. 



"M. Klein, "High-Resolution Seabed Mapping; New Develop- 

 ments," Proceedings, Offshore Technology Conference, Houston, 

 TX, May 1984, p.75. 



^•^Vogt and Tucholke, "Imaging the Ocean Floor," p. 34. 



^'P.R. Ryan and A. Rabushka, "The Discovery of the Titanic by 

 the U.S. and French Expedition," Oceanus, vol. 28, No. 4, wfinter 

 1985/86, p. 19. 



"D.E. Pryor, "Overview of NOAA's Exclusive Economic Zone 

 Survey Program," Ocean Engineering and the Environment, Oceans 

 85 Conferenfce Record, sponsored by Marine Technology and IEEE 

 Ocean Engineering Society, Nov. 12-14, 1985, San Diego, CA, pp. 

 1186-1189. 



