Ch. 4— Technologies for Exploring the Exclusive Economic Zone • 123 



kilohertz) than long-range systems and because they 

 are towed closer to the bottom.'* However, higher 

 resolution is obtained at the expense of swath width. 



SeaMARC I data is relatively expensive to ac- 

 quire, given the smaller area that can be surveyed 

 in a given time; however, SeaMARC I coverage 

 in specific areas is a logical follow-on to GLORIA 

 regional coverage, as the information it provides 

 is of much higher resolution. For example, little is 

 known about the small-scale topography of sea- 

 mounts and ridges where cobalt crusts are found. 

 SeaMARC I surveys (or surveys by a similar deep- 

 towed system) will be needed to determine this 

 small-scale topography before appropriate mining 

 equipment can be designed.'' 



Interferometric Systems 



By measuring the angle of arrival of sound echoes 

 from the seafloor in addition to measuring echo am- 

 plitude and acoustic travel time, interferometric sys- 

 tems are able to generate multi-beam-like bathy- 

 metric contours as well as side-scanning sonar 

 imagery (table 4-3). '« SeaMARC II developed 

 jointly by 1ST and HIG, newer versions of Sea- 



"Rowland, Goud, McGregor, "The U.S. Exclusive Economic 

 Zone — Summary," p. 18. 



"J.R. Hein, L.A. Morgenson, D.A. Clague, et al., "Cobalt-Rich 

 Ferromanganese Crusts From the Exclusive Economic Zone of the 

 United States and Nodules From the Oceanic Pacific," Geology and 

 Resource Potential of the Continental Margin of Western North Amer- 

 ica and Adjacent Ocean Basins — Beaufort Sea to Baja California, D. 

 SchoU, A. Grantz, and J. Vedder (eds.), American Association of Pe- 

 troleum Geologists, Memoir 43, in press, 1986. 



"J.G. Blackinton, D.M. Hussong, andJ.G. Kosalos, "First Re- 

 sults From a Combination Side-Scan Sonar and Seafloor Mapping 

 System (SeaMARC II)," Proceedings, Offshore Technology Confer- 

 ence, Houston, TX, May 2-5, 1983, OTC 4478, pp. 307-314. 



Table 4-3.— Swath Mapping Systems 



Image only 



Image and bathymetry Bathymetry only 



Side looking 



Swath Map 

 GLORIA 

 SeaMARC I 

 SeaMARC II 

 SeaMARC CL 

 Deep Tow 

 SAR 



EDO 4075 

 EG&G SMS960 

 EG&G 260 

 Klein 



Interferometric 



SeaMARC II 

 SeaMARC/S 

 SeaMARC TAMU 

 Bathyscan 

 TOPO-SSS 



Sector scan 



Hydrosearch 



SNAP 



Multibeam 



Sea Beam 



BSSS/Hydrochart 



SASS 



BOTASS 



Krupp-Atlas 



Honeywell-Elac 



Simrad 



Benetech 



MARC I, and several other systems have this dual 

 function capability. 



SeaMARC II is a midrange to long-range side- 

 looking sonar towed 100 meters below the surface 

 (above SeaMARC I, below GLORIA). It is capa- 

 ble of surveying over 3,000 square kilometers (875 

 square nautical miles) per day when towed at 8 

 knots, mapping a swath 10 kilometers wide (20 

 kilometers or more when used for imaging only) 

 in water depths greater than 1 kilometer. Some re- 

 cent SeaMARC II bathymetry products have pro- 

 duced greater spatial resolution than Sea Beam or 

 SASS bathymetry technologies (discussed below). 

 Currently, SeaMARC II does not meet Interna- 

 tional Hydrographic Bureau accuracy standards for 

 absolute depth, which call for sounding errors of 

 no more than 1 percent in waters deeper than 100 

 meters. Although there are physical limits to im- 

 provements in SeaMARC accuracy, the substan- 

 tial advantage in rate of coverage may outweigh 

 needs for 1 percent accuracy, particularly in deep 

 water.'' SeaMARC II's swath width is roughly four 

 times Sea Beam's in deep water, so at similar ship 

 speeds the survey rate will be about four times 

 greater. 



Two other SeaMARC systems, both of which 

 will have the capability to gather bathymetry data 

 and backscatter imagery, are now being developed 

 at 1ST: SeaMARC TAMU and SeaMARC CL. 

 SeaMARC TAMU is a joint project of the Naval 

 Ocean Research and Development Activity, Texas 

 A&M University, and John Chance Associates. 

 The unit will be able to transmit and receive sig- 

 nals simultaneously at several frequencies, which 

 may enable identification of texture and bottom 

 roughness. 



Concurrently, developments are underway to use 

 Sea Beam returns to measure backscattering 

 strength; hence, technical developments are begin- 

 ning to blur the distinction between SeaMARC and 

 Sea Beam systems. '^ Additional advances in seabed 

 mapping systems are being made in the design of 

 tow vehicles and telemetry systems, in signal proc- 



SOURCE; International Submarine Technology, Ltd. 



"D. E. Pryor, National Oceanic and Atmospheric Administration, 

 OTA Workshop on Technologies for Surveying and E.xploring the 

 Exclusive Economic Zone, Washington, DC, June 10, 1986. 



'"Vogt and Tucholke, "Imaging the Ocean Floors," p. 34. 



