Ch. 4— Technologies for Exploring the Exclusive Economic Zone • 119 



RECONNAISSANCE TECHNOLOGIES 



Side-Looking Sonars 



Side-looking sonars are used for obtaining acous- 

 tic images of the ocean bottom. Most side-looking 

 sonars use ship-towed transducers which transmit 

 sound through the water column to the seafloor. 

 The sound is reflected from the seabed and returned 

 to the transducer. Modern side-looking sonars 

 measure both echo-time and backscatter intensity. 

 As the ship moves forward, successive sound pulses 

 are transmitted, received, and digitally recorded. 

 Side-looking sonars were originally designed for 

 analog operation (i.e., for producing a physical 

 trace of the returned echo), but most now use dig- 

 ital methods to facilitate image processing. The data 

 are usually processed to correct for variations in 

 the ship's speed, slant-range distance to the seafloor, 

 and attenuation of sound in the water. The final 

 product is a sonograph, or acoustic image, of the 

 ocean floor. It is also possible to extract informa- 

 tion about the texture of some seabed deposits from 

 the sonar signal. Side-looking sonars useful for EEZ 

 exploration are of three types: 



1. long-range (capable of mapping swaths 10 to 

 60 kilometers wide), 



2. mid range (1 to 10 kilometers swaths), and 



3. short range (<1 kilometer swaths).* 



Table 4-2 displays characteristics of several side- 

 looking sonars. 



Long-Range Side-Looking Sonar 



One of the few technologies used to date to in- 

 vestigate large portions of the U.S. EEZ is a long- 

 range side-looking sonar known as GLORIA (Geo- 

 logical LOng Range Inclined Asdic) (figure 4-2). 

 GLORIA was designed by the Institute of Oceano- 

 graphic Sciences (lOS) in the United Kingdom and 

 is being used by the U.S. Geological Survey 

 (USGS) for obtaining acoustic images of the U.S. 

 EEZ beyond the continental shelf. ^ When proc- 



essed, GLORIA images are similar to slant-range 

 radar images. GLORIA'S main contribution is that 

 it gives geologists a valuable first look at expanses 

 of the seafloor and enables them to gain insight 

 about seabed structure and geology. For instance, 

 the orientation and extent of large linear features 

 such as ridges, bedforms, channels, and fracture 

 zones can be determined.^ Horizontal separations 

 as little as 45 meters (148 feet) and vertical distances 

 on the order of a few meters can be resolved. 



USGS is using GLORIA to survey the EEZ rela- 

 tively inexpensively and quickly. GLORIA can sur- 

 vey swaths of seabed as wide as 60 kilometers (al- 

 though, in practice, a 45-kilometer swath width is 

 used to improve resolution). When towed 50 meters 

 beneath the sea surface at 8 to 10 knots and set to 

 illuminate a 60-kilometer swath, GLORIA is ca- 

 pable of surveying as much as 27,000 square kilom- 

 eters (about 8,300 square nautical miles) of the 

 seafloor per day. It is less efficient in shallow water, 

 since swath width is a function of water depth be- 

 low the sonar, increasing as depth increases. 

 GLORIA can survey to the outer edge of the EEZ 

 in very deep water. 



Processing and enhancement of digital GLORIA 

 data are accomplished using the Mini-Image Proc- 

 essing System (MIPS) developed by USGS.' MIPS 

 is able to geometrically and radiometrically correct 

 the original data, as well as enhance, display, and 

 combine the data with other data types. In addi- 

 tion, the system can produce derivative products, 

 all on a relatively inexpensive minicomputer sys- 

 tem.^ It is also possible now to vary the scale and 

 projection of the data without having to do much 

 manual manipulation. 



'P.R. Vogt and B.E. Tucholke (eds.) "Imaging the Ocean Floor — 

 History and State of the Art," in The Geology of North America, 

 Volume M, The Western North Atlantic Region (Boulder, CO: Geo- 

 logical Society of America, 1986), p. 33. 



'EEZ Scan 1984 Scientific Staff, Atlas of the Exclusive Economic 

 Zone, Western Conterminous United States, U.S. Geological Sur- 

 vey Miscellaneous Investigations Series 1-1792, Scale 1:500,000, 1986. 



•iR.W. Rowland, M.R. Goud, and B.A. McGregor, "The U.S. 

 Exclusive Economic Zone — A Summary of Its Geology, Exploration, 

 and Resource Potential," U.S. Geological Survey, Geological Cir- 

 cular 912, 1983, p. 16. 



^P.S. Chavez, "Processing Techniques for Digital Sonar Images 

 From GLORIA, ' ' Photogrammetric Engineering and Remote Sens- 

 ing, vol. 52, No. 8, 1986, pp. 1133-1145. 



'G.W. Hill, "U.S. Geological Survey Plans for Mapping the Ex- 

 clusive Economic Zone Using 'GLORIA'," Proceedings: The Ex- 

 clusive Economic Zone Symposium: Exploring the New Ocean Fron- 

 tier, M. Lockwood and G. Hill (eds.), conference sponsored by 

 National Oceanic and Atmospheric Administration, U.S. Department 

 of the Interior, Smithsonian Institution, and Marine Technology So- 

 ciety, held at Smithsonian Institution, Oct. 2-3, 1985, p. 76. 



