132 • Marine Minerals: Exploring Our New Ocean Frontier 



also be considered classified under current Depart- 

 ment of Defense policy. The grid lines are often 

 only one-quarter mile apart, indicating that these 

 maps would be very accurate (although a stand- 

 ard 3.5 kilohertz echo sounding does not have the 

 resolution of Sea Beam).^* 



NOAA is currently exploring ways to utilize data 

 acquired by academic and private institutions to 

 upgrade existing bathymetric maps to avoid dupli- 

 cation. In some areas, it may be possible to accumu- 

 late enough data from these supplemental sources 

 to improve the density and accuracy of coverage. 

 However, because these data usually were not 

 gathered for the purpose of making high-quality 

 bathymetric maps, these data may not be as ac- 

 curate as needed. NOAA is adhering to Interna- 

 tional Hydrographic Bureau standards because 

 these standards are: widely accepted by national 

 surveying agencies, result in a product with a high 

 degree of acceptance, and are feasible to meet. 

 NOAA could relax its standards if this meant that 

 an acceptable job could be done more efficiently. 

 For example, if depth accuracy of the SeaMarc II 

 system (which has a much wider swath width than 

 Sea Beam) could be improved from the present 3 

 percent of depth to 1.5 percent or better, NOAA 

 might consider using SeaMarc II in its bathymet- 

 ric surveys. 



Public data sets rarely have the density of cov- 

 erage that would provide resolution approaching 

 that of a multi-beam survey. Commercial survey 

 data are not contiguous over large areas because 

 they cover only selected cireas or geologic structures. 

 Data may be from a wide beam or deep seismic 

 system, possibly uncorrected for velocity or un- 

 edited for quality. Data sets would also be difficult 

 to merge. Unless the lines are sufficiently dense, 

 computer programs cannot grid and produce con- 

 tours from the data at the scale and resolution of 

 multi-beam data.'^ 



SASS data acquired by the U.S. Navy is classi- 

 fied. NOAA neither knows what the bathymetry 

 is in areas surveyed by SASS nor what areas have 

 been surveyed. More optimistically, once the 



"C. Savit, Senior Vice President, Western Geophysical, OTA 

 Workshop on Technologies for Surveying and Exploring the Exclu- 

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



"Perry, "Mapping the Exclusive Economic Zone," p. 1193. 



Global Positioning System becomes available 

 around the clock, thereby enabling precise naviga- 

 tional control at all times, it may be possible for 

 NOAA to utilize multi-beam surveys conducted by 

 others, e.g., by University National Oceanographic 

 Laboratory System (UNOLS) ships. If the three 

 university ships currently equipped with Sea Beam 

 could be used as "ships of opportunity" when 

 otherwise unemployed or underemployed, both 

 NOAA and the academic institutions would bene- 

 fit. NOAA has already discussed the possibility of 

 funding Sea Beam surveys with the Scripps Insti- 

 tution of Oceanography. 



Reflection and Refraction Seismology 



Seismic techniques are the primary geophysical 

 methods for acquiring information about the geo- 

 logical structure and stratigraphy of continental 

 margins and deep ocean areas. Seismic techniques 

 are acoustic, much like echo sounding and sonar, 

 but lower frequency sound sources are used (fig- 

 ure 4-8). Sound from low-frequency sources, rather 

 than bouncing off the bottom, penetrates the bot- 

 tom and is reflected or refracted back to one or more 

 surface receivers (channels) from the boundaries 

 of sedimentary or rock layers or bodies of differ- 

 ent density (figure 4-9). Hence, in addition to 

 sedimentary thicknesses and stratification, struc- 

 tural characteristics such as folds, faults, rift zones, 

 diapirs, and other features and the characteristic 

 seismic velocities in different strata may be deter- 

 mined (figure 4-10). 



Seismic reflection techniques are used extensively 

 to search for oil, but they are also used in mineral 

 exploration. Reflection techniques have been and 

 continue to be refined primarily by the oil indus- 

 try. Seismic refraction, in contrast to seismic reflec- 

 tion, is used less often by the oil industry than it 

 once was; however, the technique is still used for 

 academic research. Ninety-eight percent of all seis- 

 mic work supports petroleum exploration; less than 

 2 percent is mineral oriented. 



The depth of wave penetration varies with the 

 frequency and power of the sound source. Low-fre- 

 quency sounds penetrate deeper than high-fre- 

 quency sounds; however, the higher the frequency 

 of the soynd source, the better the resolution pos- 

 sible. Seismic systems used for deep penetration 



