144 • Marine Minerals: Exploring Our New Ocean Frontier 



towed instrument packages like SLEUTH are en- 

 abling systematic surveys of large ocean areas to 

 be undertaken. 



Nuclear Methods 



Nuclear methods consist of physical techniques 

 for studying the nuclear or radioactive reactions and 

 properties of substances. Several systems have been 

 developed to detect the radiation given off by such 

 minerals as phosphorite, monazite, and zircon. One 

 such device was developed by the Center for Ap- 

 plied Isotope Studies (CAIS) at the University of 

 Georgia. In the mid-1970s, the Center developed 

 an underwater sled equipped with a radiation de- 

 tector that is pulled at about 3 knots over relatively 

 flat seabed terrain. The towed device consists of a 

 four-channel analyzer that detects potassium-40, 

 bismuth-214, thallium-208, and total radiation. The 

 sled has been used to locate phosphorite off the coast 

 of Georgia by detecting bismuth-214, one of the 

 radioactive daughters of uranium, often a constit- 

 uent of phosphorite. In another area offshore Geor- 

 gia, the Center's towed sled detected thallium-208, 

 an indicator of certain heavy minerals. Subsequent 

 acquisition of surficial samples (grab samples) of 

 the area confirmed the presence of heavy mineral 

 sands. '° 



A similar system for detecting minerals associ- 

 ated with radioactive elements has been developed 

 by Harwell Laboratory in the United Kingdom. 

 The Harwell system identifies and measures three 

 principal elements: uranium, thorium, and potas- 

 sium. The seabed probe resembles a snake and is 

 towed at about 4 knots in water depths up to 400 

 meters (1,300 feet). The Harwell system is now 

 commercially available and is being offered by Brit- 

 ish Oceanics, Ltd., as part of its worldwide survey 

 services." 



A second type of nuclear technique with prom- 

 ise for widespread application in marine mineral 

 exploration uses X-ray fluorescence to rapidly ana- 

 lyze surface sediments aboard a moving ship. The 

 method was developed by CAIS and uses X-ray 

 fluorescence as the final step. X-ray fluorescence 



is a routine method used in chemical analyses of 

 solids and liquids. A specimen to be analyzed using 

 this technique is irradiated by an intense X-ray 

 beam which causes the elements in the specimen 

 to emit (i.e., fluoresce) their characteristic X-ray 

 line spectra. The elements in the specimen may be 

 identified by the wavelengths of their spectral 

 lines. '^ 



The CAIS Continuous Seafloor Sediment Sam- 

 pler was originally developed for NOAA's use in 

 rapid sampling of heavy metal pollutants in near- 

 shore marine sediments. A sled is pulled along the 

 seafloor at about three knots. The sled disturbs the 

 surficial sediments, creating a small sediment 

 plume. The plume is sucked into a pump system 

 within the sled and pulled to the surface as a slurry. 

 The slurry is further processed, after which small 

 portions are collected on a continuous filter paper. 

 After the water is removed, a small cookie-like wa- 

 fer remains on the paper (hence, the system is 

 known as the "cookie maker"). "Cookies" are 

 coded for time, location, and sample number and 

 can be made about every 30 seconds, which, at a 

 ship speed of 3 knots, is about every 150 feet. An 

 X-ray fluorescence unit is then used to analyze the 

 samples. It is possible to analyze three or four ele- 

 ments aboard ship and approximately 40 elements 

 in a shore-based laboratory. The system has been 

 designed to operate in water 150 feet deep but could 

 be redesigned to operate in deeper water. '^ 



The cookie maker can increase the speed of ma- 

 rine surveys. Not only are samples quickly obtained 

 but preliminary analysis of the samples is available 

 while the survey is still underway. Availability of 

 real-time data that could be used for making ship- 

 board decisions could significandy improve the effi- 

 ciency of marine surveys. One current limitation 

 is that samples are only obtainable from the top 3 

 or 4 centimeters of sediment. Researchers believe 

 that some indication of underlying deposits may be 

 obtained by sampling the surfacial sediments, but 

 further tests are needed to determine if the tech- 

 nique also can be used for evcduating the composi- 

 tion of deeper sediments. 



'°J. Noakes, Center for Applied Isotope Studies, OTA VV'orkshop 

 on Site-Specific Technologies for Exploring the Exclusive Economic 

 Zone, Washington, DC, July 16, 1986. 



""Radiometric Techniques for Marine Mineral Surveys," World 

 Dredging and Marine Construction, Apr. 1, 1983, p. 208. 



'''Encyclopedia of Science and Technology, 5th ed., (New York, 

 NY: McGrjiw-Hill, 1982), p. 741. 



"J. Noakes, OTA Workshop on Site-Specific Technologies for Ex- 

 ploring the Exclusive Economic Zone, Washington, DC, July 16,1 986. 



