Ch. 4— Technologies for Exploring tlie Exclusive Economic Zone • 139 



ity surveying, but the information they hold is pro- 

 prietary. Little surveying has been done in very 

 shallow waters (i.e., less than 10 meters), as the 

 larger survey ships cannot operate in these waters. 



The availability of high-density gravity data (and 

 possibly also magnetic data) for extensive areas of 

 the EEZ may pose a security problem similar to 

 that posed by high- resolution bathymetry. Gravita- 



tional variations affect inertial guidance systems and 

 flight trajectories. The Department of Defense has 

 concerns about proposals to undertake systematic 

 EEZ gravity surveys, particularly if done in con- 

 junction with the systematic collection of bathym- 

 etry data, since characteristic subsea features might 

 be used for positioning missile-bearing submarines 

 for strikes on the United States. 



SITE-SPECIFIC TECHNOLOGIES 



Site-specific exploration technologies generally 

 are those that obtain data from small areas rela- 

 tive to information provided by reconnaissance 

 techniques. Some of these technologies are deployed 

 from a stationary ship or other stationary platform 

 and are used to acquire detailed information at a 

 specific site. Often, in fact, such techniques as cor- 

 ing, drilling, and grab sampling are used to verify 

 data obtained from reconnaissance methods. Other 

 site-specific technologies are used aboard ships mov- 

 ing at slow speeds. Electrical and nuclear techniques 

 are in this category. 



Electrical Techniques 



Electrical prospecting methods have been used 

 extensively on land to search for metals and min- 

 erals, but their use offshore, particularly as applied 

 to the shallow targets of interest to marine miners, 

 is only just beginning. Recent experiments by re- 

 searchers in the United States and Canada suggest 

 that some electrical techniques used successfully on 

 land may be adaptable for use in marine mineral 

 exploration.^^ Like other indirect exploration tech- 

 niques, the results of electrical methods usually can 

 be interpreted in various ways, so the more inde- 

 pendent lines of evidence that can be marshaled in 

 making an interpretation, the better. 



The aim of electrical techniques is to deduce in- 

 formation about the nature of materials in the earth 

 based on electrical properties such as conductivity, 



"A.D. Chave, S.C. Coustable, and R.N. Edwards, "Electrical E.\- 

 ploration Methods for the Seafloor," in press, 1987. See also S. Chees- 

 man, R.N. Edwards, and A.D. Chave, "On the Theory of Seafloor 

 Conductivity Mapping Using Transient EM Systems," Geophysics, 

 February 1987; and J.C. Wynn, "Titanium Geophysics — A Marine 

 Application of Induced Polarization," unpublished draft, 1987. 



electrochemical activity, and the capacity of rock 

 to store an electric charge. Electrical techniques are 

 similar to gravity and magnetic techniques in that 

 they are used to detect anomalies — in this case, 

 anomalies in resistivity, conductivity, etc., which 

 allow inferences to be made about the nature of the 

 material being studied. 



The use of electrical methods in the ocean is very 

 different from their use on land. One reason is that 

 seawater is generally much more conductive than 

 the underlying rock, the opposite of the situation 

 on land where the underlying rock is more conduc- 

 tive than the atmosphere. Hence, working at sea 

 using a controlled-source electromagnetic method 

 is somewhat analogous to working on land and try- 

 ing to determine the electrical characteristics of the 

 atmosphere. In both cases, one would be looking 

 at the resistive medium in a conductive environ- 

 ment. The fact that seawater is more conductive 

 than rock appeared to preclude the use of electri- 

 cal techniques at sea. Improvements in instrumen- 

 tation and different approaches, however, have 

 overcome this difficulty to a degree. A difference 

 which benefits the use of electrical techniques at sea 

 is that the marine environment is considerably 

 quieter electrically than the terrestrial environment. 

 Thus, working in a low-noise environment, it is 

 possible to use much higher gain amplifiers, and 

 it is usually not necessary to provide the noise 

 shielding that would be needed on land. Also, coup- 

 ling to the seafloor environment for both source and 

 receiver electrodes is excellent. Thus, electrode 

 resistances on the seafloor are typically less than 

 1 ohm, whereas on land the resistance would be 

 on the order of 1,000 ohms. 



Electricad techniques that may be useful for ma- 

 rine mineral prospecting include electromagnetic 



