Ch. 4— Technologies for Exploring the Exclusive Economic Zone * 141 



layers having different conductivities.'^ Recent ma- 

 rine DC resistivity experiments suggest that the DC 

 resistivity method may have appHcations for locat- 

 ing and delineating sulfide ore bodies. For exam- 

 ple, during one experiment at the East Pacific Rise 

 in 1984, substantial resistivity anomalies were de- 

 tected around known hydrothermal fields, and sea- 

 floor conductivities were observed that were twice 

 that of seawater.^' In this experiment the source 

 and receiver electrodes were towed from a research 

 submersible. Conversely, resistivity techniques 

 would not be expected to detect placer deposits, ex- 

 cept under the most unusual circumstances. This 

 is because seawater dominates the resistivity re- 

 sponse of marine sediments (as they are saturated 

 near the surface), and, in this case, only the rela- 

 tive compaction (porosity) of the sediments could 

 be measured. '^ 



Self Potential 



The self potential (or spontaneous polarization) 

 (SP) method is used to detect electrochemical ef- 

 fects caused by the presence of an ore body. The 

 origin of SP fields is uncertain, but it is believed 

 that they result from the electric currents that are 

 produced when a conducting body connects regions 

 of different electrochemical potential.'^ On land, 

 SP has been used primarily in the search for sul- 

 fide mineral deposits. It is a simple technique in 

 that it does not involve the application of external 

 electric fields. However, its use offshore has been 

 limited. Results of some experiments have been in- 

 conclusive, but the offshore extension of known land 

 sulfide deposits was successfully detected in a 1977 

 experiment.^" More recently, researchers at the 

 University of Washington have proposed building 

 a towed SP system for exploring the Juan de Fuca 

 Ridge. SP may prove effective for detecting the 

 presence of sulfide deposits; however, it is unlikely 

 to be of help in assessing the size of deposits. 



"M.B. Dobrin, Introduction to Geophysical Prospecting (New 

 York, NY: McGraw-Hill Book Co., 1976), p. 6. 



"T.J.G. Francis, "Resistivity Measurements of an Ocean Floor 

 Sulfide Mineral Deposit From the Submersible Cyana," Marine Geo- 

 physical Research 7, 1985, pp. 419-438. 



*°J. Wynn, U.S. Geological Survey, letter to W. Westermeyer, 

 OTA, May 1986. 



^'Chave, Coustable, and Edwards, "Electrical Exploration Meth- 

 ods for the Seafloor." 



"Ibid. 



Induced Polarization 



The induced polarization (IP) method has been 

 used for years to locate disseminated sulfide 

 minerals on land. Recent work by USGS to adapt 

 the technique for use as a reconnaissance tool to 

 search for offshore titanium placers (figure 4-11) 

 has produced some promising preliminary results. 

 The IP effect can be measured in several ways, but, 

 in all cases, two electrodes are used to introduce 

 current into the ground, setting up an electric po- 

 tential field. Two additional electrodes are used, 

 usually spaced some distance away, to detect the 

 IP effect. This effect is caused by ions under the 

 influence of the potential field moving from the sur- 

 rounding electrolyte (groundwater onshore, sea- 

 water in the seabed sediments) onto local mineral- 

 grain interfaces and being adsorbed there. When 

 the potential field is suddenly shut off, there is a 

 finite decay time when these ions bleed back into 

 the electrolyte, similar to a capacitor in an electric 

 circuit. 



If perfected for offshore use, the reconnaissance 

 mode of IP may enable investigators to determine 

 if polarizable minerals are present, although not 

 precisely what kind they are (although Umenite and 

 some base metal sulfides, especially pyrite and chal- 

 copyrite, have a significant IP effect, so do certain 

 clays and sometimes graphite). In the reconnais- 

 sance mode, the IP streamer can be towed from 

 a ship; as seawater is highly conductive, it is not 

 necessary to implant the IP electrodes on the sea- 

 floor. Consequently, it is "theoretically possible to 

 cover more terrain with IP measurements in a week 

 offshore than has been done onshore by geophysi- 

 cists worldwide in the last 30 years. "^' Best results 

 are produced when the electrodes are towed 1 to 

 2 meters off the bottom (although before IP explo- 

 ration becomes routine, a better cable depressor and 

 more abrasion-resistant cables will have to be de- 

 veloped). Electrodes spaced 10 meters apart enable 

 penetration of sediments to a depth of about 7 

 meters. The current USGS system is designed to 

 work in maximum water depths of 100 meters. 



"J.C. Wynn and A.E. Grosz, "Application of the Induced Polari- 

 zation Method to Offshore Placer Resource Exploration," Proceed- 

 ings, Offshore Technology Conference 86, May 5-8, 1986, Houston, 

 TX, OTC 5199, pp. 395-401. 



