140 • Marine Minerals: Exploring Our New Ocean Frontier 



methods, direct current (DC) resistivity, self po- 

 tential, and induced polarization. 



Electromagnetic Methods 



Electromagnetic (EM) methods detect variations 

 in the conductive properties of rock. A current is 

 induced in the conducting earth using electric or 

 magnetic dipole sources. The electric or magnetic 

 signature of the current is detected and yields a 

 measure of the electrical conductivity of the under- 

 lying rock. The Horizontal Electric Dipole and the 

 Vertical Electric Dipole method are two controUed- 

 source EM systems that have been used in academic 

 studies of deep structure. Both systems are under- 

 going further development. Recent work suggests 

 that these techniques may enable researchers to de- 

 termine the thickness of hydrothermal sulfide de- 

 posits, of which little is currently known. Changes 

 in porosity with depth are also detectable. ^^ To date, 

 little work has been done regarding the potential 

 applicability of these techniques for identifying ma- 

 rine placers. 



Researchers at Scripps Institution of Oceanog- 

 raphy are currently developing the towed, fre- 

 quency domain Horizontal Electric Dipole method 

 for exploration of the upper 100 meters of the 

 seabed. A previous version of this system consists 

 of a towed silver/silver-chloride transmitting an- 

 tenna and a series of horizontal electric field 

 receivers placed on the seafloor at ranges of 1 to 

 70 kilometers from the transmitter. Since this ar- 

 rangement is not very practical for exploratory pur- 

 poses, the Scripps researchers are now developing 

 a system in which the transmitter and receiver can 

 be towed in tandem along the bottom. Since the 

 system must be towed on the seabed, an armored, 

 insulated cable is used. The need for contact with 

 the ocean floor limits the speed at which the sys- 

 tem can be towed to 1 to 2 knots and the type of 

 topography in which it can be used; hence, this 

 method, like other electrical techniques, would be 

 most efficiently employed after reconnaissance 

 methods have been used to locate areas of special 

 interest. 



The Vertical Electric Dipole method is being de- 

 veloped by reseachers at Canada's Pacific Geo- 

 science Center and the University of Toronto. The 

 Canadian system is known as MOSES, short for 

 magnetometric offshore electrical sounding. It con- 

 sists of a vertical electric dipole which extends from 

 the sea surface to the seafloor and a magnetome- 

 ter receiver which measures the azimuthal magnetic 

 field generated by the source.^* The receiver is fixed 

 to the seafloor and remains in place while a ship 

 moves the transmitter to different locations. A 

 MOSES survey was conducted in 1984 at two sites 

 in the sediment-filled Middle Valley along the 

 northern Juan de Fuca Ridge. Using MOSES, re- 

 searchers estimated sediment and underlying ba- 

 sadt resistivity, thickness, and porosity. 



Another electromagnetic method with some 

 promise is the Transient EM Method. Unlike con- 

 trolled source methods in which a sinusoidal sig- 

 nal is generated, a source transmitter is turned on 

 or off so that the response to this "transient" can 

 be studied. An advantage of the Transient EM 

 method is that the effects of shallow and deep struc- 

 ture tend to appear at discrete times, so it is possi- 

 ble to separate their effects. Also, the effects of to- 

 pography, which are difficult to interpret, can be 

 removed, allowing researchers to study the under- 

 lying structure. The Transient EM method also 

 may be particularly useful for locating sulfides, since 

 they have a high conductivity relative to surround- 

 ing rock and are located in ragged areas of the 

 seafloor. A prototype Transient EM system is cur- 

 rently being designed for survey purposes. It will 

 use a horizontal magnetic dipole source and receiver 

 and will be towed along the seafloor. ^^ 



Direct Current Resistivity 



Resistivity is a measure of the amount of cur- 

 rent that passes through a substance when a speci- 

 fied potential difference is applied. The direct cur- 

 rent resistivity method is one of the simplest 

 electrical techniques available and has been used 

 extensively on land to map boundaries between 



"P. A. Wolfgram, R.N. Edwards, L.K. Law, and M.N. Bone, 

 "Polymetallic Sulfide Exploration on the Deep Seafloor: The Mini- 

 Moses Experiment," Geophysics 51, 1986, pp. 1808-1818. 



"D.C. Nobes, L.K. Law, and R.N. Edwards, "The Determina- 

 tion of Resistivity and Porosity of the Sediment and Fractured Basalt 

 Layers Near the Juan de Fuca Ridge," Geophysical Journal ol" the 

 Royal Astronomical Society 86, 1986, pp. 289-318. 



^^Cheesman, Edwards, and Chave, "On the Theory of Seafloor 

 Conductivity Mapping." 



