Ch. 4— Technologies for Exploring the Exclusive Economic Zone • 137 



ophysicists delineate geologic structures associated 

 with petroleum and measure the thickness of sedi- 

 ments above magnetic basement rocks.** 



Surveys also may be conducted to locate concen- 

 trations of ferromagnetic minerals on or beneath 

 the seafloor. The detection of magnetite may be 

 particularly important in mineral prospecting be- 

 cause it is often found in association with ilmenite 

 and other heavy minerals. Ilmenite also contains 

 iron, but it is much less strongly magnetized than 

 the magnetite with which it is associated (it also may 

 have weathered during low stands of sea level and 

 may have lost magnetic susceptibility). 



The precise location of a mineral deposit or other 

 object may require a more detailed survey than is 

 possible by satellite or airplane. Use of ship-towed 

 magnetometers has met with varying measures of 

 success in identifying placer deposits. Improve- 

 ments in sensitivity are needed. If enough data are 

 gathered to determine the shape and amplitude of 

 a local anomaly, the size of an iron-bearing body 

 and its trend can be estimated, a common prac- 

 tice on land. When magnetic information can be 

 correlated with other types of information (e.g., 

 bathymetric, seismic, and gravity) interpretation 

 is enhanced. 



Magnetic anomalies also can be used to locate 

 and study zones of alteration of the oceanic crust. 

 The initial magnetization of the oceanic crust is ac- 

 quired as it cools from a magma to solid rock. For 

 the next 5 to 10 million years, hydrothermal cir- 

 culation promotes the alteration of this igneous rock 

 and the generation of new secondary minerals. Ini- 

 tially, the heat of hydrothermal circulation destroys 

 the thermal remanent magnetization. Rona sug- 

 gests that this reduction in magnetization will pro- 

 duce a magnetic anomaly and signal the proximity 

 of active or inactive smokers or hydrothermal 

 vents. *^ The Deep Sea Drilling Project and Ocean 

 Drilling Program drilling results suggest that as the 

 secondary minerals grow, they acquire the mag- 

 netization of the ambient magnetic field. This ag- 

 gregate magnetization produces a signature which 



is detectable on a regional scale and might be used 

 to determine the degree and rate of regional alter- 

 ation.*' 



Variations in the intensity of magnetization (total 

 field variations) are detected using a magnetome- 

 ter. Magnetometers deployed from ships or air- 

 planes are either towed behind or mounted at an 

 extreme point to minimize the effect of the vessel's 

 magnetic field. Among the several types of mag- 

 netometers, proton precession and flux-gate types 

 are most often used. These magnetometers are rela- 

 tively simple to operate, have no moving parts, and 

 provide relatively high-resolution measurements in 

 the field. The technology for sensing magnetic 

 anomalies is considered mature. A new helium- 

 pumped magnetometer with significandy improved 

 sensitivity has been developed by Texas Instru- 

 ments and is being adapted to oceanographic work. 



Most magnetic measurements are total field 

 measurements. A modification of this technique is 

 to use a second sensor to measure the difference 

 in the total field between two points rather than the 

 total field at any given point. Use of this gradiom- 

 etry technique helps eliminate some of the exter- 

 nal noise associated with platform motion or ex- 

 ternal field variation (e.g., the daily variation in 

 the magnetic field). This is possible because sen- 

 sors (if in close enough proximity) measure the 

 same errors in the total field, which are then elim- 

 inated in determining the total field difference be- 

 tween the two points. Gradiometry improves sen- 

 sitivity to closer magnetic sources.*^ 



The most important problem in acquiring high- 

 quality data at sea is not technology but accurate 

 navigation. The Global Positioning System, when 

 available, is considered more than adequate for 

 navigation and positioning needs. Future data, to 

 be most useful for mineral exploration purposes, 

 will necessarily need to be collected as densely as 

 possible. It is also important that magnetic (and 

 gravity) data be recorded in a manner that mini- 

 mizes the effects of external sources, such as of the 

 towing platform, and that whatever data are meas- 



'^P.A. Rona, "Exploration Methods for the Continental Shelf: Ge- 

 ology, Geophysics, Geochemistry," NOAA Technical Report, ERL 

 238-AOML 8 (Boulder, CO: NOAA, 1972), p. 22. 



''Rona, "Exploration for Hydrothermal Mineral Deposits at 

 Seafloor Spreading Centers," p. 25. 



*'J.L. LaBrecque, Lamont-Doherty Geological Observatory, OTA, 

 May 1, 1987. 



*^J. Brozena, Naval Research Laboratory, and J. LaBreque, 

 Lamont-Doherty Geological Observatory, OTA Workshop on Tech- 

 nologies for Surveying and Exploring the Exclusive Economic Zone, 

 Washington, DC, June 10, 1986. 



