6. Magnetic Techniques 



a. Description 



The structure and dynamic properties of the earth give 

 rise to a static magnetic field whose characteristics are essentially 

 fixed both geographically and temporally over reasonable time periods. 

 Any object with magnetic properties placed in this magnetic field will 

 influence its direction and strength in the immediate vicinity of the 

 object. Such changes are called magnetic anomalies. 



Well-developed techniques exist for detection of magnetic 

 anomalies with magnetometers. Magnetic anomaly detector (MAD) systems 

 have been built for such diverse purposes as detecting metallic ore 

 bodies or submarines, and they can be used from surface vessel or air- 

 craft platforms. Therefore, the theoretical possibility exists that 

 MAD techniques could be used to detect the magnetic anomalies produced 

 by steel-hulled fishing vessels. 



The magnetic anomaly created by a steel-hulled vessel 

 and its metallic contents (engine, propeller shaft, etc.) depends solely 

 on the magnetic characteristics of the vessel. No method of enhancing 

 this effect by a beacon or analogous device is known. 



b. Performance Potential 



MAD systems for the detection of submarines exist and 

 there is no reason why they would not be equally effective in detecting 

 fishing vessels. However, because of naturally occurring variations 

 in the earth's magnetic field, which can only be mapped through exten- 

 sive measurement programs, and perturbations in MAD outputs caused by 

 instability in the movement of the vehicle carrying the MAD equipment, 

 the technique is effective only at very short range, typically much less 



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