6 Magnetometer 



Introduction 



To detect the presence of ferrous dipole targets of finite length, a marine 

 cesium vapor magnetic gradiometer was developed, deployed, and tested. The 

 instrument noise level was about 0.015 nanoTeslas/meter (nT/m) or 5 times less 

 than the magnetic gradients generated by relatively quiet coastal waves. 

 Numerous clusters were identified which contained responses typical of the 

 anticipated ordnance items. The magnetic gradiometer demonstrated a high 

 degree of ferrous object sensitivity, thus providing a large detection range and 

 target location capability. Potentially the gradient data can be used for basic 

 classification and discrimination of ordnance size. Underwater magnetic 

 investigations to detect ordnance were conducted in the Sandy Hook area at a 

 constructed test site, an ordnance disposal site, and at the Sea Bright designated 

 borrow area. 



Theoretical Background 



The principle of magnetic detection and location of ordnance originates from 

 the localized magnetic field variations that these objects produce. These 

 deviations from normal magnetic field conditions are the result of specific 

 characteristics of the ferrous material (iron and steel) contained in the 

 manufactured ordnance. Two physical features are present in ferrous material 

 which, in turn, cause a change in the local magnetic field. These properties are as 

 follows: 



a. Induced magnetism. This is the phenomenon that makes most ferrous 

 metal ordnance detection and classification possible with magnetic surveys. 

 The Earth's magnetic field establishes a secondary magnetic field in the 

 ordnance item. This disturbance is measurable when a sensor is within the 

 area of the ordnance's magnetic signature. The intensity and range of the 

 local magnetic field alteration is based on the magnetic susceptibility of the 

 iron or steel and the size and shape of the shell. If this value is known, the 

 mass (weight) of the ordnance can be estimated and the caliber roughly 

 approximated. 



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Chapter 6 Magnetometer 



