8 



MAGNETIC METHOD 



I. INTRODUCTION 



The magnetic method of prospecting is one of tiie oldest geophysical 

 methods. It was applied to the location of ore bodies as early as 1640. 

 Like gravitational methods and in contrast to electrical and seismic 

 methods, it utilizes a natural field of force, consisting of the field of geologic 



(a) (b) 



Fig. 8-1. Vector diagram of the earth's magnetic field (a) for the northern hemi- 

 sphere, (6) for the southern hemisphere. 



bodies and the terrestrial magnetic field. Contrary to their gravitational 

 attraction (which exists independently of the earth's gravity field), the 

 magnetic reactions of geologic bodies frequently depend on direction and 

 magnitude of the earth's field. The law controlling magnetic attraction 

 (Coulomb's law) is identical in form with that governing gravitational 

 attraction. Hence, magnetic interpretation problems may often be 

 handled by a simple adaption of the relations which apply in gravita- 

 tional work. Because of the spontaneous nature of subsurface effects, it is 

 not possible to control the depth of penetration in magnetic prospecting. 



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