CHAPTER III 



MAGNETIC METHODS 



Magnetic methods of prospecting are one of the oldest forms of appHed 

 geophysics. The mining compass, in 1640, was used to locate iron ore 

 bodies, according to a Swedish document. Prior to this, however, possibly 

 as early as 1000 B.C., two original observations had caused thoughtful men 

 to study magnetism. They were: (1) the tendency of a piece of the 

 natural magnetic mineral magnetite to orient itself in a particular direction 

 when freely suspended, and (2) the ability of a larger piece of magnetite 

 to pick up smaller pieces. 



Magnetic methods utilize a natural field of force, viz., the earth's mag- 

 netic field. They are applicable in regions where the magnetic properties 

 of rocks and formations have some known relationship to economic 

 geology. 



The magnetic fields associated with geologic bodies or geologic situa- 

 tions, which are investigated in magnetic prospecting, are superimposed 

 on the terrestrial magnetic field. For example, in the vicinity of a sizable 

 near-surface magnetic dike the local magnetic field would be increased 

 appreciably. A magnetic measurement at a station is, therefore, the sum- 

 mation or integral efifect of all the magnetic fields from magnetic bodies 

 within effective distance of the instrument superimposed upon the earth's 

 magnetic field. In a given case this might mean the magnetic contribution 

 of the dike mentioned above ; or perhaps the effect of a tobacco can in 

 the instrument man's pocket ; and the normal magnetic field of the earth 

 at the observation point. 



Since summation effects are always measured in magnetic prospecting, 

 these methods lack, in considerable degree, what is called depth control. It 

 is not possible, except in a rather limited number of cases, to determine 

 accurately or uniquely the depth to a magnetic body or the depth from 

 which a particular magnetic effect is coming. 



Interpretation of magnetic data is based on the fact that the earth's 

 normal magnetic field is uniform over areas of magnetically homogeneous 

 composition, but is measurably distorted in regions or local areas of 

 inhomogeneous composition. The amount and the shape of this distortion 

 depend on the relative magnetic susceptibilities of the subsurface materials, 

 and the relative sizes and configurations of the zones of such component 

 materials. Most magnetic anomalies (distortions from the theoretically 

 normal) are due to igneous rocks, magnetic iron ores, and to those sedi- 

 mentary deposits which contain magnetite, derived from igneous rocks. 



