74 EXPLORATION GEOPHYSICS 



For the 2nd case : „ _ w^ • Wg 1-1 1 _ n ot tr n oc 

 (r = 2cm.) ^- ^ -^—-—-0.25-// -0.25 



These elementary mathematical relationships establish a basis for phases 

 of the subjects that follow. 



Magnetic lines of force in the vicinity of magnetic poles and around a 

 bar magnet, as well as the direction in which a free (north) test pole would 

 move, are summarized in Figure 11. 



Magnetic Properties of Materials 



Any magnetic substance, when placed in a magnetic field, will become 

 magnetized or have magnetism induced in it. A bar magnet picking up iron 

 filings or causing them to line up under the influence of its magnetic field 

 illustrates this phenomenon. The iron filings become, by induction, little 

 (temporary) magnets. Since the earth acts as a spherical magnet, a string 

 of steel oil-well casing which has remained in the earth for some time 

 will become magnetized. The top of the casing will acquire a south magnetic 

 polarity, as evidenced by its ability to attract the north-seeking end of a 

 compass. 



Ferromagnetic, or paramagnetic, mineral deposits may also acquire mag- 

 netic polarity by induction in the magnetic field of the earth. Rocks mag- 

 netized in this manner can retain their magnetization for a long time even 

 if displaced or overturned. 



Magnetic Flux. — As noted above, when a magnetic body is put in a 

 magnetic field, lines of force are induced in it. This condition is also 

 described by saying that a magnetic flux is established in the body, or 

 magnetic lines force their way into it. Flux generally is represented by 

 the Greek letter </>. 



Quantitatively, the amount of flux (or force lines) in a magnetizable 

 body when it is in a magnetic field, divided by the end area A of the body, 

 defines its flux density B. In equation form : 



The end section of the body is taken at right angles to the flux lines. 



Those substances in which flux is easy to establish acquire a high flux 

 density for a given magnetizing field. Annealed iron is an example. 



Permeability (relates to flux <}>). — The magnetic permeability (fx.) of 

 a substance has been defined by Lord Kelvin as the ease with which a mag- 

 netic flux can be established in that material. Specifically, for a body 

 placed within a uniform magnetic field, its permeability is defined as the 

 ratio of the number of lines of force passing through a unit cross-section 

 of the body to the number of force lines through a like cross-section in air. 



