600 Subsurface Geologic Methods 



duced that this directional ferromagnetism may be altered or destroyed. 

 Residual or permanent magnetism should not be confused with in- 

 duced magnetism. The relationship is evident in the formula expressing 

 the total intensity of magnetization (polarization) of a rock: 



I=kH+Ip 



where / = total polarization 



k = susceptibility factor 



H = earth's magnetic field strength (horizontal component) 

 Ip = permanent polarization of a rock 



The few measurements that have been made on sedimentary rocks 

 show rather inconclusively that the permanent polarization Ip is on 

 the average but a fraction (one-fifth to four-fifths) of the magnitude of 

 the induced polarization kH. However, in some basic igneous rocks Ip 

 has been found to be several to many times as great as kH. The direction 

 of Ip is fixed at the time the rock forms and is rigidly held in that position. 

 The direction of the vector kH always agrees with that of the earth's 

 magnetic field, whereas Ip, although agreeing initially, may be and gen- 

 erally is moved out of coincidence by geologic processes. Moreover kH 

 is dependent on H, whereas Ip is entirely independent. Because kH is 

 dependent it completely vanishes when effectively shielded and symmetric- 

 ally shaped (cylinder) , where there is no axis of susceptibility, so that Ip 

 can then be accurately determined. This is the basis of magnetic core ori- 

 entation. 



Variable Geologic Factors 



The more important geologic variables that must be considered in 

 magnetic core orientation are listed and discussed below: 



1. Coarseness of sediment. 



2. Secular variation of the earth's magnetic poles. 



3. Secondary residual magnetism. 



4. Distribution of residual magnetic material in a core. 



5. Rock movements, 

 (a) Tilting. 



(6) Stress and strain, 

 (c) Temperature. 



6. Magnetic field of core barrel and drill pipe. 



7. Cores from deflected holes. 



Coarseness of Sediment 



It is an elementary geologic principle that coarse sediments are 

 normally associated with strong currents, finer sediments with weaker cur- 

 rents. We may also state that all clastic particles possessing residual 

 magnetism have a measurable magnetic force that strives to align the 



