170 EXPLORATION GEOPHYSICS 



Induced and Residual Magnetization 



The magnetic character or magnetization of a rock may be considered 

 as made up of two components : namely, the induced and the residual mag- 

 netization. The term polarisation is sometimes used for magnetization. This 

 can be illustrated by an imaginary case. 



Given a magnetizable formation which has been subjected to the earth's 

 magnetic field and has acquired magnetization and polarity by induction. 

 The magnetization thus set up in the formation is the indiiced magnetization. 



Due to tectonic forces, a portion of this magnetized formation might be 

 overturned or even reversed end for end. In this new position, the earth's 

 magnetic field would be acting on the overturned part to induce magnetiza- 

 tion in it and even to reverse or reduce the direction and amount of the 

 polarization originally set up. In time a change will be brought about, and 

 a magnetization of dififerent amount and direction would be present in the 

 segment of the formation acted upon. 



In most rocks there exists, to a greater or less degree, what is termed 

 residual magnetization, or magnetization that resists removal or reversal. 

 Residual magnetization in a body is that portion of its magnetization which 

 is permanent or fixed. Such magnetization may be appreciable in magnitude 

 and may not be in the same direction as the induced magnetization. It may 

 even be in the opposite direction to the induced component. 



Residual magnetization in rocks may account for reversed polarities in 

 certain orebodies which have been displaced or overturned. In such rather 

 rare cases a magnetic low may be found above an outcropping edge in lati- 

 tudes where a magnetic high would be normal. 



Certain simple tests can be made with a specimen of rock and a mag- 

 netometer to find out if there is any residual or remanent magnetization 

 present, and the approximate proportions of induced and residual polariza- 

 tion. More elaborate tests are made by subjecting the specimen to a com- 

 plete hysteresis cycle and recording data with which to plot the hysteresis 

 curve for measuring induced and residual magnetization quantitatively. 



Magnetic Hysteresis 



The behavior of magnetic materials when subjected to cyclic changes 

 of magnetizing fields presents some interesting phenomena.f 



Consider a rod of magnetizable material in a demagnetized condition in 

 a primary solenoid coil. A short secondary coil, surrounding the primary 

 coil, is connected to a ballistic galvanometer. When a known current is 

 passed suddenly through the primary coil, the throw of the galvanometer 

 will be proportional to the flux density B in the specimen when the circuit 

 is closed. When the circuit is broken, the galvanometer will move in the 

 opposite direction. The magnitude of the galvanometer movement in the 

 "make", in relation to the opposite direction of movement in the "break", 

 permits the magnetic properties of the specimen to be measured. 



t T. A. Ewing, Magnetic Induction in Iron and Other Metals. 



