318 



MAGNETIC METHOD 



[Chap. 8 



Disintegration of rocks is effective both chemically 

 and mechanically. Since the trivalent iron is more 

 paramagnetic than the bivalent iron, rock magnetiza- 

 tion is much reduced when magnetite disintegrates to 

 limonite or hematite. Conversely, in contact and dy- 

 namic-metamorphic processes, the iron in sedimen- 

 taries and other rocks is transformed from the bivalent 

 into the trivalent form, so that concentrations of 

 magnetic minerals are often found near intrusive 

 bodies. The mechanical effect of disintegration is (1) 

 to break up the magnetite particles and to produce 

 a more fine-grained material, and (2) to increase the 

 spacing of the particles and the path-reluctance. Both 

 result in a decrease of magnetization. 



Concentration of magnetic materials has the oppo- 

 site effect of disintegration. The relations discussed 

 above between susceptibility, grain size, chemical 

 changes, and thermal effects apply accordingly. Ex- 

 amples of an increase in magnetization b}^ concentra- 

 tion are contact-metamorphic zones and magnetite or 

 black sands in gold placers. 



Structural forces may change the position of magnetic bodies in the 

 course of their geologic history. When they have acquired remanent 

 magnetization, their overturning may produce apparent abnormal polariza- 

 tion. Similar effects may be expected from a change of position of solidified 

 and magnetized portions of magma and lava flows. 



Fig. 8-16. Mag- 

 neto-mechanical 

 hysteresis loop for 

 nickel (after Stcin- 

 haus). 



III. MAGNETIC INSTRUMENTS 



A. Construction Principles 



Instruments for the measurement of magnetic anomalies caused by iron 

 ores were developed at an early date. In the nineteenth century a number 

 of scientific institutions became engaged in the determination of the ele- 

 ments of the earth's magnetic field all over the globe. For this purpose 

 numerous types of instruments have been developed for land, oceanic, and 

 aerial observations. When, fifteen years ago, magnetic prospecting started 

 on its rapid development, extensive design experience was thus available. 

 Although the Schmidt-type magnetometers are in most extensive applica- 

 tion, a number of the other designs are employed for the same or different 

 purposes. While preference is given here to the Schmidt magnetometers, 

 other types of magnetic instruments are also discussed in view of their 

 applications in mining and engineering. 



