296 . MAGNETIC METHOD [Chap. 8 



Magnetic anomalies are frequently expressed in gammas (7); I7 = 1.10"^ 

 gauss. There is an immense variety of instruments and methods for the 

 measurement of magnetic field components, both angles and intensities. 

 The intensity measurements are of greater practical importance and are 

 readily made with the required accuracy. In magnetic prospecting this 

 is almost 1/10,000 part of the field while in gravity work it is 1/1,000,000 

 to 1/I0,000j000 part. Therefore, commercialh^ satisfactory magnetic 

 instruments may be designed v/ith comparative ease. 



Like many other geophysical methods, magnetic exploration had its 

 origin in methods of observation and research employed in earth physics. 

 While in terrestrial magnetic research the main objective of m.easurement 

 is the regional distribution of the earth's field and its time variation, mag- 

 netic exploration is concerned more with the determination of local varia- 

 tions and their geologic significance. The time variations are merely of 

 secondary importance, since magnetic field observations must be corrected 

 for them to attain sufficient accuracy. 



As in other geophysical methods, sufficient contrasts in the physical 

 properties of subsurface formations must exist to make the magnetic 

 method applicable. The properties here involved are the induced and 

 remanent magnetization. Geologic formations are either very strongly 

 or very weakly magnetic; this magnetic classification coincides almost 

 exactly with the divisions of igneous and sedimentary rocks. The m.ag- 

 netization of a rock is usually proportional to its magnetite content. Com- 

 pared with other physical properties, rock magnetization shows a greater 

 irregularity which is due not only to the bi-polar nature of magnetism but 

 to the frequently unpredictable effects of thermal and mechanical changes 

 in the course of their geologic history. 



Since most magnetic anomalies are due to igneous rocks, iron ores, and 

 sedimentary formations which have derived their magnetite content from 

 igneous rocks, magnetic methods can be applied directly to the location of 

 magnetic minerals and ores, and indirectly to the location of deposits which 

 are associated with magnetic rocks and formations. In oil exploration, 

 applications of magnetic methods in structural investigations include the 

 mapping of buried hills usually composed of igneous or metamorphic rocks ; 

 the location of anticlines where magnetic basement members are uplifted, 

 together with the sedimentaries, and where some of the sediments them- 

 selves are magnetic (or contain interbedded members of igneous origin); 

 lastly, the mapping of salt domes if they are surrounded by magnetic forma- 

 tions. The location of faults is important in both oil and mining applica- 

 tions and is possible if magnetic members are displaced, if the fault fissure 

 contains igneous sheets, or if the adjacent formations have different mag- 



