62 EXPLORATION GEOPHYSICS 



Field of Application. — In oil prospecting, geomagnetic surveys are of 

 greatest value in reconnaissance and preliminary vi^ork. They can be con- 

 ducted rapidly w^ith a small personnel at a resultant low cost. Often, such 

 magnetic studies are useful in evaluating large areas, such as geologic 

 basins, to determine the best places for the application of other geophysical 

 methods of better resolving power. 



In petroleum exploration, magnetic work is of value as a detail tool, for 

 determining the location of buried hills or ridges if they are composed of 

 granites, gneisses, or schists, and in cases where there are buried intru- 

 sions of igneous or metamorphic rocks. Anticlines can be mapped if the 

 magnetic basement members are uplifted together with the sedimentary 

 beds or if some of the formations subject to folding are themselves 

 magnetic. Salt domes and salt core anticlines can also be outlined mag- 

 netically, although other geophysical methods are often more suitable for 

 finding such structures. 



On fault problems, which relate to both oil and mining exploration, 

 magnetic investigations are applicable where the basement rocks or mag- 

 netic members of the geologic column are displaced and in situations 

 where a fault fissure itself may contain igneous sheets or magnetic material. 

 Faulting and/or formation boundaries can be located by magnetic measure- 

 ments where adjacent formations have different magnetic characteristics. 



In the field of mining geophysics magnetic methods are used in pre- 

 liminary explorations of mining regions and groups of claims, and for 

 detailed studies of igneous intrusions, igneous dikes, and contact-meta- 

 morphic zones in relation to which various types of ore may occur. Iron 

 ore deposits of magnetite and, in some cases, hematite, and also numerous 

 forms of occurrences of sulphide ore bodies, where some mineral present 

 is appreciably magnetic, can be successfully mapped. 



In an indirect way magnetic work can be used to locate non-magnetic 

 minerals if they are associated or deposited with magnetic ones such as 

 magnetite, ilmenite or pyrrhotite. This applies to gold or platinum bearing 

 placer deposits where magnetite is present. 



Meteorites are often highly magnetic; they have been located and their 

 craters studied by magnetic means. On a similar basis the magnetometer 

 has been used to find buried guns, ammunition, pipe lines, etc., and, in one 

 case, metal objects of historic value at Valley Forge. 



Classification of Magnets. — In presenting the subject of magnetic 

 prospecting on a quantitative basis, it is necessary to consider certain funda- 

 mental concepts, classifications and definitions that come under the physical 

 phase of geomagnetics. 



A magnet is defined as a body or substance capable of attracting iron. 

 Gravitational attraction is not considered in this definition. Magnets are 

 classified into two groups ; natural and artificial. Natural magnets are min- 

 erals such as magnetite particularly (earliest known magnets) and a num- 



