90 EXPLORATION GEOPHYSICS 



would be deep in the sphere and at practically the same distance from all 

 points on the surface. This bears on the geometry of the earth's magnetic 

 Held as will appear subsequently. 



The concept of a short bar magnet as the cause of the magnetic force in 

 a magnetized spherical earth is useful in another respect. Modern investi- 

 gations all pomt to the conclusion that earth magnetism is deep-seated, 

 which would be the case with virtually coincident poles as assumed above. 



From this generally accepted fact, it follows that the lines of force of 

 the field must pass through the outer crust of the earth to reach the sur- 

 face. The outer crust is made up of magnetically heterogeneous materials 

 which cause distortions in the earth's magnetic field. Measurements at the 

 surface have shown that extensive zones of rocks of different permeabil- 

 ities and great areal extent, perhaps better described as tectonic features 

 (mountain ranges, geologic basins, regional fault zones and the like), sig- 

 nificantly warp the force lines from an ideal symmetrical shape. Such 

 warping or distortion gives rise to magnetic anomalies* which will later be 

 considered in detail. 



In the foregoing nothing has been said as to the possible or probable 

 causes of earth magnetism. There are a number of theories and assump- 

 tions to account for it, although the true cause of the deep-seated magnetiz- 

 ation of the earth is not known. 



Position of the Magnetic Poles. — As is well known, the earth, like 

 any other spherical magnet, has magnetic poles. They are geometrically 

 the two points of intersection of the earth's magnetic axis with the surface. 

 Exploration has shown that they are regions of some miles in extent. They 

 are, moreover, unique localities, for at the north magnetic pole the only 

 compass direction is south and at the south magnetic pole the only 

 direction is magnetic north. The magnetic poles of the earth are best de- 

 fined as those places on the earth where the lines of force of the earth's 

 magnetic field are essentially vertical. 



Specifically the North magnetic pole was located in 1906 at latitude 71° 

 30' north and longitude 97°40' west, near the island of Boothia Phelix, in 

 Northern Canada. Shackelford's south polar expedition established the 

 South magnetic pole in 1909, at latitude 72°25' south and longitude 155°- 

 16' east, or on the edge of South Victoria Land. 



It is apparent that since the North and South magnetic poles are not 

 symmetrically disposed, as evidenced by the locations given, a line joining 

 them will not pass through the center of the earth, even allowing for the 

 fact that the poles are regions and not points. A line joining the magnetic 

 poles misses the earth's center by some 750 miles. This distortion of the 



* Defined as a difference from an average value, for example, an increase in mag- 

 netic intensity in an area, markedly greater than would be normal for the latitude of 

 the locality. Anomalies may be positive (increases), or negative (decreases), in rela- 

 tion to the average magnetic value. 



