MAGNETIC METHODS 217 



traverses across dike-type magnetic bodies. These factors are : (a) the angle 

 of incHnation of the earth's magnetic field; (b) the dip of the magnetic 

 body; and (c) the strike of the magnetic body. 



The effect of changes in the inclination of the earth's field are shown 

 by three curves after Haalck as given in Figure 97. They picture the cases 

 for three different latitudes. If the spheres shovv^n can be considered as 

 roughly representing the magnetic cores of anticlines, one explanation of 

 the shift of the position on the ground between a magnetic high and a 

 structural high is apparent. Such a shift is often encountered in field explor- 

 ation. An example of this is found in the Hobbs oil field. (See Figure 104.) 



The general effect of dip on the symmetry of magnetic anomaly curves 

 has been considered. In addition it is significant that changes in the dip, 

 or in the strike, of a magnetic tabular body may cause a considerable shift 

 of the point of maximum vertical intensity in relation to the point on the 

 surface directly above the magnetic pole. The magnetic maximum may be 

 moved either north or south of the surface point above the pole, depending 

 on conditions. 



Variation in the dip and/or the strike may also bring about differences 

 in the amount of the maximum or the minimum value of horizontal mag- 

 netic intensity. In the interpretation of magnetic surveys on ore bodies and 

 dikes for the location of test drill holes, to establish the position of such 

 bodies, these factors must be weighed. 



The type case for a fault where rocks of high magnetic susceptibility 

 are brought closer to the surface on the upthrow side is given in Figure 

 86. The effect of the fault causing an increase in vertical magnetic force 

 where more magnetic material is near the surface is the same as the effect 

 of two formations which are in contact, one being of higher susceptibility. 

 The latter case shows an increase in magnetic intensity for those stations 

 above the high-susceptibility material. 



Where the formation boundary or the fault is covered with overburden, 

 it is not possible by study of the anomaly curve alone to distinguish between 

 eft'ects caused by movement along a fault plane and those caused by con- 

 tact of two different materials. Secondary magnetite developed in and 

 along a fault zone often will produce an appreciable anomaly. However, in 

 a case of this kind the magnetite would give an anomaly curve of the same 

 general kind as a magnetic dike, or a maximum in vertical intensity in the 

 vicinity of the fault, rather than the smooth rise to higher values from 

 left to right shown in Figure 87. These considerations again bring out 

 the necessity of geologic thinking in interpretation. 



That disturbance vectors are tangents to magnetic lines of force is 

 illustrated by Figure 104 showing these two cpantities. 



Magnetic vector studiesf have been made of the oil-producing states. 

 The vectors indicate the intensity and the direction in space of the magnetic 



t W. p. Jenny, "Magnetic Vector Study of Regional and Local Geologic Structure in Principal 

 Oil States," Geophysics, Vol. 3, 1932. 



