MAGNETIC METHODS 



225 



at their gradational contacts. The effect of the high permeability of the 

 granite basement rocks is shown by the gradual rise in the magnetic trend 

 starting at about station 118 and extending to the left of the figure. 



The profile also shows the magnetic high produced by an intrusive 

 serpentine dike (at the left of the traverse). (Direct location of the dike 



Fig. 108. — Magnetic profile over contacts of lime schist and serpentines. 



could not be made by surface geology because of the covering of 50 feet 

 of soil and alluvium fill.) 



Magnetic Anomalies Over Steeply Dipping Structures. — In regions 

 containing steeply dipping structures, the crest of the deeper sedimentary 

 structure is usually over the crest of the uplift in the basement rocks. The 

 magnetic anomaly is caused by the 

 high magnetic permeability of the 

 igneous basement rock. The overly- 

 ing sediments usually have a rela- 

 tively minor effect on the anomaly. 



The peak of the magnetic ver- 

 tical intensity often is shifted later- 

 ally due to the inclination of the 

 earth's field. The amount of shift 

 depends upon many factors, includ- 

 ing depth to the basement rocks, 

 relative permeabilities, inclination of 

 the earth's field, etc. This effect is 

 illustrated schematically in Figure 

 109. 



In other cases, the magnetic and 

 the subsurface structural features 



may be displaced when: (1) the Fig. 109.— Diagram illustrating displacement 



iVnpnilS crwe iq nnt in rnnfn1-mif^r of the peak of the magnetic anomaly with refer- 

 IgneOUb core is noi m COniOimity ence to the crest of the structure. 



