218 



EXPLORATION GEOPHYSICS 



lines of force due to the first 15,000 feet of the subsurface. These studies 

 point out the areas of interest for magnetic surveys, the size of anomalies 

 to be expected, and the area to be covered. 



Fig. 104. — Showing the magnetic lines of force and the dis- 

 turbance vectors based on them for a dipping ore body. The earth's 

 magnetic field is not considered in this figure. 



Magnetic Anomalies Associated with the Larger Geological Features 



The vertical magnetometer has been vi^idely used in the United States 

 in the search for oil structures. Many anticlinal structures are situated 

 over ridges of buried granite or metamorphic rock; over these structures 

 the metamorphic rocks are nearer to the surface. Such granite and meta- 

 morphic basement rocks are usually (though not always) more magnetic 

 than overlying sediments and cause an increase in magnetic intensity. 



J. H. Wilsonf writes as follows : "In interpreting the results of a magnetic survey- 

 in terms of structure, one would want to know that the basement rocks were uniform 

 in character and that the possible producing horizons were conformable to the surface 

 of the basement rocks. If this can be assured or assumed, then the magnetometer can 

 be expected to give a faithful representation of structural conditions in the producing 

 horizons. 



"The more unconformities between the basement rocks and the producing horizons, 

 then the less likely are the magnetic results to correspond to the structure of the pro- 

 ducing horizons. In practice, one usually makes a survey over a known structure or oil 

 field and compares the results obtained to those obtained in an adjacent area of un- 

 known structure. 



"In the absence of information as to character of the basement rocks or results 

 over known structures, one may apply the following criteria to investigate the possible 

 relation of an anomaly to structure : 



1. Do the magnetic results indicate a structure of the general type that you would 

 expect from geological considerations of the area? 



2. Is the elongation of the magnetic anomaly in the direction of the dominant 

 structural trend of the area ? 



3. Is the regularity of the isogams (lines of equal intensity) such as to suggest 

 that the forces are derived from considerable depth ?" 



With a clue to the probable shape of the structure, and an idea of the 

 magnetic properties of the rocks, it is possible to compute the theoretical 



-t J. H. Wilson, Mines Magazine, November, 1928. 



