216 



EXPLORATION GEOPHYSICS 



The procedure is useful in approximating the pole depth in cases in 

 which the geological and geophysical situation permits a pole assumption. 

 The pole is, of course, some distance below the top of such an ore body. 

 In a bar magnet, as has been shown, the pole is 1/12 of the magnet length 

 from the end. How far the pole would be below the top of an ore body 

 depends upon many variables, only a few of which can be evaluated. 



FLAT-LYING CRETACEOUS 

 AND TERTIARY FORMATIONS 



FOLDED PALEOZOIC FORMATION 



Vz 



SCALE IN MILES 



/■PERIDOTITE ,■ ' PLUG ' .\ 



SURFACE OF PERIDOTITE 



ACCORDING TO WELL DATA 



Fig. 103. — Magnetic depth-finding profile and section across a peridotite plug nearRison, 

 Arkansas. I, recorded magnitude and direction of magnetic intensity; I„, magnitude and direction 

 of normal component of magnetic intensity; I,, magnitude and direction of anomalous component 

 of magnetic intensity. (After Stearn, A.I.M.E. Geophysical Volume, 1932, p. 197.) 



To apply the 1/2 and 1/3 maximum rules previously set forth, tests 

 on some vertical intensity profiles were made by Dart Wantland over 

 magnetite concentrations at the A. E. Humphrey placer at Roscoe, Colo- 

 rado. It was found that they apparently did not apply. In this case the 

 depth to bed rock was known. It is inferred from this that the rich magne- 

 tite concentrations of this placer. did not act geophysically like magnetic 

 poles. 



Three Factors Affecting the Symmetry of Anomaly Curves. — Three 

 factors which can be evaluated qualitatively affect the shape of anomaly 

 curves of the vertical and horizontal magnetic intensity, as shown by 



