212 



EXPLORATION GEOPHYSICS 



Although not suggested by Haalck, a Class 5 for anomalies of about 100 gammas 

 or less, some of which arise from serpentine plugs such as occur in Bastrop County, 

 Texas, might be set up. This could also include the small negative anomalies (50 

 gammas or less) due to salt domes. 



Magnetic Bodies That Can Be Treated As Bar Magnets Or As 

 Magnetic Poles. — In certain cases the cause of a magnetic anomaly can, by 

 implication, be treated as if it were a single magnetic pole. This assumption 

 is sufficiently valid to be of use in interpretation for certain magnetic 

 igneous dikes and tabular magnetic ore bodies. 



Pole Depth Calculation. — Where the above assumptions are justified, 

 the depth to the negative pole, which under normal conditions in the north- 

 ern hemisphere will be nearest the surface, may be made as follows. The 

 positive, or lower, pole is assumed to be at infinity (see Figure 100). 



AZ max. 



A Z =: 1/2 max. 



+ oo 



Fig. 100. — Vertical intensity anomaly curve, for 

 calculation of pole depth. 



The magnetic force at point A, or AZ4 = m/cP ; (pole strength m divided by the 

 distance squared.) 



The magnetic force at B, designated Fb, is by the same formula total force acting 

 in the direction BP divided by the square of the distance involved. This total force at 

 B is designated as ATb. 



FB = mfiBPyz= 

 since cos x rr dl BP, BP = dicos x. 



\ cos X I 



= ^ cos*;r = Ari, 



2 rf= 



(101) 



