1052 



Subsurface Geologic Methods 



netic anomalies due to structure. To the experienced geophysical inter- 

 preter this presents no problem, for he has learned to recognize the 

 small variations in regional gradient which indicate local anomalous 

 conditions. Persons not familiar with magnetic or gravity maps are 

 aided by the removal of the regional gradient. A number of methods for 

 eliminating extraneous effects are in common usage, the aim of all being 

 to leave only the anomaly due to local structure.^^ The term "residual" is 

 frequently applied to maps from which the regional has been removed. 



Henderson and Zietz ^^ have demonstrated that a definite relationship 

 may exist between residual maps and "second vertical derivative" maps. 



Figure 552. Shift of axis of magnetic anomaly from 

 center of structural disturbance. 



" Nettleton, L. L., Geophysical Prospecting for Oil: pp. 126, 222, New York, McGraw-Hill Book Co., 

 Inc., 1940. 



^' Henderson, Roland G. and Zietz, Isidore, The Computation of Second Derivatives of Geomagnetic 

 Fields: Geophysics, vol. H, no. 4, p. 516, Oct. 1949. 



