428 



MAGNETIC METHOD 



[Chap. 8 



and lateral injections into sediments in the form of laccoliths may produce 

 doming of strata, and their magnetic anomalies will give an indirect 

 indication of structure in the oil-producing section. Where basement 

 rocks are uniformly magnetized, mapping of topography has resulted in 

 the location of new oil fields and the extension of known ones. 



One of the best-known examples is the Hobbs field^ (Lea County, 

 New Mexico, see Fig. 8-73). It was located in 1926 by a midwest mag- 

 netometer party, surveyed bj'^ torsion balance, and drilled in 1927. Con- 



38 h/ 



Fig. 8-73. Magnetic contours (after Lahee), structural contours (after DeFord), 

 and torsion-balance data (after Coffin) for Hobbs field, discovered by a magnetic 

 survey. 



siderable development followed in 1929. The magnetic contours are 

 shifted in comparison with the structural contours at 4000 feet. Accord- 

 ing to Barret's interpretation, this is due to the normal induction in the 

 earth's magnetic field which places the maximum in Z south of the highest 

 point of the subsurface feature. On other structures such displacements 



"8 A.A.P.G. Bull., 16(1), 51-90 (Jan., 1932). 



