Chap. 8] 



MAGNETIC METHOD 



429 



of magnetic and structural contours have been found to occur (a) when 

 there is a shift of the structural axis; (6) when an igneous core acts as a 

 buttress against lateral forces which pushed the sedimentaries over the 

 igneous core; and (c) when the basement is not uniformly magnetized, for 

 instance, when a buried hill consists of both sedimentaries and intrusives. 

 Another instructive example of the successful application of the mag- 

 netometer in locating oil structure by mapping basement topography is 

 the survey of the Nocona field (Fig. 8-74).^^^ The magnetometer closure 



Fig. 8-74. Magnetic and structural contours of Nocona field, Oklahoma. The 

 northern extension of the field was developed as the result of the magnetometer 

 survey, (a) Magnetic contours, (b) structural contours. 



is small and the magnetic high is shifted with respect to the structural 

 high. In the Lucien field a small magnetic anomaly is caused by the 

 granite core of the Ordovician fold.*^° The buried Amarillo granite ridge 

 has been the object of considerable magnetic study. Profiles over the 

 ridge were made as early as 1925, and the results were published by 

 Adams^^ and the author. ^'^ The granite is about 3000 feet deep at the 



"s Heiland, Terr. Mag., 37(3), 343-348 (Sept., 1932). 

 IB" Jenny, Oil Weekly, 72(13), 16-18 (Mar. 12, 1934). 

 >9i Oil and Gas J., Feb. 16, 1928. 

 lo^ Colo. Sch. Mines Quart., 24(1), 64 (Mar., 1929). 



