MAGNETIC METHODS 



223 



13^ miles northwest of the magnetic high, making it apparent that the mag- 

 netometer was not faithfully recording" the sedimentary structural high. 



Geophysical afterthought strongly suggests that the magnetic high is due 

 to induced polarity in a deep (basement) anticlinal ridge which lies below 

 the producing structure. The small structural relief (about 250 feet) and 

 the uplift of the shallower sedimentary beds probably do not enter into the 

 magnetic picture. It is of interest that the geophysical exploration described 

 led to the discovery of this important oil field. 



The magnetic survey of the Nocona field, in northern Montague County, 

 Texas, is another example of the application of the magnetometer in locat- 

 ing oil structure by mapping basement topography. The magnetic closure is 

 small, about 30 to 50 gammas, and the magnetic high is shifted with respect 

 to the structural high. The northern extension of the field was developed, 

 however, as a result of a magnetic survey. f 



LOS ANGELES BASIN PLENTE HILLS SAN CABWEL 



NCLtVVOCO NO««fcl.R WMTTltft VALLEY 



Fig. 107. — Illustration of regional survey in the Los Angeles Basin. (After Ulirig and Schafer, 

 Gerland's Beitrage zur Geophysik.) 



Regional Surveys. — Magnetic investigations of a deep-seated re- 

 gional structure usually necessitate a careful interpretation based on com- 

 parisons of the observed and calculated (theoretical) values of the vertical 

 and horizontal components along traverse lines. Figure 107 shows an in- 



t C. A. Ileiland, Geophysical Exploration, p. 429, Prentice-Hal), Inc. 



