Subsurface Methods as Applied in Geophysics 



1055 



for, where the serpentine is porous, they are generally found to contain oil. 

 As the plugs are relatively shallow and small in areal extent, magnetic 

 surveys should be of a rather detailed nature in order that anomalies will 

 not be passed over. Figure 553 shows the lines of equal magnetic 

 vertical intensity and a profile across the Yoast field. Applying the 



x,/^= 2800' 



Igneous Material 

 at approx. 2500' 



-200 



-•-200 



Profile Through X ~ Y 



Figure 554. Portion of magnetic survey over Beckham County fault in Wheeler 

 County, Texas. (Adapted from Stearn. Reproduced permission Am. Inst. Min. 

 Met. Eng.) 



depth rules to this anomaly one finds that Zj equals 1,650 feet and the 

 inflection point occurs at about 1,900 feet. Using the rules for a sphere, 

 one finds the center to be roughly at 3,300 feet below the surface, which 

 places the upper edge of the serpentine about 1,400 feet below the surface. 

 The serpentine in the Yoast plug enters along a fault zone and probably 

 represents a submarine extrusive at the end of Austin time. It lies approx- 

 imately 1,500 feet below the surface of the ground and is bounded on top 

 and sides by the Taylor formation, with the Austin chalk beneath.^^ 



'^ CoUingswood, D. M., Magnetics and Geology of Yoast Field, Bastrop County, Texas: Am. Assoc. 

 Petroleum Geologists Bull., vol. 14, no. 9, pp. 1191-1198, Sept. 1930. 



