1056 



Subsurface Geologic Methods 



Magnetic Effects over Fault — Figure 554 is a portion of a magnetic 

 survey in the southeast corner of Wheeler County, Texas. A normal fault 

 with a throw of 300 to 500 feet parallels the strike of the buried Wichita 

 Mountains. Named from the adjacent Oklahoma county, the Beckham 

 County fault has been traced from well data for a number of miles, the 

 magnetic results indicating an extension of this fault to the west. By 

 applying depth rules for a fault we obtain a depth of 2,800 feet. The 

 sediments have been estimated to be 2,500 feet thick in this area, which is 

 in fairly good agreement with the magnetic data.^^ 



miles 



Xi/. = 2800' 



Ave. depth to top of salt 

 equals 285' 



Figure 555. Magnetic effects of Grand Saline salt dome in Van Zandt County, 

 Texas. (Adapted from Peters and Dugan. Reproduced permission Geophysics.) 



Grand Saline Salt Dome — A number of magnetic stations were ob- 

 served over the Grand Saline salt dome. Van Zandt County, Texas, by 

 the Magnolia Petroleum Company. (See fig. 555.) After substracting the 

 regional gradient, a weak negative magnetic anomaly of approximately 

 15 gammas existed over the salt dome. By applying the depth rules for 

 a sphere we find that the center lies at a depth of approximately 5,600 

 feet. From the inflection point in the curve the radius would appear 

 to be about 4,700 feet, which places the top edge of the salt mass about 

 900 feet below the surface. 



Susceptibility measurements were made on the salt as well as the 

 surrounding sediments. The average susceptibility of the salt was 

 —0.56X10'^ c.g.s., and the average susceptibility of the surrounding sedi- 

 ments from the Wilcox formation at the surface to the Travis Peak at 

 a depth of approximately 8,700 feet was 12X10'^ c.g.s. The susceptibility 



^ Steam, N. H., Geomagnetic Prospecting with the Hotchkiss Superdip: Am. Inst. Min. Met. Eng., 

 Geophysical Prospecting, p. 191, 1932. 



