364 



EXPLORATION GEOPHYSICS 



An anticline or buried ridge of limestone overlain by shale, with a 

 relief of about 375 feet, is illustrated in Figure 213. The top of the ridge 

 is at a depth of 1500 feet. The structure is not symmetrical, as the right 

 flank is steeper than the left. 



The gradient and curvature profiles shown are warped by the asymmetry 

 of the ridge. The zero point of the gradient and the maximum value of 

 the curvature do not lie immediately above the point of the ridge which 

 is structurally highest. They are shifted slightly to the left, down the 

 gentler slope of the structure. The amount of the shift of the gravity 

 gradient minimum is proportional to the difference in the slopes of the two 

 flanks. I 



CURVATURE 



/DENSITY ''2.70.N V - -'-" V<''iU''_; iTv -^ ''-^ - ' L'" ^ I-" iT^ - > ~ 



5av^:;;y 



1000 

 1500 

 2000 



Fig. 213. — Curvature and gradient profiles across a buried limestone ridge. The 

 example illustrates the shift in gradient minimum in relation to the high point of the 

 structure which is asymmetrical. (Afer Barton.) 



Faults. — An example of the efifect of the famous Luling fault is 

 shown by a survey made by the Rycade Oil Corporation.! Results are 

 given in Figure 214. The change in direction of the R-line value as the 

 fault is crossed is illustrated. 



Regional Effects. — Separation of the extended regional gravity 

 effects from those arising from smaller localized geologic structures is 

 shown in a torsion balance survey of the Fox Hills Oil Fields, of Okla- 

 homa.ft The gradients at the survey staions are given in Figure 215, 

 which also shows regional gravity contours (isogams) tied to a U.S. 

 Coast and Geodetic pendulum station. 



Indications of the structure at the Fox oil field are only indifferently 

 visible in this figure, but are suggested by the small size of the gradient 

 arrows and their tendency to show a local reversal about 1.5 miles south- 

 west of the town of Fox. The general NNE trend of the gradients and 

 the general increase in the value of gravity in that direction, as indicated 

 by the gravity contours, are the effects of the Arbuckle Mountains lying 



% D. C. Barton, A.I.M.E. Geophysical Prospecting, 1929, p. 439. 

 t D. C. Barton, loc. cit., p.. 451. 

 ft D. C. Barton, loc. cit., p. 456. 



