GRAVITATIONAL METHODS 367 



lying S.W. of Fox, and a linear adjustment of values on closed traverses 

 to the N. and W. and S.W. of Fox. The survey was tied into the Busby 

 pendulum station of the U.S. Coast and Geodetic Survey (No, 305), 

 which made it possible to give absolute gravity values to these isogams. 



The study gave a value for the regional gradient to be applied to the 

 results pictured in Figure 215. With this regional correction applied, the 

 anomalous isogams are shown in Figure 216, which represents the more 

 local structure of the Fox oil field. The anomalous isogams are super- 

 imposed on the generalized structure contour map of the area. (Amerada 

 Petroleum Corporation ) . 



If this had been a wildcat prospect, according to Barton, and acreage 

 had been taken on the basis of the latter map from torsion balance results, 

 a considerable part of the acreage would have been within the area proven 

 to be productive. The slight shift of the anomalous isogams to the 

 southwest from the subsurface structure is probably due to an error in 

 the estimation of the magnitude of the regional gradient due to the Arbuckle 

 mass. The type of analysis made at Fox was warranted by the very 

 complete net of stations. Although here the torsion balance demonstrated 

 ability to map structure showing rather faint gravity effects, such weak 

 indications of structure should not be relied upon in reconnaissance work. 



EXPLORATION WITH THE GRAVIMETER 



The history of gravity measurements, like that of other exploration 

 techniques, is characterized by cycles. The first stage in such a cycle is 

 the development of the instrument or method. This is followed by the 

 evaluation of its usefulness, its application to hitherto unsolved problems, 

 its peak period of activity, and, eventually, the exhaustion of its possi- 

 bilities. A resume of the early history of gravity measurements is given in 

 Chapter L 



The torsion balance was the first gravity instrument to be applied exten- 

 sively in exploration for structures containing oil (1917-1937). Successful 

 as this method proved to be, there were many advantages to be gained 

 if relative gravity measurements could be made more quickly and with 

 high accuracy. An instrument was needed which would obviate the neces- 

 sity of laboriously converting gradient values, with their more complex 

 interpretation, to the equivalent direct gravity force values. 



About 1935 a number of direct gravity measuring instruments were 

 developed having the necessary accuracy and speed of operation. These 

 developments initiated a wave of gravimeter exploration which has con- 

 tinued to the present time (1949) and is just beginning to show signs 

 of decrease. Thus the torsion balance was replaced by a new geophysical 

 tool, which, if history repeats, will some day itself be superseded. 



The next phase in gravity measurement work is not currently apparent. 

 It is possible that, like the magnetometer, the gravimeter will become air- 



