GRAVITATIONAL METHODS 273 



The principle of the practical method advanced by Kater for deter- 

 mining the distance / between a point of suspension 6" and the correspond- 

 ing center of oscillation S' will be evident from Figure 133b. The proce- 

 dure consists simply in adjusting the two weights A and B until the period 

 of vibration is the same when the pendulum is caused to vibrate first 

 about 5 and then about S\ (An alternative design applicable in certain 

 cases employs only one movable weight mounted between 5" and S'.) 

 When the periods about 5" and S' have been made equal, the distance be- 

 tween the two parallel axes of suspension through 5" and S' is equal to 

 the length of the equivalent simple pendulum. This distance can be meas- 

 ured with considerable accuracy and from it and the measured period of 

 the pendulum, the value of g can be computed. Thus, the Kater pendulum 

 can be used for absolute determinations of g in terms of measurable dis- 

 tances and times. 



The absolute value of the force of gravity may be determined at a 

 permanent base station by observing the period of oscillation of a pendulum 

 under carefully controlled conditions to insure the greatest possible accur- 

 acy. A concrete pier in the pendulum room of the U.S. Coast and Geodetic 

 Survey at Washington, D.C., is the official gravity base station for the 

 United States. The value of gravity at this base station is 980.112 dynes. 

 Absolute determinations, because of the elaborate precautions which must 

 be taken to insure accuracy, require considerable time and patience. 



Limitations of Commercial Pendulums 



About two hours' time is required for making a station reading with 

 the pendulum. This time requirement is greater than that usually neces- 

 sary to carry out a gravimeter observation. Furthermore, according to 

 Bartonf the best commercially purchasable pendulums have a probable 

 error of ± 0.5 milligals. It is this large probable error which limits the 

 use of pendulums in geophysical prospecting to surveys of regional fea- 

 tures or abnormally large anomalies. The gravitational anomalies char- 

 acteristic of structures associated with oil deposits may be less than 1.5 

 milligals. Hence, measurements of the relative value of g in oil explora- 

 tion work require a more sensitive type of instrument than the pendulum. 

 The more sensitive instruments commonly employed to determine the rela- 

 tive value of g are: (a) gravity meters, or gravimeters, for direct deter- 

 mination of the relative value of g and (b) the torsion balance and, to a 

 smaller extent, the gradiometer for indirect determinations. f 



Accuracy of Measurements 



The accuracy of absolute gravity measurements with the reversible 

 pendulum depends chiefly upon three factors: (1) measurement of length, 

 (2) accuracy with which the center of gravity is placed on the line joining 



t D. C. Barton, "Gravitational Methods of Prospecting," Science of Petroleum, Vol. I, Oxford 

 Univ. Press, 1938. 



t M. K. Hubbert and Frank A. Melton, "Gravity Anomalies and Petroleum Exploration by the 

 Gravitational Pendulum," A.A.P.G., Vol. 13, No. 9, 1928, pp. 889-898. 



