GRAVITATIONAL METHODS 



401 



arbitrary gravity value, preferably chosen great enough so that none of 

 the station values are negative, to avoid the inconvenience of carrying nega- 

 tive numbers. In all gravimeter work the absolute value of the ac- 

 celeration of gravity is not obtained, because the instrument is capable only 

 of giving the difference in gravity between two stations. Usually, however, 

 this is not a disadvantage because interpretation is based upon the varia- 

 tions or anomalies in gravity exactly as is done in the interpretation of 

 magnetic anomalies. If absolute gravity values are desired, the gravimeter 

 survey must be supplemented v/ith a pendulum gravity measurement at one 

 or more of the base stations. 



Fig. 243. — Drift curves for duplicate reading at each station, showing 

 readings taken at 13 stations. (Bryan, Geophysics.) 



An alternative procedure, which is not as rapid as that outlined above, 

 consists in occupying each station at least twice during the day. When this 

 procedure is used, the drift curve plot is similar to that shown in Figure 

 243.t For the case shown, the drift lines are practically horizontal, in- 

 dicating that on that particular day the drift was very small and gratifyingly 

 uniform. The gravity anomalies or differences between the various stations 

 are obtained, as before, from the vertical interval on the plot. 



After the gravity measurements at each station have been obtained, 

 they should be subjected to two corrections : (a) elevation and (b) latitude 



When corrected and reduced thereby to a suitable datum, the gravity 

 values are placed on a final map and usually contoured at a contour interval 

 best suited to show the variations. Following the above steps interpreta- 

 tion can proceed. 



t A. B. Bryan, "Gravimeter Design and Operation," Geophysics, October, 1934, Vol. 2, No. 4, 

 pp. 301-308. 



