374 EXPLORATION GEOPHYSICS 



carry a small rider of known weight which is used in calibrating the 

 meter. A clamp screw G locks the system. The temperature of the case 

 in which the meter is mounted is controlled accurately. The behavior of 

 the commercial type meter is indicated by Table 11, which is a summary 

 of data for five closed loops. Loop B, for example, involved a total of 20 

 miles of line, 6 gravity stations, a maximum gravity difference of 200 units 

 between the highest and the lowest station, and an error in closing of 2.6 

 units. (The closure error, or the amount by which the algebraic sum of 

 the gravity differences around a closed loop differs from zero, is a measure 

 of the combined effects of : ( 1 ) uncompensated instrumental errors such 

 as creep, elastic-after-effect, temperature changes, pressure variations, 

 etc.; (2) inaccuracies in reading of the instrument; and (3) errors due to 

 variations in calibration on scale value.) 



TABLE 11 1 



SAMPLE GRAVITY DATA OBTAINED WITH 

 TRUMAN GRAVIMETER 



Max. Gravity Closure 



Total Total Difference in Error 



Loop Miles Stations 10-' C.G.S. Units * 10-* C.G.S. Units 



Hoyt Gravimeter. t — A schematic vertical section of this instrument- 

 is shown in Figure 220. The instrument was developed by the Gulf 

 Oil Corporation§ as the result of research begun in 1933. It is unastatized 

 and a mass is supported on a helical ribbon spring, thus retaining the sim- 

 plicity of a spring balance. The advantage of this design is that gravity 

 force changes are translated into rotation of the suspended mass. 



The gravimeter consists primarily of a helical spring formed of steel 

 or of other suitable elastic material. The helical spring is suspended at one 

 end from a fixed support through an adjustable hanger 72 which allows 

 the spring to assume freely a vertical position. The other end of the 

 spring 64 is joined to another helical spring by means of a post 82. The 

 post is provided with a collar 83 which carries a spider 84, which in turn 

 carries a narrow-rimmed annular weight 85 concentric with the axis of 

 the helix. The spring and weight combination hangs freely. The weight 

 of the annulus 85 changes in accordance with the force of gravity at the 

 location where the apparatus is set up. This change in weight produces 



1" Bryan, Geophysics, loc. cit. 



* The unit employed is 0.1 millidyne or approximately 1/10,000,000 of the total 

 value of gravity. 



t Archer Hoyt, "Gravimeter," U. S. Patent 2,131,737, issued Oct. 4, 1938. (Assignor to the 

 Gulf Research and Development Company.) 



§ R. D. Wyckoff, "The Gulf Gravimeter", Geophysics, Vol. 7, No. 1, Jan., 1941, pp. 13-33. 



