Chap. 7] 



GRAVITATIONAL METHODS 



135 



Almost any kind of gravimeter may be calibrated by measuring the 

 change in reading with elevation. A sufficient difference exists in most 

 office buildings between basement and the highest floor (Fig. 7-36). 

 From the formulas given in the next section for the "free air" reduction, 

 the*change in gravity with elevation is 



A^ (in milligals) = —Ah (in meters) -0.3086. 



(7-34e) 



Because of the elastic hysteresis of the suspension material, most gra- 

 vimeters show a more or less appreciable "drift" of the zero position with 

 time. Furthermore, abrupt changes In base position may result from 

 mechanical changes in the moving systems. Both kinds of changes may 



38 



§^6 



^34 



S32 



30 



JA 64.2m. 



Beloiv- 



Above- 



J L 



J L 



I2C 



Time 



1400 



Fig. 7-36. Calibration of gravimeter on building (after Schleusener). 



be corrected for by checking with a base station or a number of them at 

 regular intervals." 



F. Corrections on Observed Gra^vity Values 



In relative gravity determinations with pendulums, the values at a field 

 station (^a) are calculated from the base station values {g^ as follows : 



Qo. = Qp 



® 



, Up (2na — l)'^ 



and gr„ = ^,_ -_ 



(7-35a) 



where T, as before, is the period and n the coincidence interval. By series 

 expansion, these equations take the more practical form: 



9a = Qp — ^Qp jfi + OQp 



Qa = gp + 2gj: 



np(2np — 1) 



P^ + 



/ 4 rt\ \fia ^P/ I 



gpiinp - 3) ^— —2 + 



nA2np — 1) 



(7-35a) 



" See, for instance, D. C. Barton and W. T. White. Am. Geophys., Union Trans., 

 18, "Geodesy," 106-107 (1937). 



