412 



EXPLORATION GEOPHYSICS 



The instrument constant k, in milligals per scale point, is obtained by 

 dividing a factor 21,3 (which relates the amount of the calibrating mass, 

 the mass of the suspended system and the force of gravity) by the scale 

 point deflection caused by adding the calibrating weight. See note, upper 

 left of form. 



For the scale range in the survey under consideration, the value of k 

 is 0.0352 milligals per scale division. This figure was determined by taking 

 an average deflection, from two tests, of 605 scale divisions and applying 

 it to the factor 21.3, as shown at the bottom of the form. This procedure 

 is similar to that used in finding the scale value of a magnetometer by 

 applying a magnetic field of known amount to the instrument and obser- 

 ving the deflection. Here, by adding a small mass, a force of a given number 

 of milligals (21.3) is applied to the gravimeter, causing 605 scale points 

 deflection. 



Columns 4 to 7 inclusive provide for instrumental corrections. Column 

 4 gives the direction, or azimuth, in which the meter truck is oriented at 

 a station, and column 5 shows the correction for that orientation in scale 

 divisions. The barometer reading is recorded in column 6, and the air 



GRAVITY COMPUTATION SHEET 



ffl/M.G = ZI.30 milligalB 

 calibration 



OEAVITT-MBTBB SURTET 



SraTlmeter Ho. Z/ 



Compnter r*- i- ^- 



