402 EXPLORATION GEOPHYSICS 



Corrections Applied to Relative Gravity Measurements 



Reduction to Datum. — The corrections applied to gravimeter measure- 

 ments at a series of field stations are the same as those already considered 

 in the reduction of pendulum gravity data (p. 256 to 258) . The derivation 

 of the constants used was given in that section. The reduction to datum, 

 to review the matter briefly, consists of two parts : the free air correction 

 or straight elevation correction, and the Bouguer correction which takes 

 into account the density of the material between the station and the datum. 

 The two are ordinarily combined, and the proper constant can be calcu- 

 lated on the basis that the elevation correction is at the rate of —0.09406 

 milligals per foot and the Bouguer correction is -1-0.0127 • d milligals per 

 foot {d = density of the material from the surface to the datum chosen.) 

 The equivalent figures in the metric system are —0.3086 milligals per 

 meter for the elevation efifect and -1-0.04815 • d milligals per meter for 

 the Bouguer efifect. 



As an example, the combined correction factor for a density of 1.8 is 

 0.07109 milligals per foot (the sign is — ), and for a density of 2.0 it is 

 0.06854 in like units. For a density of 2.2 the value is 0.06599. The value 

 of density is usually assumed from density determinations of surface 

 samples, with a judgment factor added. This assumed density then 

 represents the value from the surface to the datum. 



The datum is set at some convenient elevation below the survey base, 

 or at the elevation of the base itself, if desired. It is quite an arbitrary 

 matter, under the control of the person responsible for survey interpreta- 

 tion. A difiference in the selection of a datum elevation merely represents 

 a raising or lowering of all final map values of a survey. 



As with corrections in magnetic surveys, the effect involved and the 

 manner in which it is applied as a correction must be thought out carefully. 

 Corrected gravity values represent the readings which would be obtained 

 with the meter if it were set on the datum. The gravity at a higher eleva- 

 tion is less, due to elevation only, and the reading at such an elevated station 

 has in it a contribution caused by the effect of the mass of material between 

 the surface and the datum. 



The above can be shown by an example. If we assume a gravimeter 

 reading, corrected for drift and converted to milligals, at station X of a 

 survey as 53.8 milligals, and the station elevation at 6295 feet, with a datum 

 elevation (for convenience) at 6195, the necessary calculations are quite 

 simple. Let us assume also that the density is 2.0. 



The elevation difference, station to datum, is 100 feet. The elevation 

 correction equals 100 • 0.09406 milligals per foot, or 9.4 mg. Therefore 

 the station value 53.8 plus 9.4 gives 63.2 mg. for the value of station X on 

 the datum, elevation only considered. This is the so-called "free gravity" 

 value. 



The Bouguer effect is 0.01276 • 2 = 0.02552 mg./ft. and 100 feet times 

 this constant gives 2.6 mg. The meter reading of 63.2, on datum and cor- 



