256 



EXPLORATION GEOPHYSICS 



is denser than the average material of the earth's crust in its neighbor- 

 hood exerts a greater force on a unit mass than would have been exerted 

 if the structure had been absent. The magnitude of this gravitational 

 anomaly is dependent upon the density, geometric configuration, depth, 

 and location of the structure with reference to the point at which the 

 effect of the structure is measured. 



Reduction of Gravity Values to Sea Level Datum, — In order to 

 compare absolute values of gravity, such as are obtained from pendulum 

 observations, and to use them for geodetic purposes it is necessary that 

 they be reduced to sea level as a datum. The reduction, or correction, is 

 made up of two parts. The first part corrects for the decrease in gravity 

 due to the height (or elevation) of the field station above sea level. It is 

 called the elevation correction, or free air correction. The second part takes 

 into consideration the increase in gravity, at a station, resulting from the 

 mass of the material between it and the sea level datum. It is called the 

 Bouguer'f correction. 



Elevation Correction. — The correction for elevation is derived from 

 the law of gravitation (Equation 1) that attraction varies inversely as the 

 square of the distance. Where go = sea level gravity and g = observed 

 gravity, with H = station elevation above sea level (or elevation difference) 

 and R = the radius of the earth, it can be stated that : 



\ R R^ J 



(15) 



go (R + ny 

 9 R' 



For most exploration purposes the higher powers may be neglected, and 

 the equation may be reduced to the more simple form : 



go = g(l+2H/R) 



(16) 



STATION 



' W//////////////////////////////A ^ - 



SECTION 



Fig. 125. — Illustrating the cylindrical 

 form assumed for the material between the 

 station and datum in calculating the Bouguer 

 correction, r, radius and H, height of the 

 cylinder. 



The elevation correction is based on 

 the assumption that no mass other 

 than "free air" exists between the sta- 

 tion and the datum. The constant for 

 the elevation correction is 0.3086 milli- 

 gals per meter or 0.09406 milligals 

 per foot. After correction for elevation, 

 gravity values are known as jree air 

 gravities and are given the symbol 



(go). 



Bouguer Correction. — The correc- 

 tion for the effect of the mass of mate- 

 rial between the station and datum can 

 be derived by assuming some easily 



t P. Bouguer, See Chapter I, p. 6. 



