258 EXPLORATION GEOPHYSICS 



At the time the station was occupied and gravity observed, the cyHnder 

 of material from the station down to datum contributed its effect to the 

 observed value. This contribution is calculated in the Bouguer reduction and 

 must be subtracted from the value of gravity at the datum plane, corrected 

 for elevation only. 



When thus corrected for both elevation and Bouguer effect, the g"o 

 gravities are those that would have been obtained had the station been at 

 the datum location. 



Gravity Anomalies 



The absolute gravity anomaly (which also reflects the major subsurface 

 conditions to a certain degree) can be calculated by subtracting from the 

 Bouguer gravity value the theoretical value of gravity for the latitude of the 

 station. The theoretical value of gravity can be computed by use of Equa- 

 tion 14. Such theoretical values are usually designated as yo. After the 

 subtraction is made for a given station, the resulting gravity anomaly value 

 is symbolized I^g''o and is known as a Bouguer gravity anomaly. In brief, 

 the absolute gravity anomaly at a station is the corrected observed gravity 

 less the theoretical gravity for the station. 



The sizes of certain pendulum gravity anomalies (^g^'o values) in the 

 state of Colorado are as follows :f 



Pendulum Gravity Anomalies 

 Station Anomaly Conditions Location 



1 Denver — 0.016 In center of Denver 1 



Basin 



2 Colorado Springs —0.007 On axis deep syncline 75 miles S of 1 



3 Pike's Peak +0.021 On uplifted mass 10 miles W of 2 



4 Idaho Springs +0.022 Uplifted area 30 miles W of 1 



5 Boulder —0.013 In basin, with minor 30 miles NW of 1 



nearby fold 



6 Lafayette —0.020 In Denver Basin local 15 miles NNW of 1 



faulting 



7 Brighton —0.006 In Denver Basin 20 miles N of 1 



Gravitational Field of the Earth. — -As a consequence of the inverse 

 square law (Equation 1), there exists a physical quantity described as the 

 gravitational potential which is analogous to the magnetic potential. The 

 gravitational potential at a point is defined as the work required to move 

 a unit mass from that point to a point infinitely distant. Gravitational 

 potential is thus a scalar quantity.* 



A level surface is a gravitational equipotential surface. It is a surface 

 such that no work is done against gravity when a mass is moved between 



t J. H. Wilson, Mines Magazine, August, 1928: Article on Pendulum Apparatus. 

 D. White, "Gravity Observations from the Standpoint of Local Geology," Bull. G. S. A., 

 Vol. 35, 1924. 



* An alternative definition of potential may be stated as follows : The gravitational 

 potential at any point is that quantity whose rate of change in the direction of gravity 

 is the force of gravity at that point. 



