CHAPTER IV 



GRAVITATIONAL METHODS 



Prospecting by gravitational methods is the technique of measuring 

 the gravitational field at the earth's surface and utilizing the data thus 

 obtained to predict the subsurface structure. Gravitational methods are 

 analogous to magnetic methods in that quantitative investigations are made 

 of a natural field of force. The magnetic and gravitational methods are also 

 analogous in their fundamental relationships, as will be seen during the 

 development of the theory. 



The physical property of the subsurface materials which produces 

 the significant or diagnostic gravitational anomalies is density. It is 

 necessary that the efifects caused by the changes in density of the sub- 

 surface materials be of sufficient magnitude to manifest themselves over 

 near-surface, topographic, and regional efifects. 



The three chief types of instruments available for gravity measurements 

 are the pendulum, the torsion-balance and the gravity-meter or gravimeter. 

 The following table gives their general operating characteristics. 



Instrument 

 Pendulum 



Torsion balance 



Gravity meter 



(a) Stabilized type 



(b) Labilized type 



Quantity Measured 

 Absolute vertical component 

 of gravity 



The gravity gradient along a 

 horizontal plane, and the dif- 

 ferential curvature of grav- 

 ity equipotential surfaces 



Relative vertical component 

 of gravity 



Relative vertical 

 of gravity 



component 



Hoiv Measured 

 Time required for a given 

 number of oscillations of the 

 pendulum system. 



Angular movement of a 

 beam system supported by a 

 torsion wire. 



Extremely high magnifica- 

 tion to measure the vertical 

 movement of a spring 

 weighted system. Lineal dis- 

 placement. 



Moderately high magnifica- 

 tion to read null-point. Non- 

 lineal displacement. 



as is shown 



lineal aispiacen 



Gravity methods may be compared with magnetic methods, 

 in the following tabulation. 



Comparisons of Gravity and Magnetic Methods 



Item Gravity Magnetic 



Strength of field 1000 dynes (approximately) 0.6 gauss (dynes), average 



?asic variation Minimum at the equator; 



maximum at poles 



Vertical component mini- 

 mum at equator, maximum 

 at poles. Horizontal compo- 

 nent maximum at equator, 

 minimum at poles 



