GRAVITATIONAL METHODS 



293 



field, as indicated in Figure 153b. An equipotential surface which would 

 lie at right angles to the gravity force lines would be flat and horizontal. 



If a mass of high density, such as a granite ridge, is introduced into the 

 gravity force field pictured in Figure 153, an anomaly is created and the 

 direction of the lines of force of gravity will be deflected very slightly 

 toward this heavier mass. This will cause a curvature or distortion in the 

 lines of force of gravity, as shown in Figure 154. 



UNDISTURBED 

 LINES OF FORCE 



^'VnJ^V^N ^^EQUIPOTENTIAL SURFACE 



/•DENSE'^.COREA 



Fig. 154. — Showing the effect of a heavy subsurface mass which 

 causes a deflection of the lines of force of gravity toward it and the 

 arching of gravity equipotential surfaces over it. 



A plumb line would show this deflection if we were able to measure the 

 very minute change in direction thus caused. An equipotential surface 

 which is everywhere at right angles to the direction of gravity force would 

 be arched up above the heavy subsurface mass. These conditions are illus- 



(a) 



(b) 



- LINES OF GRAVITATIONAL 



(c) 



EQUIPOTENTIAL SURFACES 



Fig. 155. — (a) Force lines of gravity over a homogeneous uniform density area; 

 (b) force lines created by heavy subsurface mass only (theoretical case) ; (c) the resul- 

 tant direction of the lines of force of gravity where a dense mass is introduced into the 

 earth's field. Equipotential surfaces are given in each case. 



trated again in the three parts of Figure 155 which show (a) the lines of 

 force for an area where uniform conditions prevail; (b) the lines of force 

 which would exist if the mass only were effective and (c) the resultant or 

 deflected force lines, as caused by the combined attraction of the heavier 

 mass and the center of gravity of the earth. 



