154 



GRAVITATIONAL METHODS 



[Chap. 7 



subsurface quadrant is divided into compartments bounded by concentric 

 circles with radii rm and b^ angles (p^ . Then the effect of one compartment 

 is 



at/ 



bz 



= -2kh{rra+i - rni)(cos ^n+i - cos v?n)- 



(7-44a) 



A diagram constructed in this manner is shown in Fig. 7-45. The 

 effect of each field is 6.6710~'-5p microgals, where p is the scale of the 



4 >WW/M f f/ZZ/fw A 



Fig. 7-45. Diagram for the calculation of attractions due to two-dimensional masses 

 (after Jung). Unit effect: 6.67-10~^-5-p milligal; scale: l:p. 



geologic section with which the diagram is used. For subsurface masses 

 the anomaly is always positive, the diagram being inverted north of the 

 station. For masses above the horizon the anomaly is negative. Masses 

 in the. horizon are ineffective; masses vertically below the station most 

 effective. 



4. Integraphs.- Since the gravity anomalies of two-dimensional masses 

 depend on their area (eq. [7-39/]), "planimeter" methods may be used to 

 calculate the attraction of geologic features of irregular outline. However, 

 the effect of area is also dependent on distance and vertical angle. The 

 geologic section must be redrawn on distorted coordinates when used with 



