240 GRAVITATIONAL METHODS [Chap. 7 



After "far" terrain has been taken, three pegs are driven into the 

 ground, and the aluminum base plate is laid on it and so oriented that 

 the north mark is in the astronomic meridian. The "near" terrain (1.5 m 

 and 3 m radii) is then taken by placing the leveling rod successively in the 

 eight directions marked on the plate and leveling it with a carpenter's 

 level. Distances to ground are read at 1.5 m and 3 m distances with a 

 ruler (see Table 32). 



The density of the surface soil is taken by filling a 1000 cc container 

 and weighing it. If considerable contrast exists between the density of 

 the surface weathered layer and the formation below, its density may 



fc**^"^' 'r^ 





Fig. 7-87. Aerial photograph of torsion-balance stations in Mexico. 



have to be determined separately. Further details on terrain field meth- 

 ods, selection of station, calculations, and the like are given in Directions 

 for the Askania Torsion Balance. The brief directions given above apply 

 to the Schweydar method and may be modified for other terrain methods. 

 Different companies have adopted slightly different techniques in the 

 field and in the calculations (use of nomographs, and so on), but the 

 fundamentals of procedure are more or less the same. With calculation 

 diagrams involving units of equal elevation angle it may be advisable to 

 measure angles directly on the vertical circle of the transit or alidade. 



3. Correction for regional gradient. In torsion balance surveys it happens 

 that the gradient and curvature anomaly due to a structure or geologic body 



