404 



EXPLORATION GEOPHYSICS 



should be set from a certain terrain feature to render its effect negligible. 

 This suggests that a judicious choice of station sites is advisable in rougher 

 areas such as are often encountered in mining work. It is fortunately true, 

 however, that in a great many gravimeter surveys no topographic correc- 

 tions are required. In this respect the gravimeter is superior to the torsion 

 balance, with which terrain is often a limiting factor. 



Zone Chart Method of Terrain Correction. — A method for calcu- 

 lating the effect of rough terrain was presented by S. Hammer in 1939.t 

 This method permits the contribution of the terrain to the gravity value 

 measured at a station to be evaluated quantitatively to a relative accuracy 

 of 0.1 milligal, which accuracy is necessary to utilize the high precision of 

 modern gravimeters. 



ZONE CHART 



TERRAIN CORRECTION FOR GRAVITY 



SCALE 1:62.500 



(REDUCED) 



Fig. 244. — Zone chart for use in evaluating terrain corrections at gravity 

 stations. (.Geophysics, Vol. IV, No. 3, July, 1939, p. 188.) 



Zone charts are easy to construct and use. An example of one, to the 

 scale of 1 : 62500, is shown in Figure 244. A topographic map, or some 

 form of terrain survey, around the gravity station or stations to be corrected 

 is necessary. The chart is drawn in ink on a transparent sheet to the same 

 scale as the topographic map, on which the gravity stations are plotted. The 

 chart is laid on the map with its center at the station being studied. 



A zone chart divides the terrain surrounding the station into circular 

 zones or rings of a certain width, and these zones are, in turn, divided 



t S. Hammer, ibid, p. 194. 



