GRAVITATIONAL METHODS 405 



into compartments or segments by radial lines from the center of the 

 chart. Stated briefly, the gravity effect of the terrain in each of these 

 segments is determined, and the sum thereof is the terrain correction. 



The circular zones are designated as B, C, D, etc. (B, C, D and E toe 

 small and M too large to be shown.) They have inner and outer radii of a 

 definite length as, for example, 6.56 feet (2 meters) and 54.6 feet respec- 

 tively for zone B. The radii of the boundaries of zone C are 54.6 feet and 

 175 feet, and in like manner for the other zones. Zone M covers the rings 

 of terrain from 48,356 feet to 71,996 feet (or approximately 9.2 to 13.5 

 miles). It is not often used, as the difference in elevation relative to the 

 station must be over 300 feet to produce an appreciable terrain effect at 

 such a great distance. 



The compartments or segments in each circular zone are equal in size. 

 There are 4 of them in zone B and 6 in zones C and D. The number 

 increases progressively to 16 for the J to M zones. 



The zone chart is placed on the topographic map and centered at the 

 station to be corrected. The average elevation of the area within a segment 

 is determined by studying the contours visible through the chart. The eleva- 

 tion of the station is subtracted without regard to sign from the average 

 segment elevation which established the average height of the segment. 

 The area of a compartment in zone B is 2300 square feet, or the equivalent 

 of a square section of terrain about 48 feet on a side. Its average elevation, 

 as shown by surface contours, might be, for example, 4250 feet. For a 

 station elevation of 4228, the average height of the segment would equal 

 4250 - 4228, or 22 feet. 



Complete terrain correction tables (Tables 13 and 14) f have been 

 prepared which list the gravity effect for a segment in a given zone in terms 

 of its average height in feet above or below the station. For the example 

 cited, where the average elevation difference was +22 feet, the table shows 

 that for a compartment in the B zone a height of 21 to 24 feet produces 

 a gravity effect of 0.08 milligals. 



The complete terrain correction is obtained by adding together the indi- 

 vidual gravity effects from all compartments in all zones where the eleva- 

 tion difference is great enough to be effective. Often a terrain correction 

 can be made in from y^ to \ hour per station. The tables are based on a 

 density of 2.0. The total terrain correction must therefore be multiplied 

 by ^ of the density which is assumed for the local area, unless it is 2.0; 

 i.e., multiplied by the ratio of the density to the value 2. 



The terrain correction tables are calculated by the formula for the 

 gravitational attraction (d^g) of a vertical hollow cylinder (which is in 

 effect a ring of the terrain) of height (h) at a point on the axis of the 



t S. Hammer, op. cit. A similar but condensed table is given in L. L. Nettleton's "Geophysical 

 Prospecting for Oil," McGraw-Hill, 1940, p. 145. 



