336 EXPLORATION GEOPHYSICS 



The topographic or terrain correction is, in effect, a process of math- 

 ematically leveling the surface of the ground around the torsion balance 

 station. As has been demonstrated in the preceding discussion, this is ac- 

 complished by dividing the terrain into a number of sectors, computing 

 the effect of each, and adding them together. 



The effects depend on the differences in elevation between points in the 

 terrain and the instrument station. It is assumed, with usable validity, that 

 the elevation differences may be expressed by a Fourier series and that the 

 local terrain varies uniformly from point to point. 



In making the terrain correction, there is a practical limit as to the 

 number of points at which observations (rod readings) can be taken. Also, 

 the average height of segments of the terrain is used in calculating the 

 effects. It is apparent, therefore, that any terrain or topographic correction 

 is an approximation and accurate only to the degree that a particular terrain 

 fits the assumptions made. 



Field Operation, Surveying the Station Site. — After proper leveling 

 of the terrain, a transit is set up at the station where the torsion balance 

 is to be placed. The astronomic north direction is set off with the transit, 

 using its compass needle with the proper allowance for magnetic declination. 



A light rope or steel tape 50 meters long is used, and on it the out- 

 distances where rod readings are to be taken are marked. These distances 

 are specific in the terrain correction developed by Schweydar (used in the 

 example), and the coefficients in the form given are based on them. These 

 distances are 1.5, 3, 5, 10, 20, 30, 40, 50 meters. Occasionally, readings are 

 made to distances of 70 and 100 meters if extreme accuracy or rough 

 terrain indicates. 



The tape is laid out in the north azimuth. Rod readings, with the 

 transit telescope used as a level, are taken at the marked distances. The 

 rod readings at each out-distance are elevation differences from the height 

 of instrument and are entered in the form by the instrument man. Such 

 elevation differences are a requirement of the sample form given.* 



After the 8 rod readings in the north azimuth are completed, the pro- 

 cedure is repeated for the seven remaining azimuths, oriented 45° apart, 

 as indicated in Figure 187. 



A convenient form of rod, shown in Figure 188, is so designed that 

 elevation differences may be read directly. This rod is graduated in centi- 

 meters (and decimeters) to read both ways from a zero point located 125 

 centimeters from the bottom of the rod. The rod has an adjustable foot 

 which can be locked in place with a wing nut and bolt, so that the zero 



* The sample terrain correction computation, using the form as given, indicates 

 that the summation of the terrain effects is multiplied by the quantity J^ density (5/2). 

 This factor has a plus sign as applied to the two gradient components and to the east- 

 west curvature quantity. It is negative for the north-south curvature correction. 



If direct metric rod readings, rather than height differences, are used, the ^ dens- 

 ity multiplying factor must be reversed in sign in each case. This can be demonstrated 

 by running through calculations using height differences and direct rod readings. 



