332 EXPLORATION GEOPHYSICS 



the balance. Furthermore, the effect varies inversely as the cube of the 

 distance of the mass from the balance.* 



As a consequence of the topographic effects on the gradient outlined 

 above, it is not practical to use the torsion balance in rugged areas for 

 determinations of gravity gradients due to structure, unless the gradients 

 due to the structures under investigation are very large. 



The effect of a small mass on the differential curvature value approaches 

 its maximum value within 10° of the horizontal plane through the center 

 of gravity of the balance. Hence, it is difficult even in regions of moderate 

 relief to choose station sites such that the effect of topography will not be 

 excessively great. From a practical viewpoint, therefore, the differential 

 curvature is of negligible value for predicting structure in areas where the 

 topography is even mildly rugged. 



It is common practice to classify topographic corrections as terrain 

 and cartographic corrections respectively, and to apply somewhat different 

 theoretical assumptions and field methods for the evaluation of the two 

 types. The term terrain corrections, as used here, connotes corrections for 

 effects due to irregularities in topography which are present over a circular 

 area having the balance at its center and a radius equal to 100 meters. Car- 

 tographic corrections refer to corrections due to topographic irregularities 

 located at distances greater than 100 meters from the balance. As applied 

 to field methods for evaluating these effects, this classification is not rigid 

 because identical field methods frequently may be applied for evaluating 

 effects due to excesses and deficiencies of mass located at distances from the 

 balance of 3 to 300 meters.** 



Analytical treatments of topographic and cartographic corrections have 

 been given by von Eotvos, Schweydar, Numerov, Nikiforov, Lancaster 

 Jones, Heiland, Ansel and others. Graphical treatments have been given by 

 Numerov, Jung, Haalck and others. Also, a detailed description of the type 

 corrections utilized in the Imperial Geophysical Experimental Survey is 

 given by Broughton Edge and Laby. 



Corrections for mass irregularities located at distances from the balance 

 greater than 100 meters are computed most conveniently by graphical 

 methods. Corrections for topographic or terrain effects, i.e., excesses or 

 deficiencies of mass situated at distances of less than a hundred meters, are 

 computed conveniently in many cases by the Schweydar analytical method. 



* The practical significance of these facts in regard to field work may be stated 

 as follows : Whenever feasible, the site chosen for the torsion balance station should 

 be such that all appreciable mass irregularities are situated at moderate distances from 

 the balance and within ±5° of the horizontal plane through the balance. Furthermore, 

 if the balance were set up on an extensive hill sloping uniformly 1°, the effect of the 

 slope on the gravity gradient would be approximately 14 Eotvos units, the precise value 

 depending on the mean density of the topographic feature. Hence, because the gradients 

 due to structure generally are of the order of 7 to 25 Eotvos units, it is preferable to 

 choose station sites where the inclination of the ground is less than 1°. 



** It is of some interest to note that many investigators classify the corrections due 

 to topographic irregularities into three types: terrain (up to 100 feet from the site), 

 topographic (from 100 to 1000 feet), and cartographic (over 1000 feet). 



