Chap. 7] GRAVITATIONAL METHODS 241 



is frequently superimposed upon a larger anomaly due to regional geologic 

 structure or some other larger geologic body. Examples of such regional 

 eflfects are monoclines, deep-seated intrusions, regional attitude of base- 

 ment topography, geosynclines, and the like. When a local structure is 

 superimposed on such regional features, it is possible that its effect is not 

 immediately recognized in the gradient picture. For instance, in the case 

 of an anticline or dome, the familiar reversal of gradients may not appear 

 in the torsion balance map at all. It can be produced, however, by sub- 

 tracting (vectorially) the effect of the regional geologic feature (see Fig. 

 7-93). It is impossible, however, to give general directions for the applica- 

 tion of such correction. The magnitude and direction of the "regional" 

 gradient depend entirely on distance, depth, and configuration of the 

 regional geologic feature involved. The regional effect may be deter- 

 mined (1) by field measurements and (2) by calculation. Where sufficient 

 well-information is available to establish definitely the absence of local 

 structure and the presence of nothing but regional effect, the regional 

 gradient and its variation with distance may be determined from a suffi- 

 cient number of torsion balance observations. 



The regional gradient may also be derived from regional surveys with 

 the gravity pendulum or gravimeter. If the geologic feature producing 

 the regional gradient is definitely known, its effect may be calculated by 

 the formulas and procedures given in the following section on interpreta- 

 tion. An instructive example for the application of the regional gradient 

 correction is given by D. C. Barton^"^ for the Fox and Graham oil fields 

 in Oklahoma. In mining exploration the "regional" feature to be elimi- 

 nated is frequently of very restricted extent and often a geologic body 

 which in the regular usage of the term would likewise be called a "local" 

 structure, for example, a contact zone, a fault, an intrusion, or the like. 

 What is considered a regional feature depends, in other words, entirely on 

 what geologic feature is sought by the survey. The procedure of correcting 

 for a regional gradient is, therefore, comparable in every detail with the 

 subtraction of a "normal" value from magnetic anomalies. 



4. Correction for coast effect. Coast effects are due to (1) the slope of 

 the coastal shelf and (2) variations in water level (tides). The second of 

 these has been discussed before (pages 165-166) in connection with time 

 variations of gravity, and it need not be considered here. That fairly 

 appreciable gravity anomaUes accompany the continental shelf had been 

 observed by Hecker. An analysis and theoretical explanation of the 

 effect was given by Schiotz and Helmert.^"^ Whereas above a buried 

 slope of dense material the gravity anomaly rises gradually from its lowest 



'"A.I.M.E. Geophysical Prospecting, 458 (1929). 

 108 Encycl. Math. Wiss., VI 1 (7), 143 (1906-1925). 



