Chap. 7] GRAVITATIONAL METHODS 141 



After the free-air, Bouguer's, and terrain corrections have been applied, 

 the "reduced" gravity, ^o , is obtained. 



4. Isostatic correction. In the reduction of gravity values for geodetic 

 work, a very elaborate correction is applied for the isostatic compensation 

 of the visible masses. The principle of the computation is to assume such 

 configuration of the invisible masses that the remaining, or "isostatic," 

 gravity anomalies become a minimum. While this is the correct pro- 

 cedure for areas of such proportions that the curvature of the earth be- 

 comes effective and must be applied' when gravity data are used for the 

 determination of the shape of the earth, it should not be used in gravity 

 exploration. In geophysical prospecting the anomalies desired for in- 

 terpretation in terms of local geology are the Bouguer anomalies. It is 

 not desirable to modify them by generalizations of the actual geologic 

 conditions which are involved in the computation of isostatic anomalies. 



5. Correction for normal and regional gravity. In order to obtain the 

 anomalous gravity, the normal value of gravity must be subtracted from 

 the observed and corrected gravity values, since only the deviations from 

 the normal value are of geological interest. With sufficient accuracy the 

 normal gravity may be computed from the formula previously given 



Yo = 978.049 (1 + 0.0052884 sin' <p). (7-15c) 



Subtraction of normal gravity from Bouguer-corrected observed gravity 

 gives the value A^o', or the ''Bouguer anomaly." In gravimeter surveys 

 only the variation of normal gravity with latitude need be considered. 

 Differentiation of the above equation yields 



^ = i^ = Q^^f^^ . 0.0052884 sin 2^, (7-38/) 



dx Rd(p R 



which, by substitution of 3958 miles for the earth's radius, gives 



1 .3068 • sin 2^ miiiigais per mile. (7-38^) 



The variation is positive and therefore the correction negative is toward 

 north. The variations of gravity and gravity gradient with latitude are 

 shown in Fig. 7-73a. 



In addition to normal gravity, regional gravity anomahes may have 

 to be considered. In certain areas the regional anomalies and their 

 variations are quite large. Fig. 7-40 shows the distribution of the Bouguer 

 anomalies in the United States. The final results for Ag'd are plotted for 

 each station and connected by lines of equal gravity anomaly (isogams). 

 Their interval depends on the accuracy with which the observations were 

 carried out and on the magnitude of the anomalies. For most accurate 

 work, 0.2 to 0.5 milligal is a suitable interval; for intermediate accuracy, 



