GRAVITATIONAL METHODS 



349 



,', ■, BASIN-, : . - , : .: 

 LIGHT MATERIAL 



and can be evaluated only after removing the regional gravity influences, 

 that is, correcting for the regional gradient. This matter is illustrated 

 schematically in Figure 201. 



One way of determining the regional efifect is to find the average 

 value by taking the sum of the gradients 



•^ •=• ... EFFECTS 



recorded at all stations and dividing by the 

 number of stations. This can be done in 

 components and gives an average for the 

 north-south gradient and also for the east- 

 vv^est gradient. The average is then sub- 

 tracted from each station to obtain the 

 residual value for it. The residual values 

 may show features not apparent in the 

 original plotting. 



This process has an inherent error in 

 it, since the average figure obtained and 

 applied as the regional correction is not the 

 true regional. The error lies in the fact that each station entering the average 

 has to a varying degree a portion of local anomaly value in it as well as the 

 regional efifect. However, as a means of approximating the regional value 

 it is often useful. 



LOCAL 

 REGIONAL 



HEAVY MATERIAL 



Fig. 201. — Sketch of local struc- 

 ture on the flank of a basin and the 

 regional and local effects on the grad- 

 ient of gravity along a traverse line. 

 The observed gradient which is the 

 sum of the regional and the local 

 effects is shown. 



SCALE - IN METERS 

 100 200 300 



1 EOTVOS UNITS . 1 mm. 

 - =■ ISANOMALIC CONTOURS IN 



Fig. 202. 



The drawing of contours in torsion balance surveys is based on the 

 selection of stations through which a suitable closed traverse will be passed, 

 and on the computation of dififerences in gravity value between these 

 stations and an arbitrarily selected base station. The procedure may be 



