EVALUATING THE ACCURACY OF A GRAVIMETRIC SURVEY 



163 



In tlie report it is mentioned that the anomalies which were very small 

 in their dimensions and amphtudes and which are unreliably indicated 

 on the map, are also of geological interest. Table 6 giv<;s values for a and cr, 

 ensuring a reliable determination of this type of anomaly with parameters 

 i^Q = 2.5 mgal and Iq — 5 km with the condition that the isoanomalies 

 are carried out every 0.5 mgal and the relative errors in the gravity field 

 and gravity increments between the neighbouring isoanomalies are equal 

 to 15 and 29 % respectively. Table 6 gives two pairs of extreme values of a 

 and a, each of which satisfies the requirements. This example shows that 

 the same problem of determining anomalies with a given accuracy and 

 a given cross-secti(m of anomalies in some cases can have a different technical 

 and economic solution. 



The values of a and a in Table 6 are found from the nomograms (Figs. 11 

 and 13). 



Table 7 gives separately the observational and interpolational errors for 

 this case, as found on the appropriate graphs. 



It follows from the preceding that the proposed method of finding errors 

 from the parameters of an accomplished survey, or of determining the 

 parameters of a plannefl survey from adopted errors, requires in both cases 

 a knowledge of the field ^q and Iq. At the planning stage of an investigation, 

 this requirement can always be fulfilled, since the main problem of gravi- 

 metric exploration is to determine the anomalies. Consequently, their 



Table 7. 



parameters can be given at the beginning. In the majority of cases, this 

 method can be used to determine the accuracy of the finished work, since 

 the observed field is usually nondinear. Frequently, in regional surveys 

 the field does not have closed anomalies. However, if the curves for the 

 change in the force of gravity along arbitrary profiles cutting the map have 

 even only relative extreme points, this is sufficient for determining g^ and Iq. 

 In connection with this it should be stated that the field factors should be 

 determined along the profiles of the arbitrary directions in those cases where 

 the field is represented by clearly mapped maxima and minima of gravity. 



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