FLUID METHODS OF SURVEYING BOREHOLES 107 



is now largely of historic interest, the chief demerits 

 accounting for its disuse being 



a. It does not provide a continuous record on insertion 

 right down the borehole, the total of many individual 



± 



Upper ; 

 Shafi- 1 

 Depfh^ 

 foTop 



-400 ft 



SOO-ff. 



'Error 75 ff. 

 on depth deflection 



Fig. 55. — MacGeorge's guide tube. Fig. 



56. — A borehole survey by 

 MacGeorge. 



insertions having to be grouped for the final reading. 

 This is tedious, time-wasting and costly. 



b. It fails in magnetic strata and steel-cased boreholes. 



