INSTRUMENTAL SURVEY OF BOREHOLES 



49 



lated data bearing on the problem will be decided by this 

 core knowledge. Also the probable line of development 

 in the field concerned. It is singularly useful in seeking 

 index beds or marker or key beds and therefore decides 

 the spacing of holes and life of a lease. 



It is considered that shale with a dip over 5 deg. is the 

 most favorable stratum for core orientation, since dips are 

 rarer in massive formations. Hard sands are more objec- 

 tionable owing to their wearing out the cutters, and soft 

 sands tend to crumble and plug the barrel ; also false bedding 



Fig. 21. 



occurs more frequently in sands. The chief difficulty is 

 the transporting of the cores to the surface in a satisfactory 

 condition. 



At all events sufficient has been said to show that the 

 practice of borehole surveying and core orientation has 

 progressed far since the day of Dr. Newell Arber^ who was 

 rather emphatic in disclaiming the reliability of any 

 methods purporting to show the direction of dip of beds in a 

 borehole. 



In all methods of borehole surveying and core orienta- 

 tion, one of the prime factors influencing the choice is the 

 cost, since the cost consists not only in the actual expense 



1 Geology of the Kent Coalfield, Trans. Inst. Min. Eng., Vol. 47, p. 694. 



