. Deep borehole surveys and problems. Boring. 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 crum
. Deep borehole surveys and problems. Boring. 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Haddock, M. H. (Marshall Henry). New York, McGraw-Hill
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