DEVIATION AND ITS CAUSES 15 



be employed apart from the bit holder as a plumbing 

 apparatus, the principle of acoustic signals being preserved. 



However, in spite of all precautions we cannot always 

 note at once a big and gradual curvature at its commence- 

 ment from the above observations alone. The detection 

 of a suspected curvature being essentially a surface task 

 in the initial stages of deflection, the next procedure is to 

 investigate the causes previous to checking the amount 

 and direction of the deflection. The causes are numerous 

 and often local, and in many cases are due to faulty surface 

 conditions. 



The Causes of Borehole Deviation. — a. Incorrect Center- 

 ing at Surface. — This, though sometimes tending to right 

 itself in such methods as the free-fall system, of course soon 

 leads to heavy deflections. 



b. Alternating hardnesses of successive layers of hard and 

 soft rock. Inexpert handling of the drill feed whether 

 by the multiple gear or hydraulic feed here tends to cause 

 racing in the shaly and soft beds and laboring in the harder 

 strata. The tool tends to supplement this by following 

 the softer stratum unless fed or geared to meet the circum- 

 stances.^ In such cases boring has to be undertaken very 

 carefully and frequent patroning, or damming and reguid- 

 ing, has to be resorted to, thus removing immediately the 

 slightest deviation from the plumb. 



Table VI. — Moh's Scale of Hardness 



1 See also Hugh F. Marriott, discussion to Deviation of Rand Boreholes, 

 etc., p. 115. 



