20 DEEP BOREHOLE SURVEYS AND PROBLEMS 



as a rigid rod because under the influence of their growing 

 proper weight, rending, shear and turning forces arise which 

 cannot be checked aboveground. Unfortunately, regard- 

 less of any errors of observation or measurement at the sur- 

 face, the circumstances attending the wrenching off 

 of the core and the working of the rods influence the deduc- 

 tions very greatly. In solid strata the core is wrenched off 

 by a sharp jolt, otherwise we cannot tell whether the core 

 and strata are in their proper natural relation as before 

 rupture. In friable strata the core is frequently released 

 during boring operations due to the successive boring 

 shocks, and this also occurs frequently in rigid strata where 

 we have intercalated beds of clayey and shaly rocks. 

 Furthermore, the instant of jar for tearing off the core 

 often witnesses a slight rotation of the rods. The lower 

 surface of each core section should exhibit no traces of 

 shear horizontally; the fracture should be clean, for then 

 we can feel more secure that the small wrench twist is 

 absent. In order to ensure that the twist is eliminated or 

 minimized, the rod should be raised a little off the hole 

 base before the fangs of the core catcher come into action. 

 This gives the grip a better chance of making an accurate 

 engagement, because the spin of the string of tools has 

 abated. This spin definitely affects the direction of bore- 

 holes. The catcher now brought into action, a sharp 

 upward thrust will stand a better chance of yielding a core 

 with the conditions between core and strata preserved as 

 before rupture. No change from this position must occur 

 during extraction of the rods. The rod marks must be 

 carefully watched and bumping of the string of tools 

 on the borehole walls prevented. There should be no traces 

 of turning at the core grips. 



These conditions are so rigorous and so difficult of 

 application and the circumstances attending the wrenching 

 off of the core are so utterly beyond entire control that 

 absolutely exact results can not be hoped for from one core 

 alone. With cores of small diameter the small wrench twist 

 gives an error of several degrees and the smaller the diam- 



