Miscellaneous Subsurface Methods 



567 



been found that corrections above 20-degree drift must be made to the 

 bearings as the magnetic influence becomes apparent. A survey barrel 

 stopped in position on the landing ring in the top of a nonmagnetic drill 

 collar is illustrated in figure 289. 



A drop or "go devil" type of barrel has been developed recently for 



Figure 290. Trigger-type surveying bit. 



use with nonmagnetic drill collars. This barrel, equipped with exterior 

 spring and rubber shock absorbers and with the instrument suspended 

 from a rubber shock absorber, is being used successfully. A spearhead 

 at the top permits it to be retrieved from the drill pipe with an overshot 

 run on piano-wire line. If the drill pipe is to be removed from the hole, 

 the barrel is dropped and later recovered when the bit is unscrewed from 



