594 



Subsurface Geologic Methods 



tudinal axis. A lug is mounted in the angle unit of the single shot, so that 

 it is in perfect alignment with the scriber of the inner core barrel. The 

 lug and compass card are both photographed by the single shot machine. 

 The survey instrument and special barrel are run into the hole and engage 

 the inner-core-barrel shaft just previous to breaking off the core from the 

 formation. The reading taken by the instrument indicates the drift and 

 direction of the well, and the special lug shows the true direction of the 

 scribe mark on the core. This equipment generally is used in obtaining 

 oriented cores from holes that are very nearly vertical. 



In figure 306, B illustrates the equipment for taking oriented cores 



^ " i. ^^ 



Figure 308. Oil-well diamond-core bits. 



with a conventional core barrel when the hole is not vertical. No nonmag- 

 netic drill collar is used. The drift and direction of the point where the 

 core is to be cut are found by running a single-shot picture in open hole, 

 either before or after the core is cut. A special substitute containing two 

 opposite-pole magnets is made up directly above the conventional core 

 barrel. A bridger is mounted in the inner part of the substitute so that a 

 special type "M" Eastman drift indicator will be positioned with its disc 

 cup directly between the two magnets. A special disc cup, which is free 

 to rotate and contains a small magnet, is used. A small index pointer 

 contained in the disc cup marks the drift-indicator disc and thus records 

 the direction of the magnetic field of the disc cup. After the core is cut 

 and before it is broken off, the special drift indicator is run into the well 



