560 Subsurface Geologic Methods 



282). A yoke which is weighted on one side is mounted on a vertical 

 shaft. This shaft is free to turn on small ball bearings. Therefore, the 

 weighted side of the yoke seeks the low side of the hole. Two vertical 

 quadrants and a compass card are cradled in the upper portion of the 

 yoke. This sub-assembly is weighted on the bottom and is free to move 

 about a horizontal axis in the yoke. Wire-index lines are used to read the 

 drift and direction. One quadrant is graduated at 10-degree intervals of 

 drift; the vernier quadrant indicates single degrees. Direction is read 

 directly from the compass card. The 30-degree-angle unit has been super- 

 seded by the 90-degree unit in most cases. 



A few special 5-degree maximum-reading instruments are in use. The 

 angle unit is similar to that used in the 10- and 18-degree single shots 



Figure 281. Ninety-degree- 

 type single-shot disc — di- 

 rection S. 36° E., drift 

 33° 00'. 



except that the concentric rings represent 20 or 30 minutes of drift. These 

 instruments may be run in any barrel in which the other regular single 

 shots are used. 



Two types of nonmagnetic barrels are made for use with the regular 

 single-shot instrument. Both are bored to 2^-inch inside diameter, but the 

 barrels are 3 and 3^ inches in outside diameter. Both are constructed of 

 nonmagnetic duralumin and bronze and have a bull plug using an 

 0-ring-type pressure closure. A shock-absorber plunger contained in the 

 bull plug is spring and rubber buffered to protect the instrument from 

 severe vertical shock. 



The 3J-inch barrel (fig. 283) is used for running the instrument in 

 open hole on sand line after the drill pipe has been removed from the 

 well. It is equipped with a tubing collar at the top so that it may be made 

 up on two joints of tubing. The tubing is attached to the sand line with 

 a rope socket or bailer top. A tool-joint connection is provided on the 

 top of the barrel if it is necessary to run the instrument on drill pipe. A 



