552 Subsurface Geologic Methods 



disc by the plumb-bob stylus. This turning action causes the plumb bob 

 to swing violently. Forty seconds later (after the plumb bob has come 

 to rest) the disc and cup again are raised and turned by the cam until 

 a second punch mark is made. The disc-cup assembly retracts immedi- 

 ately, after which the instrument may be removed from the well. If the 

 instrument was at rest at the bottom of the well when the records were 

 made, the punch marks are on opposite sides of the disc (180 degrees in 

 azimuth) and at approximately the same drift as indicated by the con- 

 centric rings on the disc. If the marks on the disc are not in this relation- 

 ship, the instrument was moving when one or both of the records were 



Figure 274. Instrument-contact watch 

 (surface watch identical) . 



made. This double-marking feature guarantees that an incorrect reading 

 will not be accepted. 



High bottom-hole temperatures do not affect the operation of this 

 mechanical instrument or its record disc. The disc need not be protected 

 from light since it is not printed on sensitized paper. 



The instrument is dropped or run on a piano-wire line inside the 

 drill pipe. Two types of steel barrels are used (fig. 275). Both types are 

 similar in that they are equipped with spring-type bottom shock absorbers, 

 and a mud tight closure is insured by use of an O-ring. The instrument 

 is attached to a rubber shock absorber inside the barrel to cushion it. 

 The drop or "go devil" barrel (fig. 275 A) is equipped with a feather- 

 head at the top to center the barrel in the drill pipe and to retard the 

 fall of the barrel by friction. When the instrument is lowered into the 

 drill pipe on a piano wire or sand line, the barrel shown in B of figure 

 275, equipped with a rope socket, is used. A bridger, or baffle plate, is 

 placed above the bit before the drill pipe is run into the hole. The 



