The four spring-actuated arms of the tool contact the walls of the bore hole 

 when they are released. The motion of these arms is transmitted to a rheostat 

 inside the oil-filled chamber by means of a flexible bronze cable-and-pulley 

 system in such a manner that the change in resistance of the rheostat is always 

 proportional to the change in average diameter as measured by the four arms. 

 Owing to the spring tension in the arms, the tool will be approximately centered 

 in the well, unless the hole is considerably off vertical. 



The arms are held in a closed position by a steel band when going in the 

 hole. This band can be broken at will, either by firing a brass projectile located 

 underneath or by spudding on bottom. 



The chamber of the tool is filled with oil and kept hydrostatically balanced 

 by means of a large rubber diaphragm, which acts as a volume equalizer between 

 the oil and the mud, compensating for the difference caused by the motion of 

 the push rods and by changes in temperature. 



A constant direct current is supplied to the rheostat in the tool, and the 

 resultant potential drop across it is measured and recorded as a caliper log. 

 These logs take the form of a continuous galvanometer trace recorded on film 

 showing the average diameter of the bore hole, recorded as a function of depth. 



A study of numerous caliper logs soon leads one to the conclusion that 

 caving patterns exist. Certain generalizations may be made as to the relative 

 ability of rocks to stand up to bit size. These generalizations are shown in 

 graphic form in Table 16-1. 



TABLE 16-1 

 Ability of Rocks to Stand Up to Bit Size 



Although similar drilling conditions and similar muds tend to standardize 

 caliper logs, some astonishing long-range correlations may be made by using 

 them, even for wells drilled under entirely different circumstances. In the west 

 Texas area, a number of horizons may be recognized on caliper logs, from Upton 

 County, Texas, to the Hobbs Field, Lea County, New Mexico, a distance of 120 

 miles. Here caliper-log correlations are more trustworthy than those made with 

 electric logs. 



348 



