92 DEEP BOREHOLE SURVEYS AND PROBLEMS 



It is essentially a multiple-photograph orientation appara- 

 tus consisting of a pendulum and compass device, the posi- 

 tions of which are recorded on a long strip (Fig. 45).^ 

 The drill rod is orientated by external joint scribing at 

 every 10 ft. and aligned on a surface reference mark. 

 Photography was chosen as the azimuth recording medium 

 because then the pendulum and compass needle can swing 

 freely during recording, the photograph automatically 

 averaging the mean point about which swing, if any, 

 occurs. The recording instrument^ is inside the inner barrel 

 to minimize relative displacement from the core. The 

 record is of the nature shown in Figs. 45 and 45a. The most 

 recent development of this apparatus (Figs. 46, 46a) ^ 

 has a long photographic strip on a reel which records 

 the position of the pendulum and compass needle at regular 

 intervals, thus permitting a complete survey of the well 

 and allowing for records of several positions of the core 

 barrel during coring. In interpreting a record it will be 

 observed that the white lozenge-shaped shadow of the 

 magnetic compass card is eccentric to the large dark 

 circle of the exposure. The amount of eccentricity meas- 

 ures the deviation of the hole from vertical at each exposure 

 because the eccentricity is caused by the compass being 

 suspended as a pendulum. The inner core barrel is marked 

 (and it may also mark the core) and is attached to the 

 instrument in a recorded position so that the relative 

 positions of core and record are fixed and known. The 

 inner core barrel is swiveled inside the outer barrel. The 

 outer core barrel is rotated by drill pipe or drill rod to cut 

 around a core and the inner barrel is forced longitudinally 

 over the core and at the same time shaves the core a frac- 

 tion of an inch smaller so that the inner barrel takes a firm 

 friction hold on the core. A spring on the inner barrel 



^ From a private communication. 



2 Macready, G. a., Orientation of Cores, Bull. Amer. Assoc. Petroleum 

 Geol, p. 571, 1930. 



^ By the courtesy of the Bulletin of the American Association of Petroleum 

 Geologists. 



