BORE-HOLE INVESTIGATIONS 



1081 



the S.P. curves are often not sharp enough to permit the S.P. dipmeter to be used with 

 safety. The numerous resistivity anomaHes between, for example, shale and limestone, 

 found in most wells in these areas provide satisfactory levels for dip determinations at 

 almost any position in a well. 



THE PHOTOCLINOMETERf 



The photoclinometer is an instrument for measuring the angle a drill 

 hole deviates from the vertical, known as the drift, and the direction of 

 the deviation, known as the azimuth. In common with other instruments 

 using a magnetic compass, the indications of azimuth are not reliable, 

 inside of steel casing. The indications of drift in cased portions of the hole 

 are valid, however, as regards the deviation of the casing from vertical. 



By means of either an extension rod with weight at the lower end, or guides on the 

 instrument case, the axis of the instrument is kept parallel to the axis of the hole. The 

 inclination of the instrument itself is then the quantity measured. 



Photoclinometar PIctur* 



Morkcr Shooing 

 Orientolion of 

 Elactroda ** I 



Groduoled Curved 

 Gloil 



Compoo NeadI* 



ond Su$p*ntion 



(not (ho«n) 



Fig. 674. — Schematic arrangement of parts of the 

 photoclinometer. 



Inside the instrument are contained the elements pictured in Figure 674. A mag- 

 netic compass is mounted on gimbal rings and gives (in uncased holes) the reference 

 datum as to direction. A metal ball, rolling free on a graduated, spherically-curved 

 glass, indicates by the position it seeks at rest the amount of the drift, and also by 

 comparison with the direction of the compass needle, the azimuth. A photographic 

 lens focuses the images of the compass needle, metal ball, and calibrated glass onto a 



t A predecessor of the photoclinometer was the electromagnetic teleclinometer which deter- 

 mined orientations by comparing the magnetic fields of the earth and of a solenoid vertically sus- 

 pended in the instrument. The measurements were made by means of an induction compass, which 

 is essentially a rotating coil in which an E.M.F. is induced due to the magnetic field linking the 

 coil. (C. and M. Schlumberger and E. G. Leonardon, "Electrical Coring, a Slethod of Determining 

 Bottom-Hole Data by Electrical Measurements," A.I.M.E. Tech. Pub. No. 462, pp. 34-36.) 



