1082 EXPLORATION GEOPHYSICS 



35 mm. photographic film, so that the resulting picture has the general appearance of 

 Figure 675. 



Since the compass needle is painted half black and half white, an image of the 

 white end only registers on the film to indicate magnetic north. The radial lines, which 

 are 10° apart, facilitate reading differences of azimuth. 



Sometimes it is necessary, as in the case of a dipmeter survey, to know also the 

 orientation of the instrument. For this purpose on one edge of the graduated glass a 

 triangular marking is provided which conventionally indicates on the film the side of 

 the instrument on which the #1 dipmeter electrode is located. 



Nominol size of gloss 



Compass Ntedl* 



Seriol number 

 of instrument 



Boll 



Fig. 675. — Photoclinometer picture. 



The position of the ball with reference to the successive concentric circles of the 

 graduated glass indicates the drift. Interchangeable in any given instrument are various 

 sizes of graduated glasses: usually a glass of 5° nominal size which indicates drift 

 angles up to 7° ; a glass of 10° nominal size which indicates drift angles up to 15° ; and 

 a glass of 20° nominal size which indicates drift angles up to 27°. 



Field Procedure. — Photoclinometer surveys are run on multiconductor cable of the 

 same type used for electrical logging; it is usual to perform this operation in uncased 

 holes after an electrical log has been run, using the same truck. It is possible to run a 

 photoclinometer survey in conjunction with a section gauge survey on the same trip in 

 the hole. 



Referring to Figure 675, in which a 5° glass has been used, it can be seen that each 

 successive circle is 1° apart in drift, and that the position of the center of the ball 

 represents a drift of 6°. 



In the same figure the azimuth angle measured in a clockwise direction from 

 magnetic north to a radial line through the center of the ball is 177°. The azimuth 

 angle with respect to true north, or true bearing, is now calculated by correcting for 

 the declination. It is general practice to give the true bearing in degrees east or west 

 of north or south. 



Presentation of the Results. — The "horizontal footage" is the distance the hole 

 deviates laterally, irrespective of direction, between successive stations. It is computed 

 by the formula : 



Horizontal Footage — length of interval between stations • sine of drift angle 



The vertical footage is the vertical distance between successive stations corrected 

 for shortening due to drift. Unless the drift angle of the hole is very large, this quan- 

 tity differs little from the nominal vertical distance between successive stations. The 

 vertical footage is computed by the formula : 



Vertical Footage = length of interval between stations • Cosine of drift angle 



