volved to the S position and mentally moved to the center. Electrode I will 

 now be 30 degrees to the left of N and II will be 30 degrees to the right. After 

 plotting the apparent dips of I and II, we apply procedure 2, and the true dip 

 D is found to be 49 degrees on azimuth 271 degrees. But these angles must be 

 corrected for well deflection. 



Correcting for Well Deflection (9) 



The direction of well deflection (az. 35 degrees) has been already marked 

 on the stereogram. To apply procedure 4, the well will be treated as a vertical 

 normal at the center of the net. The normal d to the bedding plane found above 

 is, of course, on azimuth 91 degrees. With the well direction revolved to the E 

 point, the well is tilted into its true position w', and the bedding plane normal 

 d moves 8.5 degrees right on the arc of a small circle to d' . The dip of the 

 bedding plane is now found to be 54 degrees on azimuth 266 degrees. One 

 more correction must be made. 



Correcting for Magnetic Declination (10) 



In most instances, the dipmeter instruments record all directions with 

 reference to magnetic north. The simplest way to make the correction is to 

 mark the magnetic declination for the locality of the well on the stereographic 

 net, then to revolve the stereogram to place the N point on magnetic north. In 

 this example, the magnetic declination is 16 degrees east. Final corrected dip 

 is therefore 54 degrees on azimuth 282 degrees. 



INTERPRETATION OF The use of diamond drilling or core drilling 



CORES FROM NON- for exploration often presents the problem of 



PARALLEL DRILLHOLES interpreting structural information contained 



in the cores (Fisher, 1941; Bucher, 1943; 

 Gilluly, 1944; Phillips, 1954). In most cases the cores are unoriented. If a 

 key bed can be recognized in three holes not in a straight line the dip and 

 strike can be easily determined. In the absence of key beds, if the local strike 

 of beds can be estimated with reasonable accuracy, stereographic procedure 5, 

 for orientation of unoriented cores, may be used. 



If neither of the above methods is applicable, the method of drilling two 

 or more nonparallel holes may be used, and the information obtained com- 

 bined. Here the number of possible answers resulting may be one, two, three, 

 or four. Selection of the correct one may be made from the probable strike, or 

 perhaps by another carefully located drill hole. 



Stereographic Procedure for Nonparallel Drillholes (11) 



In Figure 25-13 it is seen that the normal to a bedding plane makes an 

 angle with the axis of the hole equal to the measured dip of the bedding plane. 



546 



