A. On the stereographic net this determination requires rotation of the sphere. 

 (Fisher, 1938; Johnson, 1939; Bucher, 1944; Phillips, 1954) 



Figure 25-6 illustrates the solution of the above example by both cyclo- 

 graphic and polar projection. The first method is easier to visualize, but the 

 second requires less construction. The stereogram is first drawn of the bedding 

 planes of A and B. Now the sphere is to be rotated until plane B coincides with 

 the plane of the stereogram. With point B on the W-E line, move it 

 to the right 35 degrees. At the same time all points on curve A will move 35 

 degrees to the right along the projected small circles or latitudinal arcs of the 

 net. Two convenient points will be suffcient to locate the new position of the 

 great circle, designated A'. The plane represented by A' indicates that the 

 original dip of the beds of series A before the secondary tilt was 74 degrees on 

 azimuth 244 degrees. 



Turning now to polar projection, one may observe that the normals to 

 planes A and B are represented by the points a and b. With the stereogram ori- 

 ented to place b on the W-E line, the sphere is mentally rotated until b is vertical, 

 having moved 35 degrees to the center. At the same time a moves 35 degrees in 

 the same direction along the arc of a small circle to position a', representing 

 the normal to the plane A', which has the attitude found above. 



DETERMINATION OF TRUE A number of methods have been developed 

 DIPS IN WELLS for orienting cores taken from wells and 



drillholes (Johnson, 1939; McClellan, 1948; 



Phillips, 1954) . Where it is possible to use 

 these methods, a determination of the direction and amount of dip is made. 

 Unless the hole is very nearly vertical (as shown by a survey), it is necessary 

 to apply a correction for the deflection of the hole. With the growth of im- 

 proved methods of controlling the direction, many wells are now directionally 

 drilled, and the nature of offshore conditions forces deflected holes in that 

 province, so that the correction of dips for well deflection is growing in im- 

 portance. The stereographic net offers the best method that is accurate for any 

 angle of well deflection. In addition there is a method for orienting any core 

 if the local strike of the beds can be estimated. 



Correction for Well Deflection of Dip in Oriented Core (4) 



We have a given deflection of the well at the cored point of 55 degrees on 

 azimuth 155 degrees. The measured dip in the core is 20 degrees, found by 

 some method of orientation to be in a direction 259 degrees azimuth. What is 

 the true direction and amount of dip? As pointed out by Johnson (1939) there 

 is a basic similarity between the problem of two tilts and the correction of core 



538 



