had only one answer, which indicated structural conditions far different from 

 those expected. Later dipmeter runs confirmed the work of the stereographic 

 net. When a field is being drilled, the strike is often accurately known; and, 

 if the structure is complicated by faulting and the wells deflected, this method 

 finds frequent use. 



The following example will illustrate the method. At the coring point a well 

 is deflected 55 degrees from vertical, in a direction S25E. Therefore the core 

 top is tilted 55 degrees in the opposite direction. The measured dip in the core 

 is 20 degrees. The estimated direction of true dip is N47W. The angle between 

 the direction of tilt of the core top and the estimated direction of true dip is 

 22 degrees. 



For this problem, the stereographic net is turned with N point at the right, 

 degrees on the stereogram (fig. 25-8). The angle 22 degrees is traced on a 

 great circle, measuring from the upper side of the stereogram. The observed 

 dip, 20 degrees, is traced on a small circle, measuring from the point. On a 

 polar stereographic net, the angle of well deflection and tilt of the core top, 55 

 degrees, is marked off by radii. The stereogram is placed on the polar net, and 

 an arc of a small circle is found that cuts the 22-degree arc and the 20-degree arc 

 exactly 55 degrees apart. By this process we are rotating the horizontal core top 

 into its true position in the well. At the same time the measured dip is rotated 

 counterclockwise into its true position. The amount of true dip is found at d' ' , 

 and in this instance, two possible positions for d' satisfy the conditions. If the 

 estimated direction is correct, the true dip, measured by the small circles of the 

 meridional net, may be either 62 degrees or 44 degrees. Other data in the area 

 may indicate which is correct. 



If a nonparallel well is available within a reasonable distance, the stereo- 

 graphic procedure for nonparallel drill holes (see p. 546) may be applied for 

 additional information on the most probable strike and dip. 



THE DIPMETER One of the most valuable methods for obtain- 



ing true dip in wells is by use of the dipmeter. 

 This special adaptation of electric logging has become particularly useful with the 

 introduction of the continuous dipmeter. 



Several articles describe the dipmeter in detail (Chambrier, 1953). It has 

 three electrodes in contact with the wall of the hole at the same level and 120 

 degrees apart. On passing a tilted bed, the three electrodes will record it at 

 different depths. On the plane determined by these three points, it is a fairly 

 simple matter to calculate the dip. A correction for hole deflection must be 

 made, and usually a correction for magnetic declination. 



No doubt a considerable number of geologists and engineers have independ- 

 ently developed methods for dipmeter interpretation. The writer uses a method 



541 



