PROBLEMS 273 



Third Alternate Graphic Solution. — This method, which 

 is an extension of the third computation method above 

 and of the method for two vertical boreholes dealt with 

 previously, is the quickest graphical solution of the three- 

 borehole problem (Fig. 182). Plot the horizontal posi- 

 tions of the boreholes, i.e., draw A,B and C or A,Bi and Ci 

 in their true relative positions in plan (Fig. 182). Let hy 



Di reef I on G of Strike 



Fig. 182. 



be the difference in depth of A and C, and h% the difference 

 between B and C (here hi is CCi of Fig. 179 above and h^ 

 is EC of the same figure). At A as center draw the circle 

 of radius r^ = hi cot 5 and at B draw a circle with radius 

 7-2 = hi cot 5, the full dip angle 5 being got from the cores. 

 Draw the tangents to both circles from the shallowest 

 point C and they together will provide one line, so giving 

 the strike bearing and thus the dip bearing by +90 deg. 



Special Cases of the Three-borehole Problem 

 Two special cases arise in practice, viz. : 



1. When all the boreholes hit the stratum at the same 

 altitude respecting the datum; we shall not deal with this 

 case which presents no features of note. 



2. When Two of the Holes Hit the Stratum at the Same 

 Altitude, the Other Being Either Higher or Lower Than 

 These (Fig. 183). — ^Let A and B be the two boreholes 

 of similar depth respecting the datum. Pass a horizontal 

 plane through AB which is the strike of the stratum and it 

 will cut the C borehole, here assumed shallower, in Ci. 

 Drop perpendiculars from C and Ci on to A B at F. The 



