184 DEEP BOREHOLE SURVEYS AND PROBLEMS 



This method was used to survey a hole in the Heidelberg 

 district of the Transvaal which ultimately deviated 58 

 deg. off the vertical, the hole being 6,656 ft. deep. A 

 special pilot wedge device (Fig. 122) 2 in. in diameter 

 and 18 in. long with oblique face 6 in. long was first lowered 

 (wedge face upward) and its being solid on the floor 

 assured by letting down the rods. Another rod 3 ft. long 

 screwed at both ends was used to get the wedge position. 

 This last rod (Fig. 123) had a spiral spring on one end 

 and a 2-in. cup with a M-in. diameter brass pin through it 

 at the other end. This cup was filled with lead which 

 projected about 1 in. beyond its edge and turned to its 

 diameter. The end of the rod with the spiral spring was 



Pin T^ P'"'^^^ 



Prick I ^ Prick . \ 



Fig. 121. 



screwed into the instrument base instead of the lower plug. 

 The top end of the instrument was screwed into a brass 

 tube 10 ft. long and then again screwed to the ends of the 

 drill rods. It was then lowered in on to the wedge. 



A chisel cuts an impression in the lead, a photograph 

 being taken of the magnetic needle at the same time. 

 A disc of lead is sawed off on gaining the surface and the 

 direction of the wedge calculated. The guide wedge 

 (Fig. 124) is an exact counterpart of the pilot" wedge and 

 is screwed into the said main deflecting wedge, which is 

 solid, 2 in. in diameter and 7 ft. long. These wedge 

 devices are not an essential part of the equipment but are 

 added because they enable other sections of the reef to be 

 taken in the same borehole, saving the expense of extra 

 holes. 



