14 DEEP BOREHOLE SURVEYS AND PROBLEMS 



k is open and the motor with its connections stops. If, 

 on the other hand, the bit holder has deviated to the right 

 (Fig. 5) the working circuit is closed, the motor actuating 

 worm di. Now worm wheel di with pins e engages hammer 

 /i to strike the wall of casing a^ as each pin passes lug /. 

 These blows on the casing are clearly perceived at the sur- 

 face and counted by means of a listening earpiece on the 

 rods or any simpler device. 



At the same time as worm wheel d2 starts, the screw spin- 

 dle hi moves slide i to the left in opposition to the action of 

 spring i2. The pendulum contacting on the slide follows 

 this motion until it hangs free, breaking contact k and 

 stopping motor d. Until this happens we get four hammer 

 blows per revolution of worm wheel di, so the observed 

 total number of blows indicates to what extent slide i 

 has moved to the left in order that pendulum t hangs free 

 and vertical; that is to say, it is a measure of the deviation 

 of the bit holder and bit. 



The surface observer has now only to stop the boring 

 blows from time to time and listen to the blows of hammer /i 

 against the boring rods in order to ascertain the extent of 

 the deviation. 



Turning the chisel 90 deg. gives the inclination compo- 

 nent in the plane of the reader's vision as against that of the 

 drawing and where the component is greatest is the direc- 

 tion of maximum inclination. Otherwise two independent 

 pendulums in planes at 90 deg. to one another can be used. 

 Having got this line of major inclination the deep edge of 

 the beveled chisel is turned to deal with it and correct the 

 deviation. 



Though the device gives only the inclination component 

 relative to the chisel and not to the geographical position 

 of the borehole, twisting of the rods need not be heeded so 

 long as the transmitter does not twist relative to the 

 chisel. 



In this way incipient or initial deviations can be quickly 

 detected and corrected. The device can, with suitable 

 modification, be applied to rotary boring and it can also 



