FLUID METHODS OF SURVEYING BOREHOLES 117 



average error in \p will be about + 5 deg. and for ^ = 10 deg. 

 about + 3 deg. 



The manner of proceeding with a survey depends on the 

 accuracy possible in measuring the angle \}y. First get the 

 general dip of the hole by lowering an electrolytic registering 

 apparatus on a steel wire and measuring 6 at every 250 

 ft. or thereabouts. 



1. If 6 is at least 15 deg. the accuracy in measuring \p 

 will be such that we may proceed by the method of succes- 

 sive bearings, i.e., a string of drill rods 30 to 100 ft. long is 

 lowered into the hole. At the end of each is an electrolytic 

 registering device and these are rigidly orientated to one 

 another by a scribed mark or generatrix on the rods. 

 The first registration is made with the upper apparatus 

 in the mouth of the hole so that the traverse can be referred 

 to a fixed reference mark on the surface. Then the whole 

 string is lowered until the upper apparatus is in the position 

 formerly occupied by the lower one and a new registration 

 is made. In this way the hole is surveyed by successive 

 determinations length after length. 



2. If d is less than 15 deg., i.e., the hole is more or less 

 vertical, the following method is used because the successive 

 bearings method would be risky, since any errors would be 

 cumulative and might render the survey void. To prevent 

 this, orientation from the surface is used. In this method 

 a string of drill rods is let down from the surface and it has 

 an externally scribed generatrix. Part of it is kept out 

 of the hole for orientating the said mark. Registering sets 

 are put in every 200 ft. and also orientated to the mark. 

 Instead of this mark it is often found more reliable to have a 

 device known as an ''orientating coupling." This coupling 

 (Fig. 65) is so made that it does not allow torsion between 

 the rods. Ordinary drill rods make up the rod string and 

 the joints are pitched and nailed to hold them tight. This, 

 with the orientating couplings at each end, keeps the rods 

 straight and orientated to the surface reference mark. 



When the rod string and apparatuses have been lowered 

 into the hole the current from a surface battery is switched 



