202 



DEEP BOREHOLE SURVEYS AND PROBLEMS 



Anderson's sand-line method a set of expansible steel-spring 

 guides is run both above and below the instrument in its 

 shell to prevent rotation in azimuth. A practically 

 frictionless swivel connection is made from the end of the 

 sand line to the instrument container. 



Readings are taken at each stand length and the station 

 distances measured on the drill lengths, the operator taking 

 the instrument as delivered from the well and interpreting 



Fig. 142. — Plan of very deep borehole surveyed by Anderson. 



the results on a special orientating stand (Fig. 141). 

 Orientation is thus measured mechanically the direction 

 of drift being referred to a north-south line on the derrick 

 floor so that at each exposure the directional deflection 

 is known at the surface. 



Interpretations will average within about 7 ft. of arc 

 of being correct for vertical angles and 30 ft. for azimuth. 

 The instrument is also self-checking in that all recorded 

 points must fall on a curve when plotted. Various surveys 



