Miscellaneous Subsurface Methods 



575 



can be rotated in relation to the base, and an orienting lug similar to 

 that shown on the film is mounted on the protractor base. In reading the 

 film the time interval noted on the field sheet at which the instrument was 

 held motionless in the well indicates the correct picture to be projected. 

 Magnetic surveys are read in the same way that a group of single-shot 

 pictures would be solved for drift and direction. An oriented survey is 

 read by aligning the orienting lug photographed in the angle unit with 

 the lug on the protractor. The compass card of the protractor is turned 



Figure 296. Derrick man aligning drill-pipe clamp and telescope upon distant target. 



until the correct azimuth resulting from summing the pipe-rotation angles 

 for that particular reading is directly under the lug. Then the direction 

 of the well is determined by superimposing a line from the center of the 

 picture through the intersection of the plumb-bob cross hairs to the edge 

 of the compass on the protractor. Drift determinations are made by count- 

 ing the concentric rings from the center of the picture and interpolating 

 between rings. 



Drift and direction readings are derived in the same manner for each 

 point at which the drill pipe was allowed to remain motionless during 

 the running of the field survey. The multiple-shot instrument exposes the 

 film every 90 seconds. As the drill pipe is being run into the well, many 



