96 DEEP BOREHOLE SURVEYS AND PROBLEMS 



can be accelerated by strengthening it. In a straight 

 hole this outline is a circle and an ellipse in an inclined hole. 

 The angle which the plane of this ellipse forms with the axis 

 of the horizontal plane is equal to the angle between the axis 

 of the flask and the vertical, i.e., the sought 

 angle of deviation (Fig. 47). Let MN be the 

 vertical and AB the vessel axis then the angle 

 j8 formed between these lines is the required 

 deflection angle. In the vertical position of 

 the flask the surface is at CD. 



Actually the surface is at EF due to the 

 deviation with CD forming the angle 8 

 with EG. 

 Now § can be solved from 



FG .^. 



tan 5 = ^ (7) 



where FG is the double difference between the highest and 

 lowest positions of the fluid surface while EG equals the di- 

 ameter. For carrying out the method by means of HF a 

 flask half filled with dilute acid is let down to the place 

 where we wish to measure the deviation. In modern 

 practice a larger tube is used closely fitting the borehole, 

 say a core tube, which, however, should not be shorter 

 than 15 ft. In a tube of less length or of essentially smaller 

 diameter than the borehole we can not assume that the axes 

 of the hole and tube coincide or at least any line parallel 

 thereto. If, on the contrary, the tube is about 5 m. long 

 and its diameter only a few millimeters less than that 

 of the hole, the angle between the tube and borehole axes 

 will be so small that it may be neglected for practical 

 purposes. In order to protect the flask in the tube against 

 the pressure of the water column present in the borehole, 

 the tube must be made airtight above and below. The 

 upper joint has a rod thread and both joints are provided 

 with neck and hook flasks, whereby they can be rapidly 

 screwed on or off. The airtight joint is absolutely 

 necessary because otherwise water would penetrate into 

 the tube and at the greater depths the pressure would 



