. Deep borehole surveys and problems. Boring. 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, , 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
. Deep borehole surveys and problems. Boring. 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, , 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. Please note that the
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