pass bearing of the line from the center of the disc through the plumb-bob 

 mark is the direction of drift. Most photographic single-shot instruments also 

 have the protection that the slow exposure characteristic of the sensitized disc 

 makes an erroneous reading improbable — a moving instrument will cause a blur- 

 red image on the disc. 



Single-shot instruments are designed to be run in an empty hole or inside 

 the drill pipe. As they are generally magnetically oriented, they cannot be used 

 in a cased hole. The instruments for use inside the drill pipe are very popular 

 and are designed either to protrude through the drill bit into the open hole where 

 there is no magnetic influence, or to seat in a non-magnetic drill collar (K-Monel 

 or similar metal). Both of these arrangements have the advantage that the 

 instrument is held rigidly in place, and there is no possibility of a tilted instru- 

 ment such as there might be in an open hole. The instrument run inside the 

 drill pipe is retrieved by pulling it out with an attached sand line or by pulling 

 up the drill string. The greatest advantage of these instruments that are run 

 inside the drill pipe is that it is not necessary to make a round trip with the 

 drill string in order to take a reading. 



After the well has been completed, or before the casing is set at an inter- 

 mediate depth, a multiple-shot survey of the entire well is frequently made. 

 This method of well surveying has the advantage that a complete survey of the 

 hole is made in one operation. The operation of the multiple-shot instrument in 

 an uncased hole is essentially the same as that of the single-shot instrument, 

 with the exception that the record is made on a roll of film which is advanced 

 automatically after each reading is taken. Either the camera shutter and film- 

 winding mechanism are operated automatically at a given interval of time by 

 a clock mechanism, or a conducting cable is run from the instrument to the 

 surface and the operation controlled by the operator. During the course of the 

 survey the instrument is lowered to the bottom of the hole, usually on a wire 

 line, but occasionally on drill pipe or tubing; then it is raised slowly to the 

 surface, stopping at regular intervals, commonly every 100 feet, for a reading. 

 The multiple-shot survey may also be taken with the surveying instrument 

 lowered into position within a non-magnetic drill collar; and in this instance, 

 readings are taken as each stand of drill pipe is removed from the hole. 



Two methods have been utilized for making multiple-shot surveys in cased 

 holes. As the drift angle from the vertical can be measured by means of a plumb 

 bob in either cased or uncased holes, the only new problem in cased holes is 

 determining the compass orientation of the drift direction. The first cased-hole 

 method consists of noting the exact postion of a reference mark on the surveying 

 instrument as the instrument is lowered into the hole on a string of drill pipe. 

 The rotation of the drill pipe with respect to a fixed point is noted as each 

 stand is added to the drill string. Surveying instruments are used to make 

 accurate readings of the drill pipe rotation. The algebraic sum of the successive 



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