Miscellaneous Subsurface Methods 565 



could be run inside the drill pipe and allowed to protrude from the end 

 of the drilling bit into open hole to obtain a reading. Originally the small 

 single shot was developed to meet this need. Drillers realize a considerable 

 saving in drilling time by making an accurate single-shot survey through 

 the bit without the necessity of removing the drill pipe from the hole. 



The 2|-inch bronze instrument barrel illustrated in figure 285 is similar 

 to the open-hole barrel used for the regular single shot except that sinker 

 bars are added as needed for weight. A rubber shock absorber has been 

 substituted for the shock-absorber plunger in the large barrel. The upper 

 portion is made with a landing shoe which seats in the bit when the in- 

 strument is in the correct position for taking a reading. In taking a pic- 

 ture the retractable core barrel is retrieved from the bit with an overshot 

 run down through the drill pipe on a wire line. The drill pipe is hoisted 

 about 10 to 20 feet off bottom. A spearhead on top of the small single- 

 shot barrel is engaged by the overshot. Then the instrument and barrel 

 are lowered down the drill pipe until the barrel landing shoe fits into 

 the bit. Various lengths of nonmagnetic sinker bars are used (according 

 to the drift of the well) to position the instrument-angle unit from 6 to 17 

 feet beyond the magnetic influence of the bit and drill pipe. After the 

 picture has been taken, the barrel is hoisted out of the drill pipe, the 

 core barrel is dropped back down the pipe, and ordinary drilling is 

 resumed. Figure 288 shows the instrument barrel in position in a drilling 

 bit. 



The development of special surveying bits, among them the "trigger 

 bit," has increased the use of small single shots. These types of bits, for 

 digging both soft and hard formations, are constructed with a hole of 

 sufficient size to accommodate the 2|-inch barrel. A spring-actuated trigger 

 mounted in the bottom of the bit breaks the core when the bit is drilling. 

 When a survey reading is taken, the drill pipe is hoisted off bottom so that 

 the single-shot barrel may protrude beyond the bit. A reading is taken 

 with the small single shot by lowering the barrel on wire line through the 

 driU pipe. The trigger, which is forced back by the barrel, resumes its 

 normal position when the survey barrel is withdrawn. Figure 290 shows 

 one type of trigger bit. 



Non-magnetic drill collars used in the drilling string immediately 

 above the bit have reduced the cost and delay in surveying wells. K-Monel 

 collars from 10 to 23 feet long are used, according to the drift angle of 

 the well being drilled. The survey barrel is lowered through the drill 

 pipe on wire line and comes to rest on a landing ring in the top of the 

 collar. Some barrels are made with nonmagnetic spacer bars below the 

 bull plug, and are a length that positions the instrument correctly in the 

 drill collar. By either method the barrel is always positioned so that the 

 instrument-angle unit is in the center of the length of the nonmagnetic 

 collar. Below 18- or 20-degree drift the magnetic influence of the bit and 

 drill pipe does not affect the magnetic compass in the single shot. It has 



