880 



EXPLORATION GEOPHYSICS 



Drill Rods and Bits. — The hollow drill rods or drill pipe ordinarily- 

 used for seismograph drilling are the size "N" (2^'" outside diameter) 

 external flush rods in exact 10-foot lengths. (Figure 549-1.) f 



Numerous types of bits are used in seismograph drilling, depending upon 

 the hardness of the ground being penetrated and the diameter of the hole. 

 The most common type of bit is the ordinary fish tail or drag bit (Figure 

 549-J). These drag bits are available with either two, three, four or six 



Fig. 552. — Mud pump of mechanical feed type drill, (A) drive rod 

 or Kelly; (B) rotary table; (C) feed chains. 



wings. The cutting edge of each wing is faced with tungsten carbide alloy, 

 producing a very hard wear-resistant edge. In soft drilling the two-wing 

 fishtail is the fastest cutting. With increasing ground hardness, it is cus- 

 tomary to use bits with more wings or cutting edges. There are openings 

 through all of these bits to permit the required circulation of water or mud 



t For detailed dimensions of these rods and couplings, see U. S. Department of Commerce, 

 Bureau of Standards, Publication CS-17-30 on Commercial Standards. 



