872 EXPLORATION GEOPHYSICS 



example of the old adage "familiarity breeds contempt." Constant vigilance 

 is necessary on the part of those in charge to maintain proper control of 

 storage, transportation, and field operations where explosives are used, 



DRILLING EQUIPMENT FOR SEISMIC OPERATIONS 



Types of Drills. — Drills for seismograph shot holes are of two general 

 types : ( 1 ) the rotary drill and (2) the churn drill. 



The rotary drill has the cutting bit attached to the lower end of a string 

 of hollow drill pipe. This pipe is rotated and simultaneously forced or fed 

 downward by the drill, thus imparting rotation and pressure to the cutting 

 bit. Water or mud fluid is pumped down through the pipe to wash the 

 cuttings up out of the hole. The rotary drill is used almost universally for 

 all classes of seismograph shot hole drilling. 



The churn drill, on the other hand, uses a reciprocating chopping bit, 

 which the drill repeatedly picks up and drops with a soft* rope. At frequent 

 intervals the cuttings produced by the bit are cleaned out of the hole with 

 a bailer. The churn drill is rarely used, and then only in some areas where 

 large underground cavities tend to hamper the return flow of the circulating 

 fluid used in rotary methods. Because of its relative unimportance in 

 seismic work, no further exposition of the churn drill is given. 



Rotary Drills 



There are two general types of rotary drills used in seismograph work, 

 one having hydraulic feed and the other mechanical feed. Their difiference 

 is in the method employed for applying the downward pressure, or feed, 

 to the hollow drill pipe. Both types provide a means for rotating the pipe, 

 and both make provision for circulating fluid down through the pipe or 

 drill rods, thus washing the cuttings away from the bit and conveying 

 them up to the surface in the annular space between the wall of the hole and 

 the string of drill rods. 



Hydraulic Feed Type Drill 



The hydraulic-feed portable drilling unit for seismic shot-hole drilling 

 is a modification of the standard diamond core drill used for mineral 

 exploration and engineering test boring. Figure 546 shows the general 

 arrangement of a portable hydraulic-feed seismograph drill. The truck 

 engine is the source of power for driving the rotary mechanism, the 

 hydraulic feed, the hoist or draw works, and the mud pump. A split-shaft 

 type of power take-ofif is installed in the motor truck directly behind the 

 truck transmission. This power take-ofif provides a means for disconnecting 

 the drive to the rear truck wheels and utilizing all of the power from the 

 truck engine for driving the drill machine. The essential components of 



* Hemp or manila, with considerable stretch or spring which imparts the snap or 

 impact to the bit at the bottom of the stroke. 



