980 Subsurface Geologic Methods 



etic deposits, and vein and replacement bodies of not too steep a dip or 

 rake. For most geologic examinations, the cuttings obtained from the six- 

 to twelve-inch churn-drill hole are sufficiently large to identify rock types, 

 minerals, and ores. Logging to the nearest foot is possible. In churn 

 drilling, care must be taken to avoid erroneous samples. Contamination of 

 the sample by material from the side of the hole can be partly avoided by 

 keeping the casing near the bottom of the hole. Because there is a ten- 

 dency for gold and other heavy minerals to settle, care also must be main- 

 tained in removing the cuttings or sludge from the bottom of the hole. 

 The usual dart-type bailer will not clean the bottom 10 to 15 inches of 

 hole; and, if settling is suspected, other bailers must be used to remove 

 cuttings more completely. Samples are taken at least every five feet in 

 ore horizons; and, where abrupt changes are expected, this interval may 

 be reduced to two feet. Rarely is all of the sludge kept for a sample. 

 Most commonly, the cuttings are emptied into a splitter, which uniformly 

 reduces the amount to one-half, one-fourth, one-eighth, or one-sixteenth of 

 the original volume. The retained portion may be further split, one half 

 being sent to the assay office and the other half examined and saved by the 

 geologist. As churn-drill holes are sufficiently large for instruments, 

 common oil-industry practices such as electric logging and the measure- 

 ment of radioactivity can be, but rarely are, employed. 



Diamond-Drill Holes r^*^ ^^ The various methods outlined above, under 

 certain conditions, will accurately sample the rock below the surface, but 

 for many purposes a diamond drill is more desirable. By diamond drill- 

 ing it is possible to obtain cores that can be more carefully examined and 

 to drill shallow or deep holes either vertically or at any desired inclina- 

 tion. Because of greater flexibility both as to angle of hole and maneuver- 

 ability, the diamond drill can be employed for exploring narrow, steep 

 veins, irregular deposits, and small pipe deposits, as well as for prospect- 

 ing the same types of deposits as the churn drill. 



In diamond drilling a circular, hollow bit set with diamonds is 

 mounted at the end of a hollow rod or series of rods, which are clamped 

 in the drill. The rods and bit are rotated and advanced by a screw or 

 hydraulic feed with the rate of rotation and advance controlled by a series 

 of changeable gears in the drill. Cuttings are removed from the hole by 

 water pumped through the rods. The bottom rod contains a core barrel 

 5, 10, or 20 feet long, which receives and holds the rock core that passes 

 through the bit. When the barrel is full or when the bit "blocks," the 

 rods are pulled, the core barrel is removed, and the accumulated core is 

 taken from the barrel for examination. Standard core sizes range in 

 diameter from |- inch (EX) to 2^ inches (NX) . In solid homogeneous 

 rock 100-percent core recovery is common, but with increased fracturing, 

 alteration, and friability the core recovery decreases. However, a good, 



■'"' Forrester, J. D., op. cit., pp. 398-411, 1946. 

 " McKinstry, H. E., op. cit., pp. 82-105, 1948. 



