Subsurface Laboratory Methods 119 



gives more information than is possible here, notes that much larger 

 cuttings can be obtained by reverse circulation. 



In cable-tool drilling, a chisel-shaped bit attached to a cable is al- 

 ternately raised and dropped. After drilling five feet or so, the bit is 

 removed from the hole and the accumulated cuttings are bailed out of the 

 hole with an elongated bucket having a valve at the lower end. The bailer 

 is emptied into a bucket or barrel and the cuttings are washed free of mud. 

 The hole is cased past places where excessive caving occurs or where ex- 

 cessive water enters the hole. Cable-tool cuttings have the following char- 

 acteristics that limit their usefulness: 



1. Some contamination of the cuttings from the uncased parts of 

 the hole occurs. Usually some material from five to ten feet above is in- 

 cluded because the hole is widened as drilling proceeds. In general, cable- 

 tool cuttings are less contaminated and more representative of the interval 

 drilled than are rotary cuttings. 



2. Stretching of the drilling cable results in inaccuracies in depth 

 measurements up to twenty feet or more. Steel-line measurements at in- 

 tervals provide a method of correction. 



3. The cuttings are usually less than three-eighths inch in diameter. 



4. Poorly cemented pay sands may be blown out of the hole by 

 escaping gas, and consequently samples, if any, from such horizons may 

 not be representative. 



5. Electric logs cannot be made on cased holes and therefore are 

 not available to supplement the sample studies. 



6. In some areas cable-tool drillers are not so careful in collecting 

 and washing samples as are rotary drillers. 



Cores may be collected with rotary, cable-tool, or core-drilling ap- 

 paratus. The chief disadvantages of coring are (1) high cost, (2) selective 

 loss of weaker or more soluble rocks, and (3) storage and transportation 

 of the cores. Improvements in coring methods are reducing costs and in- 

 creasing core recovery. 



Megascopic and Binocular Examination 



To a certain extent, the old saying "the closer you look, the less you 

 see" is applicable to some properties of sedimentary rocks. The color, 

 texture, and composition of cuttings and cores and the bedding, cross- 

 bedding, and porosity in cores can often be determined better and faster 

 by megascopic examination and simple chemical and physical tests than 

 in other ways. 



By placing ten to twenty cable-tool or rotary samples on sheets of 

 paper or in cardboard or metal trays and observing their appearance mega- 

 scopically, major changes in color, texture, and composition are readily 

 apparent if the samples have been washed clean of drilling mud. The 

 boundaries of beds or formations that differ markedly from those above 

 or below often can be determined quite accurately. After the major breaks 



