Subsurface Logging Methods 489 



Sampling 



In consddering the interpretation of a spectrochemical sample log 

 it is important to keep in mind the fact that analyses are based on five- 

 milligram samples, an amount that may be placed under the little finger- 

 nail with slight discomfort. Therefore, every precaution must be taken to 

 insure that the tiny sample is as representative as possible of the ten- 

 foot (or smaller) interval from which it is taken. Where cores are avail- 

 able and core recovery is high, proper sampling is achieved simply by 

 knocking off closely spaced chips, lumping together the chips from each 

 five- or ten-foot interval. Good cable-tool cuttings yield adequate samples, 

 with rapid inspection serving to eliminate obvious extraneous materials. 

 As in other samples studied, rotary cuttings pose the most difiicult prob- 

 lems. However, by judicious use of corollary data from electric logs and 

 drilling time it is usually possible to assemble a sample containing a high 

 percentage of material representative of the indicated interval. In most 

 cases a cursory examination of the sample under binocular microscope 

 and picking of chips with tweezers are unavoidable. In other cases where 

 the history of the drilling procedure is well-known and confidence can 

 be placed in the purity of the cuttings, a portion for analysis can be 

 drawn directly from the sample sack. The writers have used both tech- 

 niques and have derived interpretable results from unpicked samples. 

 However, in view of the postsampling investment in time and effort, it 

 seems more prudent to keep the sampling method as rigid as is practical, 

 at least in the exploratory stages of any investigation. 



Sample Treatment 



In the writers' first paper on spectrochemical well logging, the 

 methods employed were limited by the equipment available and by the 

 difficulty of applying quantitative spectrum analysis to relatively re- 

 fractory and quite nonconducting rock powders. In conventional metal- 

 lurgical spectrum analysis both arc- and spark-excitation methods may 

 be used, because the material under investigation usually consists of 

 regularly shaped cast or machined pieces of metal or alloy. The regular 

 shape permits reproducibility of arc length, and the high conductivity of 

 the metal permits the specimens to form the electrodes themselves. Fur- 

 ther, the reasonably uniform composition of metallic alloys permits more 

 or less constant volatilization of the contained constituents once the arc 

 has been established. 



On the other hand, quantitative analysis of minerals or of rock 

 powders by the spectrograph presents some unusual problems. In the 

 first place the sample must be representative of the particular mineral or 

 of the total amount of the specimen gathered. In the case of opaque min- 

 erals the specimen may be polished and samples taken by using a micro- 

 drill under a microscope, insuring that only the mineral under investiga- 



