Subsurface Logging Methods 461 



close resemblance between the patterns of the two curves. Note, for ex- 

 ample, such minor features as the slightly sandy shale streaks at 3,666 

 feet and 3,691 feet, which are represented as slight bulges in the potential 

 curve. The characteristics of electric and drilling-time curves are deter- 

 mined by changes in lithology. The drilling-time break at 3,676 feet is 

 interpreted as anomalous to lithology because a connection was made at 

 this depth and part of this foot was drilled with the clutch out, causing 

 an erroneously timed foot to be registered. By marking on the log where 

 interruptions in drilling occur, such features may be recognized with ease 

 and incorrect interpretations prevented. 



The curves of figure 217, which were plotted from the same data as 

 those of figure 216, show by solid lines the loss of detail in using five-foot 

 intervals instead of one-foot intervals and by dotted lines the effect of 

 averaging when using drilling-rate values. In the upper solid curve on 

 the correlation scale the total time for five feet was used and plotted as a 

 bar curve according to the manner generally practiced on sample logs. 

 In the lower solid curve on the detailed scale, the average time per foot 

 was plotted as a point-to-point curve. In both of these curves the presence 

 of two sands and one shaly sand is observed, but the exact depths at which 

 they occur, their net thickness, and minor lithologic breaks are absent. 

 Obviously, it requires more time to plot the curves in figure 216 than in 

 figure 217, and the information to be gained is disproportionate to the 

 time saved. There is some value in large-interval drilling time and in drill- 

 ing-rate curves to be sure, but their use is restricted to problems where 

 only general impressions are needed either for correlation or lithologic 

 interpretation. In plotting sample logs on the basis of percentage of 

 lithologic types present in each sample, the position of major breaks may 

 be determined by plotting a drilling-rate curve similar to the upper dotted 

 curve in figure 217. Where difficult full-length correlation problems are 

 encountered, however, the drilling-time curves of figure 216 will be found 

 far more reliable and useful. Drilling-rate logs are useful in sample ex- 

 amination work in determining sample lag, but here again the information 

 is only exact within the limits of accuracy of the method used. Further 

 discussion of the application of drilling time and drilling rate follows at 

 the end of the next section. 



Several devices purport to record changes in rate of penetration in 

 terms of feet per hour, and mechanical instruments have been marketed 

 that provide drilling-time logs or data from which these logs may be 

 plotted manually. One of the drawbacks to the use of drilling-time logs 

 has been the time required to plot curves of the type illustrated in figure 

 216. There is no machine available that will reliably record or plot 

 drilling-time logs of this type and eliminate human errors. Considerable 

 experimental work along this line has been done, and the need for such 

 a device is great. Among the best known to the author is the extensive 

 research, laboratory testing, and field demonstrations conducted by Mr. 



