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Subsurface Geologic Methods 



rate is an average value involving the drilling of several feet. The former 

 is more exact and is useful where detailed lithologic information is re- 

 quired and where difficult correlation problems may be solved only in the 

 study of minor features that are best disclosed in the pattern of a log 

 plotted from drilling-time data. The latter method requires less time for 

 recording original data and for plotting and is useful where rapid interpre- 

 tations are required and where it is used in conjunction with other methods 

 that are based on average data as well, such as sample-examination logs. 

 As will be shown further in this section the pattern revealed by a 



Figure 216. Two curves plotted from data on drilling-time chart of figure 215. 



drilling-time log has characteristics very similar to that of an electric- 

 potential curve. The differences between drilling-time and drilling-rate 

 logs may be illustrated by the electric log of a well drilled and the original 

 drilling-time record of the same well. In figure 215 the porous sands 

 3,669-3,673 feet did not drill as fast as the upper and lower sands and may 

 the relatively close spacing of the foot marks on the drilling-time chart 

 and are confirmed by the electric log. It will be noted that the streak 

 3,669-3,673 feet did not drill as fast as the upper and lower sands and may 

 be interpreted as a shaly sand. This interpretation is supported by the 

 potential curve of the electric log. 



In figure 216 are two curves plotted from the data on the drilling- 

 time chart of figure 215, using the normal scale for correlation on the 

 left and the enlarged detail scale for lithologic interpretation on the right. 

 A close comparison of the detailed curve with the potential curve in 

 figure 215 reveals not only the corresponding sand sections but also a 



