468 Subsurface Geologic Methods 



This fact may be illustrated further by the following mathematical 



relationships: 



distance 



rate = : 



time 



distance = rate X time; or 



distance = feet per hour X hours 



distance 



time = 



rate 



If distance = 1, then rate = — or, conversely, time 



time rate 



It is important to recognize the significance of using unit-depth in- 

 tervals in these calculations. If, by any mathematical means, drilling rate 

 is calculated from drilling-time data, the rate represents only the average 

 speed of penetration for the depth interval used. Consequently, drilling 

 rate obscures any minor variations that may occur within the depth interval 

 used. As has been mentioned previously, some experimental work has been 

 done by Mr. Banning and the writer in the recording of drilling time on 

 one-tenth foot intervals. Correlation with complete core recoveries ol 

 sections thus logged showed a definite and important relationship between 

 lithology and drilling time. If, in this drilling-time logging, full foot in- 

 tervals had been used and plotted as drilling rate, the resulting average- 

 value log would fail to reflect these lithologic details. Therefore, the pur- 

 pose to be served will determine the depth intervals to be used and the 

 character of the curve to be plotted. 



Method of Preparing Drilling-Time Logs 



The experience of the writer in the use of drilling-time data has been 

 based on records obtained from "Geolograph" charts for the most part. 

 Although there are other means by which usable data may be collected, 

 perhaps the most practical source of complete drilling-time records is the 

 "Geolograph." For this reason it is considered in place here to describe in 

 detail the technique recommended in translating the original record into 

 the drilling-time log for which multiple uses have been described. 



It is unnecessary to include the maintenance of the equipment, which 

 is the responsibility of the service company. A few points should be 

 kept in mind, however, by the geologist desiring to obtain as perfect records 

 as possible. A drilling-time log is the plotted curve of two components, 

 time and depth, each requiring accuracy within the limits of observational 

 errors. The "Geolograph" machine provides two inking pens, generally 

 supplied with inks of different colors. One pen indicates by vertical and 

 horizontal lines the amount of time that drilling is in progress and when 

 it has been stopped or when the bit is off bottom. The other indicates by a 



