which may be used in the solution of many geologic and engineering problems. 

 The amount of section so logged and the scale employed are determined by the 

 requirements of the individual problem. Detailed instructions are included 

 whereby a geologist not experienced in the use of this technique, may learn 

 how to prepare and interpret drilling-time logs in any area in which he may 

 be interested. 



DEFINITIONS Early methods of determining rate of pene- 



tration were crude and approximate and 

 generally consisted in recording the time required to drill a certain number of 

 feet of hole or the number of feet drilled per hour. Data thus acquired may be 

 adequate for some purposes, but as the technique of using drilling time became 

 more widely employed, certain advantages were observed in determining the 

 specific net amount of time required to drill each foot. Before discussing the 

 application of drilling-time data, the distinction should be understood between 

 drilling rate and drilling time. 



By definition, rate of penetration is a fixed lithologic property, even though 

 it may be diagnostic only when used as a relative term. Drilling time is the 

 duration of time required in the actual drilling of a unit of depth. Drilling rate 

 is the number of units of depth drilled in a unit of time. 



The foregoing may be illustrated by comparison to an automobile speed- 

 ometer. When a car travels a mile in a certain number of minutes and seconds, 

 it is a measure of speed comparable to the time occupied in the drilling of one 

 foot of formation, which we have defined as drilling time. When a speedometer 

 indicator points to 45, it indicates that the car is traveling at the rate of 45 

 miles per hour. This is comparable to rate of penetration measured in the 

 number of feet drilled per hour, which is the definition given for drilling rate. 

 The distinction is more than academic and should be clearly understood, 

 because the interpretation of rate of penetration is strongly influenced by the 

 method of recording. 



DRILLING TIME: The relationship between rate of penetration 



DIAGNOSTIC OF and lithology has been understood for many 



LITHOLOGY years. The application of drilling time was 



recognized at least seventy years ago, and 

 the interpretative value has been appreciated for more than twenty years. The 

 use of drilling-time data has widened continuously as mechanical devices for 

 their recording have become available and new applications of the data have 

 been found. 



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