470 Subsurface Geologic Methods 



there may accumulate fractional-foot errors, which may be designated as 

 "creep." Over long-continued drilling, involving several connections or 

 even round trips, a correction may be required, and it is difficult to know 

 where it should be made. Having written the kelly-down depth at each 

 connection and the correct depth at a round trip, accurate in each case 

 to the nearest hundredth of a foot, k will be obvious what depth should 

 be assigned just before or just after making a connection. For example, 

 if a connection has been made at 6,079.04 feet in figure 219 and a foot 

 mark is shown immediately after the connection, it would be reasonable 

 to indicate the depth of the mark prior to the connection as 6,078 feet, 

 as the total number of feet drilled corresponds to the number of foot 

 marks on the chart. Had the connection been made at 6,078.97 feet, 

 however, and a foot mark shown just prior to making the connection, 

 with a normal drilling interval following the connection before the next 

 foot mark was recorded, it would be reasonable to assign a depth of 

 6,079 feet to the foot mark prior to the connection. Apparent errors of 

 this type would be the result of "creep." 



The foregoing is only suggestive of some generalizations that may be 

 made in preparing the chart for time determinations. Frequent use of 

 drilling-time charts will increase the speed and accuracy in obtaining 

 proper depth designations. The chart reproduced in plate 10 provides 24 

 inches for recording twelve hours of time, or two inches per hour. The 

 hour is divided into twelve divisions of five minutes each, and these are 

 further divided into one-minute divisions. For high speed, when the re- 

 quirements demand it at the sacrifice of accuracy, the eye can measure the 

 distance between foot marks within an accuracy of approximately twenty 

 to thirty percent. Many uses of drilling-time logs require greater accuracy 

 than this, however, particularly when minor breaks in the over-all pattern 

 are used to correlate with minor breaks on an electric-potential log. To 

 obtain greater accuracy, with an error of less than one percent, measure- 

 ments may be made with an engineer's scale, using the thirty-divisions-per- 

 inch scale for the purpose. This scale, placed on the chart having two 

 inches per hour, will provide sixty divisions per hour or one per minute. 



The use of a printed form to tabulate the time readings may be con- 

 sidered as an extra step, and some may suggest plotting directly from 

 observation of the chart. It has been found that this extra step not only 

 avoids many errors that might otherwise be made but actually saves time. 

 In addition one often wishes to plot the same data on more than one scale, 

 in which case the time saving is considerable. The same person may make 

 the readings and write the tabulation, but if one person keeps his eye on 

 the scale and chart and another writes down the time on the form, it will 

 save two-thirds of the time required for this operation. Additional time 

 can be saved in the following step, plotting the log from the tabulation, 

 by having one person read the units off as another plots the coordinates. 



