Subsurface Logging Methods 469 



slanting stroke the completion of exactly one foot drilled, or two feet if 

 set for recording on two-foot intervals. Therefore, the exact depth marked 

 by each stroke of the pen must be known and the length of time occupied 

 in the drilling must be determined. Plate 10 is a "Geolograph" chart show- 

 ing essential data used in making a drilling-time log. 



It is strongly recommended that the geologist using complete drilling- 

 time data keep his own pipe tally. Practices differ among contractors in 

 keeping pipe tallies, but inasmuch as the object here is to know the depth 

 indicated by each mark on the chart, the geologist's tally must show all 

 pipe in the hole at the time the mark is made. This becomes most important 

 when changes in the drilling string are made, as for coring. The tally 

 should show first the bit, subs, and drill collars in the string, followed by 

 each joint of drill pipe added. The kelly should be measured accurately 

 and its length recorded. The geologist should also observe whether it is 

 the practice of the driller to drill the kelly down or make his connections 

 with some length up on the kelly. The "Geolograph" automatically shows 

 when a new joint of pipe is added, and the geologist should write on the 

 chart the kelly-down depth of each connection, accurate to the nearest 

 hundredth of a foot, as shown on his pipe tally. (See connections at 

 6,017.27 feet and 6,048.61 feet in pi. 10) . 



The charts are generally changed twice a day and when received for 

 translating will show the date, time of chart change, depth of beginning 

 and end of each chart, and correct depth at each connection or begin- 

 ning of a round trip. The depth of each mark or every fifth mark be- 

 tween connections should then be written on the chart, as 6,000 feet, 

 6,005 feet, 6,010 feet, et seq., plate 10. If the drillers have been 

 very careful in throwing in the clutch at the proper time, the number 

 of marks should be identical with the number of feet drilled. Often 

 this is not the case, and observation of drilling operations and experi- 

 ence with how such discrepancies occur will show the geologist where 

 errors take place. The most common error of this type is made by 

 the driller in failing to throw in the clutch after making a connection at 

 exactly the same depth as when it was thrown out prior to making the 

 connection. Changes in drilling weight caused by settling out of rock cut- 

 tings may cause what might be called "false drilling" or the redrilling of 

 depth without drilling a new formation. Or, if the driller fails to throw in 

 the clutch as soon as the bit reaches bottom with the same weight as when 

 it left bottom, some new hole may be drilled without being recorded. 

 (See 3,676 feet in figs. 215, 216.) Errors of these kinds are readily 

 understood when one realizes that the drilling crew is most busily occu- 

 pied when connections are made. Often the correction for depth will be 

 made immediately after a connection, but the geologist must use his best 

 judgment in making all corrections so that when a depth is assigned to 

 a mark it will be correct. 



Where no "extra" marks have been made and no "skips" noticed, 



