Sometimes the upper part of a well may be drilled at such a rate that two- 

 foot timing is desirable. In this case the horizontal or time scale will be one- 

 half that for one-foot timing so that the relative amplitude of the curves will 

 remain the same. The time scale should be given on the log when changing 

 from two-foot to one-foot recording. It has been found by experience that a 

 time scale of five minutes per inch on one-foot recording is most acceptable to 

 yield an amplitude corresponding to electric-potential curves. On this scale each 

 unit represents a quarter of a minute, which is the base unit tabulated in Figure 

 18-5. If conditions in an area are such that drilling is much faster or much 

 slower than normal, the time scale may be changed to meet the requirements 

 of that area. It may be preferable, where drilling rates are very slow, to shift 

 the base line instead of changing the time scale. In either manner of plotting the 

 log, exceptionally slow feet will be encountered which will exceed the visible 

 scale on the log. Such off-scale footage may be plotted on scale using the thirty- 

 minute time as the zero base and cross-hatching the off-scale portion for con- 

 venience. The resulting log will appear similar to off-scale electric-resistivity logs. 



The question may arise as to what to do with apparent errors in time values. 

 If during continuous drilling, the brake is set while the driller is busy elsewhere, 

 the weight may drill off the bit before the drill stem is lowered. This often occurs 

 in drilling soft formations, and the geolograph will fail to record the true net 

 time required in the drilling. It is recommended that the values be plotted as 

 they actually are recorded even when they are known to be fictional. The 

 reason for this is that, if an effort is made to make arbitrary corrections, the 

 value selected may have as little relation to the correct value as the recorded 

 value. Therefore, if all values are plotted as recorded, the correction can be 

 made in the interpretative phase of the work. One very fast foot in the midst 

 of normally slow drilling, or one very slow foot in the midst of normally fast 

 drilling would present no problem in its interpretation. The writer has followed 

 this practice consistently because sometimes an apparent time error may not 

 be an error at all, but rather may represent a very abrupt lithologic change 

 over a short distance. A thin streak of shale in a soft sandstone, an ironstone 

 bed located in soft shaly sand, or a thin streak of soft sandstone interbedded 

 with a very hard limestone would appear as false recordings such as are 

 indicated above. 



As a suggestion, there are many advantages to be gained from plotting the 

 log with black ink rather than with pencil. The tabulated values are plotted 

 as coordinates on the log with respect to depth and time. A sharp, medium- 

 hard pencil is ideal for this purpose. These points may then be connected by an 

 ink line, using a ruling pen and metal-edged ruler. The inked curve is much 

 easier to examine than a penciled curve, particularly under poor lighting 

 conditions. An inked curve is preferable to a penciled curve also because it 

 facilitates reproduction by direct printing or photostating. 



386 



