Subsurface Logging Methods 473 



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 record- 

 ing 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 219. 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, excep- 

 tionally 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 

 convenience. 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 "Geolo- 

 graph" 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 inter- 

 pretative 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 sand- 

 stone interbedded with a very hard limestone would appear as false record- 

 ings 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 photo- 

 stating. 



In conclusion, it may be suggested that the free exchange of drilling- 

 time logs would prove of considerable mutual benefit to geologists in 

 much the same way that the interchange of electric and radioactivity logs 



