Subsurface Maps and Illustrations 



895 



Preparation of Subsurface Data 



The subsurface map can be only as good as the data used in its 

 preparation. In surface mapping it is usually possible to observe the 

 structural or stratigraphic behavior of formations over a considerable 

 area around an instrument or rod station. The geologic interpretation of 

 soils, topography, plant ecology, springs, and other natural conditions 

 can aid materially in bridging over areas where the bedrock formations 

 are concealed from direct observation. In contrast to the area! control 

 available on the outcrops, there is only point control for subsurface work. 

 For this reason it is necessary to prepare the data from wells with con- 

 siderable care. 



Figure 468. Geologic cross section compiled from subsurface (well) data. 



Reduction of Datum Elevation 



The elevation on the datum bed is the algebraic difference between 

 the surface elevation of the well and the drilled depth to the datum. Thus, 

 if the surface elevation is 5,000 feet and the depth to the datum is 4,000 

 feet, the datum elevation is 1,000 feet. If the depth is 6,000 feet, the 

 datum elevation is minus 1,000 feet (1,000 feet below sea level). 



If the drill hole is crooked, the apparent vertical depth to the datum 

 will be either too great or too small. In figure 469, A shows a hole that 

 has drifted down-dip and penetrated the datum at point a. Using the 

 actual drilled interval from the surface to point a and the surface loca- 

 tion of the well, the datum would appear to be at b. While the actual dip 

 is to the west, the crooked hole produces an erroneous effect of east dip. 

 In B of figure 469 the hole has drifted in a direction up the dip. In this 

 case the actual drilled interval is less than the vertical depth of the datum 

 bed. The effect is an apparently steepened dip between the two wells. 



Unless a direction survey (chapter 6) has been made, it is impossible 

 to adjust the log of a crooked hole to obtain a correct datum point. 



Dips or dips and strikes determined from cores can aid the subsur- 

 face geologist greatly; but they can also lead him far astray in his inter- 

 pretations. It is hazardous to use core dips indiscriminately in subsurface 



