subsurface. The principal difference in these sections and in surface and sub- 

 surface maps is that the surface map or section is constructed from surface data ; 

 that is, from outcrops. The subsurface section or map is constructed from data 

 supplied by wells that have penetrated recognizable formations. 



Figure 24-1. Geologic cross section compiled from surface data. 



The subsurface map can be only as reliable as the data used in its prepara- 

 tion. In surface mapping it is usually possible to observe the structural or strati- 

 graphic 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 con- 

 trast to the areal control available on the outcrops, there is only point control 

 for subsurface work. For this reason, the data from wells must be prepared 

 with considerable care. 



Reduction to 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 5000 feet and the depth to the datum is 4000 feet, the datum 

 elevation is 1000 feet. If the depth is 6000 feet, the datum elevation is minus 

 1000 feet (1000 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. Figure 24-3^4 shows a hole that has drifted 

 downdip and penetrated the datum at point a. If one uses the actual drilled in- 

 terval from the surface to point a and the surface location of the well, the datum 

 would appear to be at b. Although the actual dip is to the west, the crooked hole 

 produces an erroneous effect of east dip. In B of Figure 24-3 the hole has drifted 



454 



