gfafitvi 24 



SUBSURFACE 

 MAPS AND 

 ILLUSTRATIONS 



Julian W. Low 



PREPARATION OF The term subsurface map may be somewhat 



SUBSURFACE DATA confusing in that nearly all types of both sur- 



face and subsurface maps display features that 

 are actually concealed at the surface of the ground by soils, alluvium, and other 

 types of overburden. The geologic formation or horizon contoured on a surface- 

 structure map may lie beneath other formations over the greater part of the map 

 area, as shown in the cross section in Figure 24-1. In this figure, points 1, 2, and 

 3 are outcrops of the datum horizon where direct instrumental observations can 

 be made. Point 4 is an outcrop of bed C, stratigraphically below the datum, and 5, 

 6, 7, and 8 are outcrops of beds B and A, stratigraphically above the datum. 

 Control points a, b, c, d, and e, are computed from instrumental observations 

 obtained on the outcrops. The position of the datum surface from 1 to 2 and 

 3 to e is restored above the actual surface of the ground, and at all other places 

 the datum is covered by other formations. 



Figure 24-2 is a subsurface cross section. Neither the datum nor the two 

 key beds A and B crop out at the surface. The control points, a, c, and e, are 

 determined from the logs of wells which penetrate the datum bed. Control 

 points b and d are computed from the drilled points on key bed A. The similar- 

 ities in the section shown in Figures 24-1 and 24-2 are obvious. Indirect methods 

 are used in the construction of both; yet one is a surface section and the other a 



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