928 Subsurface Geologic Methods 



graphic horizon is selected as a reference line or datum; this line is 

 drawn straight across the sheet. All other formational boundaries are 

 referred to this line according to the thicknesses of the formation. Thick- 

 nesses are usually indicated by numbers starting with zero at the datum 

 horizon. In the drawing cited above, the datum is the top of the Permian, 

 and thickness numbers start at this point. Figure 494, C, shows the same 

 stratigraphic section, but with the vertical scale two times as large as the 

 horizontal. 



Exaggeration of the vertical scale, in either a stratigraphic or struc- 

 tural section, introduces certain distortions which may suggest nonexistent 

 geologic conditions. This is particularly true in straight-line sections like 

 those shown in figure 494. Sometimes the vertical scale is exaggerated as 

 much as fifty times. Structural sections with greatly exaggerated vertical 

 scales tend to exhibit thinning of the formations where dips are steep, 

 giving the effect of attenuation on the flanks of structures and thickening 

 on the tops of anticlines, bottoms of synclines, and at any other places 

 where the dips are flat. Rates of dips are likewise increased according to 

 the amount of exaggeration of the vertical scale. 



Despite the distortions resulting from exaggerating the vertical scale, 

 there are also some advantages. Local structures in areas of very low dips 

 may not be discernible in sections drawn to natural scale. Where dips 

 are extremely low, there is little distortion except in the rates of the dips; 

 therefore, some increase in the vertical scale is obviously desirable. 



The sections just discussed are drawn along a series of short lines 

 connecting the wells. Occasionally, it is possible to select wells situated 

 on a nearly straight line, but more often the line of section is somewhat 

 zig-zag. This fact alone often gives erroneous impressions of actual ge- 

 ologic conditions, because the line of section at some places may parallel 

 the structural strike or stratigraphic strand lines, whereas at others it may 

 cut across these lines. Where the distribution of wells permits a choice 

 of those to be used in the section, an effort should be made to select the 

 ones that will give the straightest line of section. 



A true cross section is a straight line on the map and shows structural 

 or stratigraphic variations between the wells. Such a cross section is more 

 difficult to draw and requires considerably more data for correct presen- 

 tation. 



The best way to construct a true cross section involves the use of 

 structural and isopach maps. Incidentally, such a section is an excellent 

 check on the accuracy of a complete series of isopach and structural maps. 



Figure 495 consists of one structural map and three isopach maps; 

 the cross section is drawn on the basis of data taken from the maps. 

 The method employed is as follows: 



The line of section is drawn on each of the maps. In the case illus- 



