912 



Subsurface Geologic Methods 



the peg model, it cannot readily be altered to incorporate structural data 

 that might become available at a later date, and for this reason it is not 

 generally used for other than illustrative purposes. 



Section Models 



The section model is a series of parallel cross sections drawn or 

 painted on any thin, rigid, boardlike material (fig. 482). These sections 

 in turn are set in properly spaced slots in a solid base. A thick, hard card- 

 board is satisfactory for the sections, although transparent material such 

 as cellulose acetate or cellulose nitrate is sometimes used because it is 

 then possible to view one cross section through another. The section model 

 is more practicable than the solid model in that any one of the sections 

 can easily be removed and revised without affecting the others. 



Figure 482. Models of parallel structural cross sections. 



Miscellaneous Models 



Various types of working models have been made for the purpose 

 of studying the behavior of fluids in permeable reservoirs. Others have 

 been constructed to determine the deformation of rocks under different 

 kinds of stresses. The models discussed earlier are similar to maps in 

 that they are built as an aid to visualizing geologic conditions as they 

 exist today. In contrast, the working models attempt to determine the 

 series of events that bring about these conditions. 



IsoPACH Maps 



An isopach or isopachous map is one which shows by means of con- 

 tours the variations in stratigraphic thickness of a stratum, formation, 

 or group of formations. As in the case of structural maps, isopach maps 

 may be either surface or subsurface, depending upon the class of data 

 used in their construction. The subsurface isopach map is based primarily 

 upon formation thicknesses determined from well cuttings. 



While contours must be drawn to agree with thicknesses plotted on 



