Subsurface Maps and Illustrations 



917 



is a north-plunging nose on the Devonian, and if the map were extended 

 on the south, a large closure would be evident in the Devonian. 



In figure 489 the combined thickness of the Pennsylvanian and Mis- 

 sissippian can be determined at any contour intersection simply by sub- 

 tracting the lower value from the higher value, if both data are either 

 above sea level or both are below sea level. Where one datum is above 

 sea level and the other below, the contour values are added to obtain the 

 thickness. 



This method of reducing structural maps from higher to lower hori- 

 zons should be applied wherever the rate of convergence (in feet per 



Figure 486. Subsurface structural map on top of Mississippian. 



mile) between the structure datum and the prospective oil horizon ap- 

 proaches the rate of dip (in feet per mile) on the flanks of the structure. 

 In some regions persistent and sharply defined seismic-reflecting hori- 

 zons are encountered several thousands of feet above the prospective oil- 

 producing formations. Because of the fact that much of the wave energy 

 is reflected here, it is sometimes impossible for the little remaining en- 

 ergy of the shot to reach the lower horizons and be reflected back to the 

 seismometers in sufficient strength to produce usable records. Therefore, 

 a detailed seismic structure map may be obtained on the upper horizon, 

 but sparse data or none on the lower. Now, if a few deep wells provide 

 the necessary convergence data, an isopach map can be constructed from 

 the subsurface information, and, by means of this map, the seismic strue- 



