Subsurface Maps and Illustrations 



901 



normal fault with a low-dipping plane invariably results in a zone where 

 the datum surface is absent. This zone is called a "datum gap." The 

 breadth of the datum gap is determined by the degree of dip in the strata, 

 the dip of the fault plane, and the amount of throw. The datum gap can 

 be worked out on a subsurface structure map if there are sufficient datum 

 points to control the general contouring of the structure and at least three 

 wells that have penetrated the fault plane in a triangular arrangement (not 

 in a straight line) . 



It is first necessary to determine the dip and strike of the fault plane 



j.w.Lo„ :■• 



Figure 474. Structure contour map on top of Cambrian. 



from the three or more wells. The method is shown in the fault-plane 

 detail of the figure cited. The procedure is exactly the same as that used 

 in obtaining a three-point dip and strike on a bed with the plane table. 

 In the illustration wells numbered 1 to 5 have been plotted in their correct 

 relative positions in a separate drawing in order better to illustrate the 

 method. In actual practice, the determination of the dip and strike of the 

 fault plane would be made directly on the map. 



Referring again to the fault detail drawing: Wells 1, 2, and 3 pene- 

 trated the fault plane at elevations 4,100, 4,600, and 4,300 feet respectively. 

 These wells are joined by straight lines. The difference in elevation (on 

 the fault plane) between 2 and 3 is 300 feet. Divide the line into three 

 equal parts. The difference in elevation between each of these points is 



