Where a vertical seismic section intersects a stratigraphic feature the interpreter can normally find a small amplitude or character anomaly. The expression of a sand-filled channel or bar, for example, is therefore normally so subtle that it takes a considerable amount of interpretive skill to detect it. In contrast, a horizontal section reveals the spatial extent of an anomaly. The interpreter can thus observe characteristic shape and relate what he sees to geologic experience. A shape or pattern which is unrelated to structure may prove to be interpretable as a depositional, erosional, lithologic or other feature of significance. Klein (1985) and Broussard (1975), among others, have provided depositional models on which the interpreter can base his recognition of depositional features. The study of horizontal sections and horizon slices can provide a bird’s-eye view of ancient stratigraphy, analogous to the view of modern stratigraphy obtained out of an airplane window.
Figure 4-1 shows five adjacent vertical seismic sections from a small 3-D survey in the Williston basin of North Dakota. Note that the reflections indicate largely flat-lying beds. At 1.8 seconds there is a very slight draping of reflections which is only just discernible. Figure 4-2 shows two single-polarity horizontal sections superimposed on each other. The data from both levels reveal the same almost circular shape. This is the outline of a carbonate buildup measuring approximately one kilometer in diameter.