All seismic interpretation is of course directed toward geological understanding of the subsurface. In the previous chapter, the objective of the interpreter was to make maps of surfaces, mainly in order to delineate traps by mapping the top of a reservoir. However, how does he or she recognise where the reservoirs are likely to be in an undrilled area? What reflectors are most likely to be the top of a reservoir body? If there are some well data available, perhaps reservoirs have already been encountered, but what is their lateral extent likely to be? What lateral changes in reservoir quality are likely, and how should they be related to changes in seismic appearance? These questions are of course just as relevant for 2-D seismic as for 3-D, but the dense data provided by 3-D seismic offers more scope for defining the external geometry and internal architecture of reservoir bodies. The detailed map view derived from 3-D seismic is often more instructive than an individual section can be.

Before embarking on a more detailed discussion, it is important to understand the limitations on achievable seismic resolution; this is discussed in section 4.1. The principles of seismic stratigraphy are briefly explained in section 4.2, including the recognition of seismic facies. Some tools to allow the interpreter to look for the expression of different sedimentary facies are described in section 4.3, and some examples of the results presented in section 4.4.