Figure 1. P and S reflection images recorded along the same surface track across a gas reservoir. P-wave reflections (a) occur at gas-water fluid boundaries (labeled "Flat spot" and "High-amplitude reflection"), but S-wave reflections (b) do not. Both images show a high-amplitude reflection at a known lithological boundary (labeled "lithology"). In this example, an interpreter could use well control to identify which P-wave reflection marks a fluid-contact boundary. In areas with little or no well control an interpreter will benefit by having both P and S images and using a comparative method such as demonstrated here to identify reflections associated with fluid-contact boundaries. Data examples taken from Ensley (1984).