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Figure 2 – A second stratigraphic condition illustrating why a synthetic seismogram response may not match the reflection response seen in surface-based seismic data. The gap between the sand bodies is of the order of the dominant wavelength λ of the illuminating wavefield. Thus diffractions span the gap and tend to leave the impression of a continuous reflection event, even on migrated seismic sections. However, log data acquired in the well do not measure an impedance change, and a synthetic seismogram calculated from these logs will show no reflection.