In order to verify that the first-order Born scattering approach used by Biondi et al. 1996 gave correct amplitudes, Born synthetic seismograms were compared with plane-layer reflection coefficients. Results showed good qualitative agreement between the two approaches. Two sets of results were tested in this way. The first set covered a range of possible AVO signatures, and the second set covered the possible shale/sand interfaces in the heterogeneous reservoir model described in the main paper. The plane-layer models were then used to gain an initial insight into the AVO response of the heterogeneous model.