Synthetic Gathers

I calculate the synthetic seismograms using the elastic properties displayed in Figure . The forward modeling is done using a generalized Kirchhoff body force scattering theory by Lumley and Beydoun 1993. This technique combines Zoeppritz plane-wave reflection and Rayleigh-Sommerfeld elastic diffraction responses and generates correct AVO responses for locally planar reflectors. The method computes only primary reflections, which is, however, sufficient for this case. Because of the thick water layer and limited offset range, the real data do not show multiple contamination or converted shear waves.

The resulting synthetic gathers for hydrate models A, B and C in the case of hydrate being underlain by free gas are shown in Figure . All three gathers display strong, negative BSR reflections and reflections from the bottom of the gas zone underneath. It is obvious that both models A and B result in increasingly negative BSR amplitudes with increasing offset. The cementation model, on the other hand, yields decreasing BSR amplitudes with increasing offset, as expected from the way the hydrate was affecting the elastic sediment properties. All the models show clear reflections off the top of the hydrate and the bottom of the gas.

 

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Figure 7 Synthetic gathers for hydrate models A, B, and C in the case of hydrate overlying gas-saturated sediment.

Figure shows the synthetic seismograms for the case of hydrate-bearing sediments overlying brine-saturated sediments. As before, all three schemes of hydrate deposition in the pore space produce clear reflections from the top and bottom of the hydrate zone. However, the amplitudes appear significantly weaker as was the case with underlying gas sediments. The cementation model (model C) again shows decreasing amplitudes with offset, while both models A and B seem to produce more constant amplitudes with offset.

 

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Figure 8 Synthetic gathers for hydrate models A, B, and C in the case of hydrate overlying brine-saturated sediment.

A comparison of these synthetic gathers with the real reflectivity gather (Figure ) makes it obvious that the amplitudes produced by model C (the cementation model) do not match the general amplitude trend displayed by the data. This suggests that the hydrate is most likely to be deposited either as part of the fluid or as part of the solid in the pore space. It also suggests, that, at least at the surface location of this CMP gather, the amplitudes generated by hydrate overlying brine appear too weak to reproduce the data. An explicit look at the AVO behavior at the BSR should further support these indications. Moreover, the seismic data do not show a strong or clearly identifiable response from the top of the hydrate. Reflections at about 4.5 s two-way traveltime could be attributed to the top of hydrate, but proof for such cannot be given from seismic. Furthermore, the top of hydrate has not been resolved in any seismic data at present. This would suggest that the top of hydrate is gradational.