Expected BSR AVO Responses

Changes in the AVO responses, as shown in Figure , can be directly related to changes in Poisson's ratio, indicating pore fluid transitions. Three different possibilities can be imagined for the transition from hydrate-bearing sediment to the sediments underneath:

• Case 1: Poisson's ratio in the hydrate layer (layer 1) is smaller than Poisson's ratio in the layer underneath (layer 2)
• Case 2: Poisson's ratio in the hydrate layer is approximately the same as the one in the layer underneath
• Case 3: Poisson's ration in the hydrate layer is larger than the one underneath

 

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Figure 2 Expected BSR AVO curves for hydrate overlying brine and hydrate overlying gas.

Figure shows the theoretical AVO curves corresponding to these three different scenarios. Since the BSR is characterized by both a negative reflectivity and a drop in P-wave velocity (see Chapter 2), the zero-offset reflection amplitude and the resulting P-impedance contrast can be expected to be negative. In Chapter 2, I have shown that the presence of hydrate increases the P-wave velocity of the sediment. If, additionally, hydrate-bearing sediment possesses a high S-wave velocity with respect to the surrounding sediment, amplitudes should decrease as angle increases (case 1). Consequently, the BSR would be caused mainly by the strong effect of the hydrate on the elastic moduli of the saturated in-situ sediments. The BSR would thus be characterized by both a strong negative P-impedance contrast and S-impedance contrast. If, on the other hand, hydrate-bearing sediment does not have a significant impact on Poisson's ratio and is underlain by brine sediment with approximately the same Poisson's ration, the amplitudes should remain constant with increasing angle (case 2). In this case, the BSR would be the consequence of hydrate overlying brine-saturated sediment, and would be characterized by a negative P-impedance contrast and a slightly negative S-impedance contrast. Domenico 1976 showed that gas saturation can significantly decrease Poisson's ratio with respect to brine saturation. Such a decrease in Poisson's ratio should manifest itself in an increasingly negative amplitude versus offset Ostrander (1984). This assertion is based on the drop in P-wave velocity and density caused by the gas saturation and the simultaneous slight increase in S-wave velocity (case 3). In this case, the BSR would be the result of hydrate-saturated sediment overlying gas-saturated sediment and be characterized by negative P-impedance and positive S-impedance.