3.2　Velocity Model Based on Seismic Wave Propagati...

3.2　Velocity Model Based on Seismic Wave Propagation in Three-phase Effective Medium

The relationships of AVA and the concentrations of gas hydrate from the velocity model based on seismic wave propagation in three-phase effective medium are shown in Fig.3.AVA corresponding to the contrast between water saturated sediments and free gas-bearing sediments with the concentration of 10%is shown in the upper thick line of Fig.3（a）and in the lower dash line of Fig.3（b）.And AVA corresponding to the contrast between gas hydrate-bearing sediments with the concentration of 10%and free gas-bearing sediments with the concentration of 10%is shown in the upper dash line of Fig.3（a）and in the lower dash line of Fig.3（b）.AVA corresponding to the contrast between water saturated sediments and free gas-bearing sediments with the concentration of 20%is shown in the lower thick line of Fig.3（a）and in the upper thick line of Fig.3（b）.And AVA corresponding to the contrast between gas hydratebearing sediments with the concentration of 10%and free gas-bearing sediments with the concentration of 20%is shown in the lower dash line of Fig.3（a）and in the upper dash line of Fig.3（b）.P-P reflection coefficient increases with the angle for these four contrasts.AVA of water saturated sediments is similar to that of gas hydrate-bearing sediments with the concentration of 10%，so it's difficult to distinguish one from the other situation.The AVA shape of free gas-bearing sediments with the concentration of 1%is the same as that with the concentration of 20%，but the value of the former is different from the later.So it is important to obtain absolute reflection coefficient.P-S reflection coefficients of these four situations are very small and the values increase slightly with the angle.

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Fig.3　Amplitude-versus-angle of P-P（a）and P-S（b）reflections of modelof seismic wave propagation in three phases effective medium