3.3 Elastic Modulus Model
The relationships of P-P reflection coefficients and the concentrations of gas hydrate based on elastic modulus model are shown in Fig.4.Fig.4（a）represents AVA computation based on elastic modulus model A for the contrast between gas hydrate-bearing sediments with the concentration of 0%～100%and free gas-bearing sediments with the concentration of 2%.P-P reflection coefficient increases with the concentration of gas hydrate and the angle，while the shapes of AVA are similar.Fig.4（b）represents AVA computation based on elastic modulus model B for the contrast between gas hydrate-bearing sediments with the concentration of 0%～100%and free gas-bearing sediments with the concentration of 2%.The AVA character corresponding to the gas hydrate concentration of 0%～60%of model B is similar to that of model A，but the AVA character corresponding to the gas hydrate concentration of 80%～100%of model B is different from that of model A.The absolute value of P-P reflection coefficient decreases with the angle with the concentration of 100%for model B.While it decreases in the range of small angles and increases in the range of large angles with the concentration of 80%for model B.P-S reflection coefficients of model A are very small and they are less than 0.001.P-S reflection coefficients of model A are shown in Fig.5.The value of P-S reflection coefficient increases with the concentration of gas hydrate，while the shapes of AVA are similar in the shape of parabolic curves.
Fig.4 Amplitude-versus-angle of P-P reflection of model A（a）and Model B（b）of elasticmoduli Contrast between gas hydrate-bearing sediment with the concentration of 0%～100%and free gas-bearing sediment with the concentration of 2%
The P-wave AVA curves of the contrast between gas hydrate-bearing sediment with the concentration of 0%～100%and water-saturated sediment are shown in Fig.6and are similar to those in Fig.5.Thcrc is siginificant change of AVA character for the situation of the gas hydrate concentration of 100%in Fig.6（b）.The absolute value of the reflection coefficient decreases with the angle of incidence.There is similar phenomenon in Fig.4（b）.They are related to very low Poisson's ratio of gas hydrate model B with the concentration of 100%while AVA curve are greatly influenced by Poisson's ratio.The PS-wave AVA curves with water saturated beneath BSR are similar to those with free gas-bearing sediments of the concentration of 2%beneath BSR.
The P-wave AVA curves with free gas-bearing sediments of different concentrations beneath BSR are shown in Fig.7.Fig.7（a）represents the situation of water saturated sediments above the contrast and Fig.7（b）represents the situation of gas hydrate-bearing sediments with the concentration of 10%based on model A above the contrast.P-P reflection coefficient increases with the angle and the free gas concentration for both situations.The shapes of AVA with different free gas concentrations are similar.And the shapes of AVA in Fig.7（a）and Fig.7（b）are similar，but there are small difference for the reflection coefficient values.The reflection coefficients in Fig.7（a）are slightly smaller than those in Fig.7（b）.P-S reflection coefficients are very small and less than 0.008for both situations and they increase slightly with the angle and the concentration.Other AVA curves are calculated from different contrasts such as gas hydrate-bearing sediments with the concentration less than 10%above BSR.The basic characters of AVA are similar and the values are different.(https://www.daowen.com)
Fig.5 Amplitude-versus-angle of P-Sreflection of Model B of elastic moduli Contrast between gas hydrate-bearing sediment with the concentration of 0%～100%and free gas-bearing sediment with the concentration of 2%
Fig.6 Amplitude-versus-angle of P-P reflection of model A（a）and Model B（b）of elastic moduli Contrast between gas hydrate-bearing sediment with the concentrati on of 0%～100%and water-saturated sediment
