含天然气水合物介质地震空白带成因研究

基于弹性波动方程,结合随机介质理论,以渗透型天然气水合物地震空白带的研究为背景,建立了含天然气水合物的非均匀双相孔隙介质模型。利用高阶交错网格有限差分方法，正演计算了地层孔隙度一定时天然气水合物饱和度分别为2%,10%,15%,20%,25%,30%,40%, 50%,60%,70%,80%等11种模型的地震响应。通过对正演偏移剖面作定性分析以及相对能量、频率衰减梯度的定量分析可知:当天然气水合物在地层中的赋存状态为不均匀分布时,水合物对反射波的散射效应使反射波能量减弱,形成地震空白带。当赋存的天然气水合物饱和度达到一定值时,由于水合物不均匀体产生的散射波能量增大,在地震空白带内出现“亮点”。同时,地层中非均匀分布的天然气水合物会影响地震剖面中似海底反射(BSR)的形成,这也可以用于解释为什么会在一些勘探实例中存在天然气水合物而无似海底反射的现象。

Based on the elastic wave equation and stochastic medium theory,a heterogeneous biphasic porous medium model is established to study on the cause of seismic response blanking zone for gas hydrate.High-order staggered grid finite difference method is used to simulate seismic responses in 11 kinds of models,in which the gas hydrate saturation is respectively 2%,10%,15%,20%,25%,30%,40%,50%,60%,70%,80% for certain porosity.Through qualitative analysis of simulated migration profile and quantitative analysis of reflected wave energy and frequency attenuation gradient as wave field characteristics,the following conclusions can be drawn:if distribution of gas hydrate in the layers is non-uniform,the scattering effect caused by gas hydrate reduces the energy of reflected wave and then the seismic blanking zone occurs.As the saturation of gas hydrate reaches to a certain value,“bright spots” can be found in the seismic blanking zone.Moreover,the non-uniform distribution of gas hydrate in the layers will affect bottom simulating reflector occurrence,which can be used to explain the phenomena that in some cases there are gas hydrate in the layers without bottom simulating reflector occurrence.