天然气水合物似海底反射层(BSR)AVA特征:双相介质模型(英文)

Reflection and transmission of bottom simulating reflectors in gas hydrate-bearing sediments: Two-phase media models

天然气水合物似海底反射层是一个由固体、气体和液体等多相介质组成、受温度和压力影响的物理分界面,所以基于弹性介质Zoeppritz方程计算的反射和透射系数都不能满足实际的分析需要,因此需要研究多相介质分界面上的反射和透射特征。但是由于多相介质理论的复杂性,在研究中可以将多相介质近似看作是双相介质,也就是说可以将含天然气水合物沉积地层简化为双相介质,来研究波在其BSR分界面上随频率和入射角度的变化所反映出的反射和透射的特征,并且改变BSR上下层介质中水合物含量和游离气含量,进一步研究波在介质分界面上的反射和透射特征与流体饱和度的关系。对模型计算发现第一类纵波和横波对频率的变化不敏感,速度改变不大;而第二类纵波受频率的影响较大,速度变化范围较大;随着水合物和游离气含量的变化,各种反射和透射波都会随之改变,变化的范围和幅度说明了波对介质组分变化的敏感性。

The bottom simulating reflector （BSR） in gas hydrate-bearing sediments is a physical interface which is composed of solid, gas, and liquid and is influenced by temperature and pressure. Deep sea floor sediment is a porous, unconsolidated, fluid saturated media. Therefore, the reflection and transmission coefficients computed by the Zoeppritz equation based on elastic media do not match reality. In this paper, a two-phase media model is applied to study the reflection and transmission at the bottom simulating reflector in order to find an accurate wave propagation energy distribution and the relationship between reflection and transmission and fluid saturation on the BSR. The numerical experiments show that the type I compressional （fast） and shear waves are not sensitive to frequency variation and the velocities change slowly over the whole frequency range. However, type II compressional （slow） waves are more sensitive to frequency variation and the velocities change over a large range. We find that reflection and transmission coefficients change with the amount of hydrate and free gas. Frequency, pore fluid saturation, and incident angle have different impacts on the reflection and transmission coefficients. We can use these characteristics to estimate gas hydrate saturation or detect lithological variations in the gas hydrate-bearing sediments.