断裂带压缩体与斯通利波声-孔-弹耦合的新模型
A new model for acoustic-poroelastic coupling of compressional body and Stoneley waves at a fault zone
摘要：
在垂直地震剖面法（VSP）中，由于井内流体与周围孔隙弹性地层的耦合作用，会产生斯通利波（管波）。管波在过去已被用来推断现场水力特性。为了更好地理解断层带产生管波的物理机制，我们发展了一个新的模型，计算井中的压力响应。该模型结合了弹性阻抗边界，流体渗透从多孔弹性地层，和不规则的井眼半径的同时影响。解析管波振幅来自新的模型，假设一个正常入射的平面P波，验证了完整的数值解Biot的动态孔隙弹性理论。我们发现，由弹性阻抗边界激发的上行和下行管波具有相反的极性，而由薄的多孔弹性层激发的管波具有不同的波形。该模型还可以预测在日本的一个主要断层带（野岛断层）的VSP响应。我们的定量评估表明，管波产生的弹性阻抗边界和钻孔不规则性周围的主要剪切带的故障，以及由于存在几个多孔层。我们还发现，模拟的振幅与观察吻合得很好，特别是当假设一个不均匀的渗透率分布。所开发的模型和所提出的结果将是至关重要的定量解释VSP数据，以估计断裂带的水力性质。
英文摘要：
In vertical seismic profiling (VSP), Stoneley (tube) waves are generated due to the coupling between the borehole fluid and the surrounding poroelastic formation. The tube waves have been exploited in the past to infer the in-situ hydraulic properties. In order to understand better the physical mechanisms underlying the generation of tube waves at a fault zone, we develop a new model that calculates the pressure responses in a borehole. The model incorporates simultaneous effects of elastic impedance boundaries, fluid infiltration from poroelastic formation, and irregularities in the borehole radius. The analytical tube-wave amplitudes are derived from the new model assuming a normally incident plane P wave, verified by complete numerical solutions for Biot's theory of dynamic poroelasticity. We find that the upgoing and downgoing tube waves due to an elastic impedance boundary have opposite polarities, and those excited by a thin poroelastic layer have different wave shapes. The model also enables the prediction of a VSP response at a major fault zone in Japan (Nojima fault). Our quantitative evaluation suggests that tube waves are generated by elastic impedance boundaries and borehole irregularities around the main shear zone of the fault, as well as due to the presence of several porous layers. We also find that the modeled amplitudes agree well with the observation, especially when assuming a heterogeneous permeability distribution. The developed model and the presented results will be crucial in quantitatively interpreting the VSP data in order to estimate the fault zone's hydraulic properties.
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