考虑毛细管压力的纵波速度衰减特征分析——以球状斑块饱和模型为例

地震波在斑块饱和岩石中传播往往会引起地震波速度的频散与衰减,不同的斑块大小及分布引起速度的频散与衰减不同.毛细管压力作为影响斑块物理特征的主要因素之一,其对速度的频散与衰减的影响知之甚少.为了研究毛细管压力的影响,笔者利用斑块膜刚度来表示毛细管压力的宏观响应,通过改变球状斑块饱和模型的边界条件将毛细管压力考虑到球状斑块饱和模型中,得到了一种新的改进球状斑块饱和模型,并基于该模型对比了考虑毛细管压力前后的地震频段的速度频散与衰减变化.数值模拟结果表明,在地震频段内,与原始球状斑块饱和模型相比,新模型的速度较原始模型速度大,频散降低,衰减减小.除此之外,利用新模型解释了已发表的不同饱和度情况下速度和衰减系数的实验室测试结果.与原始球状斑块饱和模型相比,新模型能够更好的解释不同饱和度下速度与衰减系数的变化趋势,对于斑块饱和岩石的速度饱和度,衰减系数饱和度关系解释具有重要的指示意义.

An analysis of P wave attenuation in consideration of capillary forces:Exemplified by spherical patchy saturation model

The attenuation and dispersion of seismic wave will occur when seismic wave propagates in patchy-saturated media and is influenced by fluid patch distribution and size.The patch characteristics in turn can be influenced by capillary forces mainly.The effect of capillarity on wave attenuation in patchy-saturated rocks is not fully understood.In view of such a situation,the authors make use of the concept of patch membrane stiffness as a macroscopic expression of capillarity,incorporate the capillary forces into spherical patchy saturation model by changing a pressure discontinuity at patch interfaces,get a new model and compare the variation of wave attenuation and dispersion before and after the consideration of the capillarity based on the new model.It is suggested that considering capillarity will increase seismic velocity and decrease the dispersion and attenuation coefficient.The authors applied this capillary-extended spherical patchy saturation model to interpret velocity-saturation and attenuation-saturation relations.A comparison with the original model shows that the new model can fit the velocity-saturation and attenuation-saturation experimental relation trend well,which suggests that capillary-extended spherical patchy saturation model can have important implications for interpreting velocity-saturation and attenuation-saturation relations in patchy-saturated rocks.