饱和度变化对弹性波在非饱和砂岩表面反射和透射的影响

EFFECT OF WATER SATURATION ON REFLECTION AND TRANSMISSION OF ELASTIC WAVES AT INTERFACE OF PARTIALLY SATURATED SANDSTONE

通过将等效流体模型和非饱和土Darcy定理引入Biot-Geertsma-Gassmann方程,研究弹性波由饱和孔隙介质向非饱和孔隙介质传播时,饱和度变化对弹性波反射系数、透射系数,以及弹性波携带的能量分配的影响。分别推导了振幅反射系数、透射系数以及能流密度的反射系数、透射系数的理论表达式,并在此基础上给出数值算例。计算结果表明,饱和度的轻微变动不仅对波的反射系数、透射系数产生巨大的影响,还将极大地影响到入射波的能量分配。得出的结论一方面在理论上揭示了土动力学研究中应更加重视饱和度变化的影响,另一方面对工程地震勘探中常用的地震反射波法、地震折射波法以及声波测井数据处理有实际工程意义。

After effective fluid model and Darcy＇s law in partially saturated porous media are introduced to Biot-Geertsma-Gassmann（BGG） equation, the elastic wave propagating from a saturated porous medium to a Biot-type porous medium saturated by a liquid-gas mixture is investigated. Theoretical formulation is developed for the computation of amplitude reflection and transmission coefficients, and energy density flux reflection and transmission coefficients at the interface, which are considered as the functions of saturation degree and incidence angle. All the coefficients are studied in a wide range of frequencies and angles of incidence. Numerical results are given to illustrate the influence of saturation degree on amplitude reflection and transmission coefficients and energy splitting at plane interfaces. It＇s found that even a slight decrease of complete saturation may lead to a substantial change of the reflection and transmission coefficients. The reflected and transmission type II waves of Biot at the contact interface of two media carry little energies at low frequencies, but they carry much more energies when frequency arises. The energy carried by reflected or transmission shear wave is affected slightly by frequency. The results imply that more attention should be paid to saturation degree effects in the interpretation of field observations.