含流体孔隙介质中面波的传播特性及应用

Characteristics and application of surface wave propagation in fluid-filled porous media

基于单相介质中地震波理论的高频面波法已广泛应用于求取浅地表S波的速度.然而水文地质条件表明,普遍的浅地表地球介质富含孔隙.孔隙中充填的流体会显著地影响面波在浅地表的传播,进而造成频散和衰减的变化.本文研究了地震勘探频段内针对含流体孔隙介质边界条件的面波的传播特性.孔隙流体在自由表面存在完全疏通、完全闭合以及部分疏通的情况.孔隙单一流体饱和时,任何流体边界条件下存在R1模式波,与弹性介质中的Rayleigh波类似,相速度稍小于S波并在地震记录中显示强振幅.由于介质的内在衰减,R1在均匀半空间中也存在频散,相速度和衰减在不同流体边界下存在差异.Biot固流耦合系数(孔隙流体黏滞度与骨架渗透率之比)控制频散的特征频率,高耦合系数会在地震勘探频带内明显消除这种差异.介质的迂曲度等其他物性参数对不同流体边界下的R1波的影响也有不同的敏感度.完全闭合和部分疏通流体边界下存在R2模式波,相速度略低于慢P波.在多数条件下,如慢P波在时频响应中难以观察到.但是在耦合系数较低时会显现,一定条件下甚至会以非物理波形式接收R1波的辐射,显示强振幅.浅表风化层低速带存在,震源激发时的运动会显著影响面波的传播.对于接收点径向运动会造成面波的Doppler频移,横向运动会造成面波的时频畸变.孔隙存在多相流体时,中观尺度下不均匀斑块饱和能很好地解释体波在地震频带内的衰减.快P波受到斑块饱和显著影响,R1波与快P波有更明显关联,与完全饱和模型中不同,也更易于等效模型建立.频散特征频率受孔隙空间不同流体成分比例变化的控制,为面波方法探测浅地表流体分布与迁移提供可能性.通常情况孔隙介质频散特征频率较高,标准线性黏弹性固体可以在相对低频的地震勘探频带内等效表征孔隙介质中R1波的传播特征,特别在时域,可在面波成像反演建模中应用.

High frequency surface wave method based on seismic wave propagation in single-phase media has been widely applied for acquiring near surface shear wave velocity in several fields.However,the near surface earth media,consolidated and unconsolidated,for the general hydrogeological conditions,bear plenty of pores.Fluid in pores affects the surface wave propagation in the media remarkably,which is represented in dispersion in velocity and attenuation.The pore fluid behaviors through the free surface make the boundary more complex as fully drained,fully sealed and partially drained conditions.These effects are taken into account in surface wave secular equation derivation and closed-form dynamic response investigation.Complex searching algorithm and fast coverage quadrature are applied in the dispersion and response calculation.Typical near surface earth media are selected.For weakly consolidated media in which the surface wave propagates in an extremely low velocity,the effects of the motion of source are obtained.The partial saturation of mesoscopic loss significant in seismic frequency band is introduced beyond the Biot model.The effective viscoelastic model that has been used for body waves in porous media is extended to solve the boundary value problem of surface wave propagation by coupling body wave representations on the free surface.For one fluid saturated,R1 mode wave can propagate under each boundary condition,which is similar with classic Rayleigh wave in elastic media.Its phase velocity is a little less than the S wave and amplitude is strong in seismograms.Due to the intrinsic attenuation,R1 wave is dispersive in the homogeneous half space.The velocities and attenuation coefficients are different under different fluid boundary conditions.Biot solid-fluid coupling coefficient controls the critical frequency of dispersion as for the body waves.High solid-fluid coupling can eliminate the differences.Tortuosity affects R1 waves for different fluid boundaries with different sensitivities.R2 mode appears and propagates under sealed and partially drained conditions.Its phase velocity is a little less than P2 wave.In most cases,it is difficult to be observed in dynamic responses.However,when the solid-fluid coupling is low,it may obtain the radiation from R1 wave as a non-physical wave with strong amplitude under partially drained surface.When the velocity of surface wave is low in weak media,the movement of source impulse with regards to the receiver makes remarkable effects on surface wave responses.Radical velocity to the receiver makes Doppler frequency shift.Lateral velocity makes distortions of wave responses in time and frequency domains.For partial saturation with heterogeneous multiphase fluid saturated patches,the mesoscopic flow induced by wave can interpreter wave attenuation in seismic frequency band,which is accordance with the practical data.Because the fast P1 wave is dominantly affected by this strong loss mechanism,R1 wave is more distinctively related to P1 wave considering partial saturation,quite different to the Biot fluid saturated model.The critical frequency for mesoscopic mechanism is controlled by fluid compositions in the patches,which in turn presents the correlation for surface wave signals to subsurface fluid distribution and flowing.Because the Biot critical frequency is always high,standard linear viscoelastic solid can effectively represent R1 wave propagation in porous media in seismic frequency band by the coupling body wave fits and ignoring the slow wave perturbation,especially for the dynamic responses in the time domain.While in patchy saturation model,the P wave propagates under the interactive wave induced flow between the two P wave modes,which also make it simple to effectively represent.The fluid phase in near surface media can be regarded as an important factor for surface wave data acquisition and processing.The effects of different fluid free surfaces need to be considered for correction in several unconsolidated and high permeability rocks.The low velocity of the surface wave makes the movement of source be an important impact in the frequency responses.For passive surface wave observation usually with low frequency signal extraction,this impact needs to be carefully estimated.The dominated mesoscopic mechanism in seismic frequency band shows obvious attenuation in the P wave,which also dominates the surface wave propagation.For near surface attenuation estimation by surface wave,the P wave must be paid attention to.The developed effective medium saves the storage and time consumption for surface wave in porous media,which can be applied in the inversion modeling for surface wave image in an economic way.