Possible weaken of Rayleigh waves in weak porous media: Effects of surface drainage

In unconsolidated weak porous media, the slow P2 wave may overtake the S wave across the characteristic frequency f_c as a fast slow P2 wave (FSP2). The FSP2 can destructively interfere with the first Rayleigh wave (R1), which is generated by the fast body waves (P1 and S waves) along the free surface interface. These phenomena are discovered under sealed closed pore (cp) hydraulic contact on the free surface, as described by poroelastic theory. However, under permeable open pore (op) hydraulic contact, the FSP2 wave does not affect the R1 mode. It is speculated that the interference may be dominated by the second Rayleigh wave (R2), which is only excited under cp conditions. In weak porous media, the R2 wave may destructively interfere with the first Rayleigh wave (R1) when the FSP2 appears, causing it to transition to a inverse-dispersion surface wave mode. This induces strong attenuation of the R1 wave, thereby weakening pore pressure build-up and displacement penetration. This discovery suggests that preventing fluid transport on free-surface interfaces can effectively absorb the impact of vibrations caused by Rayleigh surface waves, providing a new perspective for seismic protection of weak near-surface media.