Dynamic wave responses near surface in unsaturated porous media

Understanding the dynamic wave signatures in near surface unsaturated soils is crucial for the application of seismic observations in hydrology. We first derived and analyzed the half-space spatiotemporal Green's functions that correspond to the raw seismic recordings, incorporating fluid, pressure jump and free surface boundary conditions represented by the surface stiffness. We described the dynamic wave propagation in solid frame displacement and excess fluid pressures in response to an excitation wavelet impulse, and our results show that the surface hydraulic contact does not obviously alter the wave signatures in solids, regardless of how strong or tiny the solid-fluid viscous coupling, however, in fluids due to the near surface fluid diffusion across the boundary; in this state, the proportional term or the imagery reactance term causes the intermediate state to alter between the open pore (OP) and closed pore (CP) surface. The fast P-P1 body and the dominant fast Rayleigh-R1 surface waves can always be observed near the surface, as can the second Rayleigh-R2 surface wave at the high frequency limit with tiny solid-fluid viscous coupling. Furthermore, the surface waves in the fluid pressures do not penetrate as deeply as in the solid due to the shear motion transferring in them. A high internal permeability may generate the second P-P2* wave by the excited air acoustic wave propagation in fluid pore pressures, showing the strong reverberations under the CP surface. These changes in dynamic waves can be attributed to petrophysically plausible scenarios of flow across the free surface by imperfect hydraulic contacts or capillarity reactance.