Horizontal and vertical motion at the surface of seawater-gas hydrate bearing sediment-seabed under obliquely incident P waves

Abstract

This study extends the seismic dynamics of two-phase/three-phase poroelastic sediments in coupled ocean-sediment-substrate systems to gas hydrate-bearing sediments. The ocean sediment floor is described as a finite-thickness porous medium with two solids and a compressible viscous fluid (porous skeleton, gas hydrate, and fluid), which sits on an elastic seabed. In the coupled system of seawater, gas hydrate-bearing sediment layer and seabed, analytical solutions for the P wave reflection and transmission coefficients are derived. Two key factors are derived: displacement amplitude and magnification factor. The relationship among two key factors with incident angle, frequency, sediment thickness, cementation parameter, contact parameter, temperature and saturation in the sedimentary layer containing gas hydrate is analyzed. The research shows that when the P wave is vertically incident (θip = 0°), the incident frequency has a greater influence on the vertical displacement amplitude, and when the P wave is grazing incident (θip = 90°), the influence is not observed. The displacement of the sedimentary layer fluctuates in different degrees with the increase of the layer thickness. When the temperature increases, the saturation is higher, that is, the hydrate content is lower, and the influence of the contact parameters gradually disappears.