Characterization of Subsurface Parameters with Combined Fluid-pressure and Particle-velocity Measurements

The idea of combined 3C-geophones and hydrophones (4C-receivers) often used in marine Ocean-Bottom Cables, can also be used for seismic surveying on land in saturated soils. The fluid-pressure data can be used to interpret the particle-velocity data. It can, however, also be used to extract more information about the subsurface, e.g., information about 2-phase behaviour. To gain insight in the behaviour of various wave modes in the different components, the three-dimensional point-force response of a homogeneous isotropic half-space has been calculated. Because the subsurface is considered as a poroelastic medium in 4C-experiments, Biot’s theory for poroelastic media needs to be used; this explicitly accounts for 2-phase behaviour. The responses show strong dependence on porosity and permeability. It is the combination of particle velocity and fluid pressure that reveals poroelastic characteristics of the subsurface, especially for the Rayleigh wave. Its dispersive behaviour, met in 4C-experiments, is not entirely understood yet. Furthermore, it is shown that it makes sense to use Biot’s theory to model seismic responses when dealing with 4C-measurements. The slow compressional mode may even contribute significantly to the Rayleigh wave.