Seismic attenuation in fractured porous media: insights from a hybrid numerical and analytical model

Abstract

Seismic attenuation in fluidsaturated porous rocks can occur by geometric spreading, wave scattering or the internal dissipation of energy, most likely due to the 'squirt-flow' mechanism. In principle the pattern of seismic attenuation recorded on an array of sensors then contains information about the medium, in terms of material heterogeneity and anisotropy, as well as material properties such as porosity, crack density and pore-fluid composition and mobility. In practice this inverse problem is challenging. Here we provide some insight into the effect of internal dissipation from analysing synthetic data produced by a hybrid numerical and analytical model for wave propagation in a fractured medium embedded in a layered geological structure. The model is made up of one anisotropic and three isotropic horizontal layers. The anisotropic layer consists of a porous, fluid-saturated material containing vertically-aligned inclusions representing a set of fractures. This combination allows squirt-flow to occur between the pores in the matrix and the model fractures.. Our results show that the fluid mobility, and the associated relaxation time of the fluid-pressure gradient controls the frequency range over which attenuation occurs. The induced attenuation increases with incidence angle and azimuth away from the fracture strike direction. Azimuthal variations in the induced attenuation are elliptical, allowing the fracture orientations to be obtained from the axes of the ellipse. These observations hold out the potential of using seismic attenuation as an additional diagnostic in the characterisation of rock formations for a variety of applications, including hydrocarbon exploration and production, subsurface storage of CO2 or geothermal energy extraction.