Imaging and characterization of a subhorizontal non-welded interface from point source elastic scattering response

Abstract

The inverse scattering of seismic waves can reveal the spatial distribution of the elastic compliances along a non-welded interface, such as a fracture surface. The spatial heterogeneity along the surface of a fracture is a key determinant for fracture-associated hydraulic properties. In this paper, we demonstrate that the inverse scattering solution can be successfully applied to the point source response of a subhorizontal fracture. In the scale of seismic exploration, it is more appropriate to consider spherical waves from a point source than plane waves. Further, from only the P-wave point source response it is possible to estimate both normal and tangential fracture compliances. The synthetic seismic wavefield due to a P-wave point source in a 2-D elastic medium was computed using a time-domain finite difference approach. On this spherical wave data set, the correct estimation of the position and dip of the non-welded interface was possible through reverse-time migration followed by least-square fitting of the maximum amplitude of the P-P reflection. In order to estimate the heterogeneity along the non-welded interface, we first extract the elastic wavefield at the interface position. The extrapolated wavefield is then rotated such that the horizontal axis aligns along the fracture plane. Next, using this extrapolated and rotated wavefield, we solve the linear-slip boundary condition to obtain the distribution of normal and tangential compliances. Our result shows that the estimates of normal compliance are very accurate around the dominant frequency of the incident seismic wavefield. At lower frequencies, the estimated compliance distribution is less accurate and rather smooth due to the presence of evanescent waves. Extracting the distribution of the tangential compliance requires a larger stabilization factor. For a correct estimation of the tangential compliance, one needs S-wave sources or multiple sources providing more grazing angles to avoid the shadow zone. © The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Society.