Seismic modelling and inversion of nonwelded interfaces using the boundary integral equation

Abstract

Seismic wave propagation across a fracture is represented by a nonwelded interface or the linear-slip boundary condition. The normal and tangential fracture compliances control the magnitude of discontinuity of elastic-wave-induced displacement across the interface. The fracture compliances are functions of aperture, roughness and elasticity of the infilling materials. Therefore, imaging and characterizing the nonwelded interfaces have the potential to characterize in situ the mechanical and hydraulic properties of the fractured reservoirs. Furthermore, the nonwelded interface is useful to represent a thin, compliant zone in rock, e.g., fractures, joints, fault branches and geological faults. We explore new approaches to model, image and characterize nonwelded interfaces using elastic waves. We discuss modelling seismic responses of nonwelded interfaces using the boundary integral representation of the wavefield. We investigate the Born approximation of the boundary integral equation and the effects of higher order terms. Furthermore, we develop a new Born inversion approach to image and characterize the nonwelded interfaces, and we discuss advantages of the new approach over the recently proposed AVO inversion. We show that while the AVO inversion accurately estimates the fracture compliances assuming that the fracture geometry is known and the primary reflections are isolated without interference of other events, the Born inversion simultaneously images and characterizes a nonwelded interface using the scattered wavefield without assuming fracture geometry. We also illustrate that the Born inversion handles correctly the multiple fractures. We discuss the effectiveness of these new methods using numerically modeled datasets and laboratory experimental datasets. We show that the Born inversion successfully images the nonwelded interfaces and characterizes the spatially varying fracture compliance, simulating heterogeneous inclusion of water/fluid in a fracture.