True-Reflection Imaging: What is the Major Factor Influencing the Image Amplitude

Traditional migration methods only use the kinematic information in seismic data, so they can only provide the geometrical location of the subsurface structures and provide unreliable reflection/scattering strength (or image amplitude) of subsurface structures. True-reflection imaging tries to produce not only correct geometrical location but also correct image amplitude of the subsurface structures. There are many factors that influence the image amplitude in true-reflection imaging and geophysicists still don't understand very well their relative importance to the image amplitude. We apply the local angle domain migration method based on one-way wave equations to investigate the influence of propagator correction and acquisition aperture correction on the image amplitude in true-reflection imaging. WKBJ solution for one-way wave equations in smooth c(z) media derived from the principle of energy flux conservation for acoustic media can be extended to general c(z) media by introducing the concept of "transparent boundary condition" and "transparent propagators", which neglect all the scattering/reflection loss during propagation. Although the transparent boundary condition may not reflect the physical reality, it can be useful and preferred for true-reflection imaging because it can conserve all the energy collected by the receiver array to the maximum degree. Further along this line, "transparent propagators" can be generalized to 3D heterogeneous c(x,y,z) media in local angle domain. The numerical results demonstrated that the one-way wave propagator with WKBJ correction provides amplitude that agrees very well with that from the full wave equation method in smooth c(z) media. Specially designed numerical test is used to compare the effect of WKBJ correction and acquisition aperture correction in a smooth c(z) media. The numerical results indicate that the propagator and acquisition aperture both influence the image amplitude, however the acquisition aperture has much stronger effect on image amplitude than the propagator WKBJ correction does for migration with limited acquisition aperture. To demonstrate the applicability of true-reflection imaging, we apply this method to the field data acquisition geometry examples (e.g. the IRIS-PASSCAL Cascadia 1993 experiment), in which the receiver number and density permit the application of wave equation based imaging method.