Imaging seismic reflections


Op 't Root, Timotheus Johannes Petrus Maria (2011) Imaging seismic reflections. thesis.

open access
Abstract:The goal of reflection seismic imaging is making images of the Earth subsurface using surface measurements of reflected seismic waves. Besides the position and orientation of subsurface reflecting interfaces it is a challenge to recover the size or amplitude of the discontinuities. We investigate two methods or techniques of reflection seismic imaging in order to improve the amplitude. First we study the one-way wave equation with attention to the amplitude of the waves. In our study of reverse-time migration the amplitude refers to the amplitude of the image.
Though the approach in the thesis is formal, our results have practical implications for seismic imaging algorithms, which improve or correct the amplitudes.
The one-way wave equation is a 1st-order equation that describes wave propagation in a predetermined direction. We derive the equation and identify the steps that determine the wave amplitude. We introduce a symmetric square root operator and a wave field normalization operator and show that they provide the correct amplitude. The idea to use a symmetric square root is generally applicable. Our amplitude claims are numerically verified. The one-way wave equation is an application of pseudo-differential operators.
Reverse-time migration (RTM) is an imaging method that uses simulations of the source and
receiver wave fields through a slowly varying estimate of the subsurface medium. The receiver wave is an in reverse time continued field that matches the measurements. An imaging condition transforms the fields into an image of the small scale medium contrast.
We investigate the linearized inverse problem and use the RTM procedure to reconstruct the perturbation of the medium. We model the scattering event by the scattering operator, which maps the medium perturbation to the scattered wave. We propose an approximate inverse of the scattering operator, derive a novel imaging condition and show that it yields a reconstruction of the perturbation with correct amplitude. The study extensively uses the theory of Fourier integral operators (FIO). Besides that we globally characterize the scattering operator as a FIO, we also obtain a local expression to calculate the amplitude explicitly. The claims are confirmed and illustrated by numerical simulations.
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