Acoustic Waveform Inversion: Case histories
“Acoustic Waveform Inversion: Case histories: This chapter presents four case histories of acoustic full-waveform inversion (FWI). These cases demonstrate that inverting each type of data requires a different series of processing steps to suppress the nonacoustic effects in the recorded data and to avoid cycle-skipping problems. The first example is the result of applying acoustic FWI to land data, in which the traces are time windowed about the early arrivals to avoid accounting for the elastic surface waves and converted waves. Inverting only the early arrivals minimizes the problem of cycle skipping and is denoted as early-arrival waveform tomography (EWT). The second example applies FWI to streamer data from the Gulf of Mexico, in which both shallow and deep reflection arrivals are emphasized uniformly by embedding an inner loop of least-squares migration (LSM) within the outer loop of nonlinear iterations. The LSM loop corrects for geometrical spreading (and attenuation if an anelastic modeling code is used) effects. Thus, it increases the amplitudes of deep reflections to be nearly the same magnitude as those from shallow reflections, which facilitates imaging below the diving waves. Finally, acoustic FWI is combined with viscoacoustic modeling to take into account the effects of attenuation in crosswell data. In this example, inverting the first arrivals provides a starting velocity model that is suficiently accurate so the later reflections mostly are not cycle skipped. Unlike the surface-land experiment, the crosswell experiment allows inversion of the “diving waves” at all depths.”
Figures & Tables
This book describes the theory and practice of inverting seismic data for the subsurface rock properties of the earth. The primary application is for inverting reflection and/or transmission data from engineering or exploration surveys, but the methods described also can be used for earthquake studies. I have written this book with the hope that it will be largely comprehensible to scientists and advanced students in engineering, earth sciences, and physics. It is desirable that the reader has some familiarity with certain aspects of numerical computation, such as finite-difference solutions to partial differential equations, numerical linear algebra, and the basic physics of wave propagation (e.g., Snell’s law and ray tracing). For those not familiar with the terminology and methods of seismic exploration, a brief introduction is provided in the Appendix of Chapter 1. Computational labs are provided for most of the chapters, and some field data labs are given as well. Matlab and Fortran labs at the end of some chapters are used to deepen the reader’s understanding of the concepts and their implementation. Such exercises are introduced early and geophysical applications are presented in every chapter. For the non-geophysicist, geophysical concepts are introduced with intuitive arguments, and their description by rigorous theory is deferred to later chapters.