MS

thesisAcoustic waveform tomography can provide a velocity model with higher accuracy than traveltime tomography because the forward modeling is based on the complete wave equation rather than the high-frequency approximation of ray tracing. Moreover, each trace contains both shallow and deep reflection arrivals and so is richer in information content than the first arrival traveltime. As a result, waveform tomography provides a more detailed estimate of the velocity medium than does traveltime tomography. However, acoustic waveform tomography is a highly nonlinear inverse solution that requires a good starting velocity model compared to the smooth starting model that is sufficient for traveltime tomography. To overcome the nonlinear problems in waveform tomography, I develop a novel multiscale waveform tomography method and apply it to both synthetic and marine seismic data from the Gulf of Mexico. The inversion process is carried out using a multiscale acoustic method with a dynamic early-arrival muting window to mitigate the local minima problem of waveform tomography and elastic effects in the data. This multiscale approach is denoted as multiscale waveform tomography (MWT), and is first validated with synthetic acoustic data from the 2D SEG/EAGE salt model. Using the traveltime velocity tomogram as an initial model, MWT fails to converge to the global minimum. I found that the flooding technique normally used in subsalt migration can be used to significantly improve the convergence of MWT. The velocity model recovered by MWT using the flooding technique is more accurate and highly resolved than that obtained using conventional MWT. For the marine data, MWT can provide a more accurate velocity model than the initial model from traveltime tomography. The accuracy of the waveform velocity model is verified by comparing migration images and common image gathers. I also present ray-based spatial resolution formulas for migration and inversion with numerical tests on both homogeneous and heterogeneous media. Spatial resolution formulas are validated for both homogeneous and heterogeneous velocity models. For both models, the resolution of reverse-time migration images are consistent with the estimated resolution limits with respect to the Rayleigh's resolution criterion. A long wavelength corresponds to a high velocity value and therefore both vertical and horizontal resolution limits tend to degrade with depth in the heterogeneous model â€" deeper regions have higher velocity values