Elastic wavefield migration and tomography

Abstract

Wavefield migration and tomography are well-developed under the acoustic assumption; however, multicomponent recorded seismic data include shear waves (S-modes) in addition to the compressional waves (P-modes). Constructing multicomponent wavefields and considering multiparameter model properties make it possible to utilize information provided by various wave modes, and this information allows for better characterization of the subsurface. In my thesis, I apply popular wavefield imaging and tomography to elastic media, and propose methods to address challenges posed by elastic multicomponent wavefields and multiparameter models. The key novelty of my research consists of new elastic imaging conditions, which generate elastic images with improved qualities and clear physical meaning. Moreover, I demonstrate an elastic wavefield tomography method to obtain realistic elastic models which benefits elastic migration. Migration techniques, including conventional RTM, extended RTM, and least-squares RTM (LSRTM), provide images of subsurface structures. I propose one imaging condition that computes potential images (PP, PS, SP, and SS). This imaging condition exploits pure P- and S-modes obtained by Helmholtz decomposition and corrects for the polarity reversal in PS and SP images. Using this imaging condition, I propose methods for conventional RTM and extended RTM. The extended imaging condition makes it possible to compute angle gathers for converted waves. The amplitudes of the scalar images indicate reflectivities, which can be used for amplitude verse offset (AVO) analysis; however, this imaging condition requires knowledge of the geologic dip. I propose a second imaging condition that computes perturbation images, i.e., P and S velocity perturbations. Because these images correspond to perturbations to material properties that are angle-independent, they do not have polarity reversals; therefore, they do not need dip information for polarity correction. I use this perturbation imaging condition for LSRTM to increases the image resolution and attenuates artifacts. Since the quality of the wavefield-based migration image greatly depends on the accuracy of the material property models, I also propose elastic waveform inversion methods for multiparameter model estimation. Waveform inversion solves a non-linear problem and aims to obtain a model that best matches the predicted and observed data. My contribution to elastic waveform inversion is a petrophysical constraint term in the objective function, which imposes plausible relations between model parameters; this feasible region is assumed to be prior information which can be obtained from laboratory measurements or well logs. Such petrophysical constraint term enforces appropriate physical relationships between the model parameters. With the constraint term, I obtain realistic models that can be used for migration and reservoir characterization.