Theory and application of source independent full wavefield elastic converted phase seismic imaging and velocity analysis

Abstract

The recorded seismic signal contains full information about the source that generated the seismic waves and the path along which the seismic waves travel and interfere with subsurface. However, source information is not an explicit part of the seismic record and thus is a large source of uncertainty in seismic imaging and velocity analysis applications. In this thesis, we develop source-independent methods for seismic imaging, seismic trace interpolation and velocity analysis using the interference between pure (PP and SS) and converted-phase (PS and SP) waves. For seismic imaging, we develop amplitude-balancing source-independent converted-phase seismic imaging conditions and introduce a concept of conversion ratio coefficients to provide a physical and mathematical foundation for source-independent converted-phase (SICP) imaging. For seismic trace interpolation, we develop a scheme for migration/de-migration to suppress migration-based artifacts due to sparse station deployments. For velocity analysis, we present first a source-independent space-lag domain Extended SICP imaging condition (ESICP-IC). Then, we mathematically derive an optimization scheme for source independent converted-phase wave equation migration velocity analysis (SICP-WEMVA). We investigate numerically the stability and convergence of SICP-ICs, SICP interpolation and SICP-WEMVA with synthetic data. Finally, using the developed methodologies, we investigate the subsurface structure of the Hengill geothermal area in Iceland using the abundant micro-seismic activity of the region. The constructed SICP seismic images show detailed subsurface structure of the Hengill area that is well correlated with previous seismic and resistivity studies. Also, we find that the amplitudes of the images are well correlated with a low resistivity region of the geothermal area. The reason for this correlation is not fully understood, but may provide an additional tool for investigation of the Hengill site.