Comparative Analysis of Seismic Wave Propagation in Sedimentary and Igneous Terrains

Thesis Abstract

Seismic wave propagation characteristics vary significantly across different geological terrains, influencing the accuracy of seismic imaging and earthquake hazard assessments. This study addresses the limited comparative understanding of seismic wave behaviors within sedimentary versus igneous terrains, which are critical for improving subsurface imaging, resource exploration, and seismic risk mitigation. The primary aim is to perform a comprehensive comparative analysis of seismic wave propagation in these contrasting geological environments, emphasizing differences in velocity, attenuation, and wave mode conversions. Specific objectives include quantifying seismic velocities, analyzing attenuation coefficients, characterizing wave mode conversions, and evaluating the influence of subsurface heterogeneity on wave propagation parameters in sedimentary and igneous terrains. The research adopts a quantitative, cross-sectional design, utilizing seismic data collected from twenty field measurement sites—ten within sedimentary formations and ten within igneous formations—distributed across a tectonically stable region with diverse geological settings. Data collection involved deploying broadband seismometers and geophones over a period of twelve months to record natural and controlled seismic events, supplemented with data from regional seismic networks. The sample size encompasses approximately 2,400 seismic records, with signal processing conducted using Seismic Analysis Code (SAC) and custom MATLAB scripts. Key analytical techniques include spectral analysis to determine seismic velocities, Modulus Reduction and Time Attenuation analysis for attenuation coefficients, and Waveform Inversion methods for mode conversion assessment. To interpret the data, the study employs statistical methods including Analysis of Variance (ANOVA) to identify significant differences in wave velocities and attenuation between the two terrains, alongside regression analysis to examine relationships between geological heterogeneity indices and seismic parameters. The theoretical framework integrates the elastic wave theory and the multiple scattering theory, alongside the Biot’s poroelastic model for sedimentary formations and the elastic-plastic model for igneous rocks, providing a basis for understanding wave behaviors in heterogeneous media. The study further explores the applicability of the Random Medium Theory to account for heterogeneity effects on wave attenuation and velocity dispersion. Expected findings include statistically significant disparities in seismic velocities, with igneous terrains exhibiting higher average P- and S-wave velocities due to their denser and more elastic nature, while sedimentary terrains demonstrate greater attenuation and wave mode conversions attributable to their higher porosity and heterogeneity. These findings are anticipated to substantiate the hypothesis that the nature and heterogeneity of geological media significantly influence seismic wave propagation characteristics. The study aims to fill existing gaps in the empirical understanding of these phenomena by providing detailed, quantitative comparisons grounded in field data and advanced analytical methods. This research contributes novel insights into the heterogeneity-dependent behavior of seismic waves, offering improved models for seismic hazard assessment, resource exploration, and subsurface characterization. It emphasizes the importance of considering geological setting-specific parameters in seismic interpretation frameworks. The main conclusion affirms that terrain-specific properties must be incorporated into seismic models to enhance predictive accuracy, particularly in regions with complex subsurface structures. Based on these findings, the study recommends tailored seismic imaging strategies for sedimentary and igneous environments, including optimized sensor placement and frequency selection, and advocates for further exploration of heterogeneity effects through three-dimensional modeling and laboratory simulations. Future research avenues involve extending the comparative analysis to include metamorphic terrains and integrating anisotropic models to better understand directional wave behaviors.