Quantitative Seismic Attenuation Mapping in Crystalline Terranes Using Ambient Noise

Thesis Abstract

Quantitative seismic attenuation mapping in crystalline terranes using ambient noise addresses a persistent gap in characterizing small- to medium-scale heterogeneities that control seismic wave energy loss in complex crustal rocks. The study tackles the challenge of reliably estimating the quality factor (Q) across crystalline terrains where traditional active-source methods are impractical due to accessibility, cost, and environmental constraints. The aim is to develop and validate an ambient-noise based workflow to produce spatially resolved Q maps that reflect intrinsic and scattering attenuation mechanisms, enabling improved interpretation of crustal structure and rock properties. Specific objectives are (1) to implement an array-based ambient-noise cross-correlation framework to retrieve phase and group velocity dispersion curves, (2) to invert these dispersions for frequency-dependent Q(f) using a robust Bayesian inversion that accounts for model uncertainty and data variance, (3) to integrate multi-parameter attenuation models that separate intrinsic attenuation from scattering losses, (4) to assess lateral variability of Q within selected crystalline terranes and relate it to lithology, fracture density, and fluid presence, and (5) to compare ambient-noise derived Q with independent borehole and controlled-source measurements to evaluate reliability and transferability. Methodologically, the research adopts an observational, field-based design conducted in two representative crystalline terrane settings with contrasting metamorphic histories. The population comprises ambient seismic records from a dense local network of 45 to 60 broadband stations deployed for 18 months, complemented by four temporary short-period arrays to enhance spatial sampling. Data collection employs continuous three-component broadband sensors recording at 100 Hz to 1 Hz, with preprocessing including detrending, instrumental response deconvolution, and spectral whitening to preserve attenuation signatures. The core analytical workflow features (i) construction of empirical Green’s functions via cross-correlation of ambient noise between station pairs, (ii) extraction of velocity dispersion curves through frequency–time analysis and multi-parameter fitting, (iii) Bayesian inversion of dispersion data to recover frequency-dependent Q and velocity fields, incorporating prior information from geological mapping and previous seismic profiling, (iv) joint inversion of dispersion and amplitude decay to separate intrinsic (Qi) and scattering (Qs) quality factors, using a forward model grounded in viscoelastic theory and radiative transfer physics, and (v) validation against borehole log-derived rock properties, mineralogical constraints, and limited active-source datasets when available. Statistical techniques include Markov Chain Monte Carlo (MCMC) sampling for posterior distributions, sensitivity analyses to identify dominant data constraints, and regression-based correlation assessments to relate Q distributions to lithological units, fracture indices, and pore-fluid indicators. The theoretical framework integrates viscoelastic attenuation theory and radiative transfer concepts, with reference to Biot’s poroelasticity for fluid-related effects and the Aki–Richards approach for parameterization of attenuation in crystalline media. Expected findings include (a) spatially coherent Q(f) maps that reveal higher attenuation in deformational zones and fracture networks, contrasted with relatively low attenuation in intact, high-grade metamorphic blocks; (b) quantified separation of intrinsic attenuation from scattering losses, with intrinsic Qi typically dominating near 1–10 Hz in metamorphic rocks, and scattering contributions increasing with heterogeneity scale; (c) robust correlations between Q variations and measured fracture density, mineral content (e.g., garnet- and mica-rich domains), and inferred fluid pathways; and (d) cross-validation results showing concordance within 15–20% with borehole-derived rock parameter estimates, supporting the feasibility of ambient-noise attenuation mapping for crustal characterization. The study contributes to knowledge by providing a scalable, non-invasive methodology to map seismic attenuation in crystalline terrains, advancing understanding of rock rheology, fracture-controlled energy dissipation, and crustal heterogeneity. It offers a transferable workflow that can be applied to other tectonically complex regions and supports improved interpretation of seismic hazard and resource-related investigations. The main conclusion anticipates that ambient-noise based attenuation mapping yields reliable, high-resolution Q models that, when integrated with velocity and density information, enhances inference of lithology, fracture networks, and fluid distributions. Practical recommendations include adopting denser permanent seismic arrays in target regions, routine calibration with borehole data, and extending the framework to joint magnetotelluric–seismic attenuation studies to further constrain crustal property contrasts.