Subsurface Characterization Using Passive Seismic Interferometry and Ambient Noise Tomography in a Folded Tectonic Setting

Project Abstract

Passive seismic interferometry and ambient noise tomography are applied to characterize subsurface structures within a folded tectonic setting, leveraging continuous ambient seismic fields and covariance analysis to extract impulse responses between dense arrays without active sources. This study integrates long-term seismic recordings from a distributed network of broadband and short-period stations, performing cross-correlation of ambient noise to retrieve the Green’s functions and estimate time-lapse changes in wave propagation media. The methodology combines robust cross-correlation techniques, deconvolution, and phase-sensitive tomography to resolve velocity perturbations, anisotropy, and fracture networks across multiple depth horizons, offering a detailed velocity model of the crust and upper mantle in structurally complex regions. A key objective is to quantify the effects of folds, faults, and lithological contrasts on seismic wave speeds, using ambient noise-derived phase delay measures to infer lateral and vertical variations in shear and compressional velocities. The workflow begins with meticulous data preprocessing, including noise filtering, daily stacking, and spectral whitening to enhance signal coherence, followed by interferometric techniques that convert ambient tremor and microseismic energy into pseudo-impulses. Subsequent tomographic inversion employs regularized least squares with model parameterization informed by geological and geophysical constraints, enabling simultaneous resolution of isotropic and anisotropic velocity fields, gradient-based depth migrations, and sensitivity kernel analyses. To validate the approaches, synthetic tests using realistic folded geometries and heterogeneities are conducted, assessing resolution, trade-offs, and non-uniqueness, while field applications target a corridor of interest characterized by pronounced folding and fault-bounded blocks. The study also probes temporal stability by dividing the dataset into sub-intervals to detect subtle velocity shifts associated with stress accumulation, fluid migration, or seasonal loading, enhancing the capability to monitor active tectonics and geomechanical processes. Results reveal high-contrast velocity anomalies corresponding to lithological boundaries within folded layers, with pronounced anisotropy indicating preferred fracture orientations aligned with fold axes and reverse faulting planes. Ambient noise tomography yields coherent dispersion curves across a broad frequency band, enabling depth-dependent imaging that resolves shallow near-surface variations and deeper crustal features. The integration of passive interferometry with ambient noise tomography demonstrates improved spatial resolution and robustness against sparse access or limited active-source campaigns, offering a cost-effective and continuous monitoring framework for seismic hazard assessment, reservoir characterization, and tectonic deformation studies. Uncertainties are quantified through bootstrap resampling and alternative regularization schemes, with sensitivity analyses highlighting the most influential stations and frequency ranges for resolving target structures. The outcomes provide a comprehensive, multi-scale velocity model and deformation indicators that advance understanding of how folded tectonics modulate seismic wave propagation, fracture networks, and fluid pathways, informing hazard mitigation, resource exploration, and geomechanical modeling in complex crustal environments.

Project Overview

What This Project Is About

---

A plain-language overview of how scientists use natural vibrations in the Earth and ambient background sounds to map underground structures, without drilling. The project combines two methods to learn about subsurface properties in folded geological areas, which are zones where rocks have been bent and stacked. These methods help us see features like layers, faults, and variations in rock stiffness, which influence where resources or hazards may be found. The goal is to create a clearer picture of what lies beneath the surface using non-invasive data.

---

---

The Problem It Addresses

---

In folded regions, rocks are deformed, making traditional survey methods harder and more costly. There is often limited information about how seismic waves travel through these zones, which leads to uncertainty in hazards, resource opportunities, and geotechnical decisions. The project tackles this gap by combining two passive approaches that require little to no active energy sources, reducing cost and environmental impact while improving subsurface images.

---

---

Objectives of the Project

---

1. Explain the basic ideas of passive seismic interferometry and ambient noise tomography in simple terms.
2. Apply these methods to a folded tectonic setting using real or synthetic data.
3. Identify key underground features such as layers, faults, and variations in material properties.
4. Compare results from the two methods to assess consistency and reliability.
5. Discuss limitations and potential improvements for field studies.

---

---

What You Will Do Step by Step

---

1. Learn the core concepts of ambient seismic noise and how interferometry works.
2. Gather or access ambient seismic data from a suitable folded region.
3. Process the data to extract phase and travel-time information.
4. Construct simple subsurface models for interpretation.
5. Cross-validate findings between interferometry and tomography results.
6. Interpret geological implications in terms of layers and structures.
7. Assess practical limitations and uncertainties.
8. Prepare a concise report and presentation of findings.

---

---

Expected Outcome

A clear, easy-to-understand subsurface image showing major structural features in the folded area, plus a discussion of uncertainties and practical implications for exploration and hazard assessment. The project should demonstrate how passive methods can complement traditional surveys and offer a cost-effective approach for initial site characterization.