PASSIVE SEISMIC TOMOGRAPHY OF THE PALACIO DE MINERÍA FOUNDATIONS USING METRO-GENERATED NOISE
Characterizing the shallow geotechnical structure beneath historic buildings in densely populated urban areas is challenging because conventional active-source seismic surveys are often impractical. We present a non-invasive passive seismic workflow to image the subsurface velocity structure beneath the Palacio de Minería, in the Historic Center of Mexico City. Rather than assuming a fully diffuse ambient-noise field, the method exploits the high-energy, repeatable vibrations generated by pneumatic-tired trains operating on Metro Line 2, which passes adjacent to the building.
Continuous microtremor data were acquired during two surveys: a perimeter array in 2024 and an internal survey in 2025. Vertical geophones recorded data continuously at 250 Hz. An automated triggered-stacking workflow, based on the analytical Hilbert envelope at a master station, was used to identify and isolate high signal-to-noise-ratio windows associated with train passages. The locations of these moving subsurface sources were estimated using a hybrid array-processing approach that combines two-dimensional frequency-wavenumber beamforming, to characterize local wavefield kinematics, with three-dimensional matched-field processing using a linear Bartlett processor.
Twelve perimeter stations were used as virtual sources to recover empirical Green’s functions through phase-normalized cross-coherence stacking. In parallel, an amplitude-preserving processing branch was used to estimate the intrinsic spatial attenuation coefficient, α, along stable far-field paths (r>5 m). To resolve vertical and lateral variations in stiffness, causal Rayleigh-wave packets from high-quality empirical Green’s functions were separated into three frequency bands (4.5–6.5 Hz, 6.5–8.5 Hz, and 10.0–15.0 Hz) and inverted using a phase-shift multichannel analysis of surface waves approach.
The resulting frequency-dependent group-velocity maps were restricted to the convex hull of the active array and used to construct a high-resolution pseudo-three-dimensional model of the shear-wave velocity (Vs) distribution. The tomographic images delineate structural boundaries, variations in the upper lacustrine clay sequence, and contrasts between the stiffened perimeter foundation zones and heterogeneous anthropogenic fill beneath the central portion of the building. These results show that vibrations generated by pneumatic-tired urban rail systems can provide a stable and repeatable source for high-resolution passive seismic imaging in dense urban settings, despite damping associated with the rubber–concrete wheel–track interface.