Near-Surface Monitoring with Rayleigh Wave Ellipticity

Passive seismic methods have gained significant attention in recent decades due to their cost-effective and non-invasive nature, making them ideal for continuous monitoring of subsurface dynamics. Among these methods, ambient noise coda-wave interferometry is widely used for detecting time-lapse changes in seismic velocities and has been successfully applied in diverse geological settings. However, its effectiveness might be limited by the complexity of coda wave composition, which complicates the estimation of the depth sensitivity of velocity changes, and by the instability of noise sources, which can introduce artificial velocity changes. These limitations highlight the need for alternative methodologies. 

In this study, we evaluate the potential of Rayleigh wave ellipticity as a non-interferometric tool for detecting near-surface variations. Using the degree of polarization method, combined with time-frequency analysis to isolate Rayleigh waves, we analyze time-lapse variations in ellipticity from multiple field case studies. To quantify these variations, we employ normalized cross-correlation and cross-covariance metrics. Unlike other widely used noise-based methods, such as spectral analysis or horizontal-to-vertical spectral ratios, our approach examines the full ellipticity function. This allows for the detection of velocity changes over a broader depth range without being limited to specific features of the curve.

Our results demonstrate the robustness of Rayleigh wave ellipticity in detecting shallow subsurface changes, which is source-unbiased. This approach addresses key limitations of existing geophysical methods and expands the toolbox for seismic monitoring, enabling a more comprehensive analysis of subsurface dynamics. Potential applications include monitoring groundwater variations, assessing infrastructure stability, and contributing to the understanding of subsurface dynamics in complex environments, making it a versatile and powerful tool for environmental and geological studies. Rayleigh wave ellipticity emerges as a robust, independent alternative, offering a refined approach for monitoring subsurface changes and addressing the challenges faced by other noise-based methods.

This work has received funding from the AGEMERA project, financed by the European Union’s Horizon Europe research and innovation programme under grant agreement N° 101058178.  As well, this work has benefited from partial support of the STONE project (CPP2021-0090072), financed with funds from the Ministry of Science and Innovation through the State Agency for Innovation (MCIN/AEI/10.13039/501100011033) and the European Union-Next Generation through the Recovery, Transformation and Resilience Plan (PRTR).