Radial-Component Predominant-Mode Inversion of Rayleigh Waves: Application to DAS-based Site Characterization

TL;DR

This paper introduces a new inversion framework for DAS-based Rayleigh-wave data that accurately retrieves shear wave velocity profiles by focusing on dominant radial modes, reducing subjective interpretation.

Contribution

The RCPM inversion method explicitly accounts for source-receiver directivity and modal sensitivity, improving the accuracy of Vs profiles from radial-component DAS data.

Findings

RCPM matches dominant radial dispersion trends with the theoretical mode of maximum participation.

Synthetic tests show RCPM accurately captures modal behavior in complex stratigraphy.

Field data applications demonstrate RCPM's Vs profiles agree with borehole measurements.

Abstract

Distributed Acoustic Sensing (DAS) has emerged as a transformative technology for near-surface site characterization. When a vertical source is activated along the fiber, DAS measures only the in-line (radial) component of Rayleigh-wave motion. Dispersion data extracted from radial-component waveforms may differ from those obtained from vertical-component measurements, particularly under complex stratigraphic conditions. Hence, a component-consistent forward problem is desired when inverting radial-component DAS dispersion data to retrieve accurate shear wave velocity (Vs) profiles. This study presents a radial-component predominant-mode (RCPM) inversion framework designed for DAS-based surface-wave analysis that explicitly accounts for source-receiver directivity and modal sensitivity of the Rayleigh-wave radial component. The proposed approach matches measured dominant radial dispersion trends with the theoretical mode exhibiting the maximum modal participation. As a result, the RCPM framework eliminates the need for explicit modal indexing, provides a component-consistent interpretation of radial-component dispersion data, and substantially reduces reliance on subjective analyst-driven modal interpretations. The RCPM approach is systematically evaluated using three synthetic ground models and two field DAS datasets. The synthetic results demonstrate that modal energy distribution differs significantly between vertical and radial components in the presence of strong velocity contrasts and velocity reversals, and that conventional inversion approaches may misinterpret modal behavior, resulting in less accurate Vs profiles. In contrast, the RCPM method consistently captures the correct modal response and yields reliable Vs profiles. Application to two field DAS datasets further demonstrates good agreement between the inverted Vs profiles and independent invasive borehole measurements.