arXiv subjects

Mrinal Bhaumik

Publications and source records attributed to Mrinal Bhaumik.

3 recordsLinked to original sources

arXiv2026

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

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.

physics.geo-ph↗

arXiv2025

Predominant-Mode Inversion of Surface Waves: Inherently Addressing Inconspicuous Low Frequency Mode Jumps

Inversion of Rayleigh-wave dispersion data is particularly challenging at sites with strong impedance contrasts, where modal energy often transitions smoothly from the fundamental to higher modes at low frequencies. Analysts may misinterpret this transition as a continuation of the fundamental mode, leading to an overestimation of shear-wave velocity (Vs) in deeper layers and/or a misinterpretation of bedrock depth. Although effective-mode inversion can theoretically account for such behavior, it requires precise source-receiver geometry and cannot be applied when target dispersion data are formed by combining multiple active shots with passive-array recordings that have unknown source locations. This study introduces a predominant-mode inversion framework that addresses low-frequency mode jumps by automatically identifying, at each frequency, the Rayleigh-wave mode with the maximum vertical surface amplitude. This enables inversion without explicit mode indexing or assumptions about fundamental-mode dominance. The predominant-mode forward model is derived and implemented using the thin-layer method. The forward model is integrated into a particle-swarm-optimization global search algorithm and applied to invert three synthetic models exhibiting low-frequency mode jumps, using multiple layering parameterizations. Across all cases, the predominant-mode method accurately recovers major velocity contrasts and interface depths, whereas fundamental-mode inversions consistently overestimate Vs and mislocate deeper boundaries. The method is further validated using real field data from active and passive surface-wave measurements at the I15 Downhole Array Site. Inverted Vs profiles show strong agreement with downhole PS logs and with empirical transfer functions. Overall, the predominant-mode framework provides a robust approach for surface-wave inversion at sites with strong impedance contrasts.

physics.geo-ph↗

arXiv2024

Active Sourced Wavefield Modeling for Layered Half-Space

Traditional free vibration-based forward models generate theoretical dispersion curves under the assumption of planar waves, neglecting the influence of the actual source-receiver configuration. While 2D/3D numerical wavefield modeling approaches mimic real-field scenarios with source-receiver information, they suffer from computational inefficiency. This study introduces a semi-analytical wavefield modeling approach incorporating source-receiver data acquisition layouts. The method considers a cylindrically spreading wavefield described by the Hankel function instead of the planer wave assumption. The approach considers both propagating waves characterized by real wavenumbers and decaying waves with complex wavenumbers, allowing for calculating surface displacements in both the far and near fields. The proposed model captures the complete wavefield, including source-offset effects and leaky waves, while maintaining computational efficiency comparable to any free vibration-based approaches. The method entails solving the eigenvalue problem constructed through the higher-order thin-layer method. Subsequently, it calculates the frequency domain vertical and radial surface responses at any desired location in space generated by a vertically positioned active source. The overall performance of the proposed method is investigated on diverse profiles, including regularly dispersive media, low-velocity layer models, and thin plate structures. The vertical and radial component dispersion images are validated against the numerical approach. The proposed method is at least two orders of magnitude faster than the numerical method. Notably, it captures the smooth transition of modal energy from the fundamental mode to higher modes, occurring due to modal osculation at low frequencies. The present approach offers a valuable tool to enhance the efficiency of active surface wave methods.

physics.geo-ph↗