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Modeling Noise Hvsr in Media With Lateral Irregularity Using the Diffuse Field Assumption and Ibem for an Irregular Soft Layer

Description: 

Modeling noise horizontal-to-vertical spectral ratio (HVSR) using the diffuse field assumption (DFA) requires the imaginary part of Green’s tensor when source and receiver coincide at the point of interest, precisely the point of data gathering. This approach has been applied for horizontally layered medium to model HVSR. The ensuing inversion leads to vertical profiles of mechanical properties. For irregular profiles, the issue was explored by Matsushima et al. (2017) in Onahama, Japan. Data processing revealed significant directional differences in HVSR. They used the spectral element method to compute the time domain response and Fourier analysis of results close to the load point allowed retrieval of the imaginary part of Green’s function. This exercise was not a formal inversion but the modeling showed improved matching of calculations and observations when the known irregularity was accounted for. A similar case for the Ijen volcano, Indonesia is due to Perton et al. (2017). The exposed layering at the crater edge allowed to model HVSR.

The scarcity of results reflects the difficulties for computation the imaginary parts of Green’s tensor at load points. Looking for a practical approach we used the indirect boundary element method (IBEM) in 3D to study the case of a single layer over a half-space. We reproduced the semi-analytical solution using an adaptive mesh that changes with frequency. This result reveals the crucial role of diffraction at low frequencies and the asymptotic behavior in high frequencies. These results clearly show the strong localized behavior of the solution which also holds for irregular profiles. We model HVSR for a layer over a half space with lateral variations. We obtained the directional HVSR and observe both the changes in peak frequency and the variations in amplitude for both longitudinal and transverse directions.

ACKNOWLEDGEMENT. This research was partially supported by the UNAM through projects DGAPA-PAPIIT IN107720 and IN105523.

Session: Single-station Passive Exploration Methods: Status and Perspectives

Type: Oral

Date: 4/18/2023

Presentation Time: 04:30 PM (local time)

Presenting Author: Francisco J. Sánchez-Sesma

Student Presenter: No

Invited Presentation: 

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