Method to simulate waveelds from ambient-noise sources

Mamdoh Alajmi, Jose M. Carcione, Ayman N. Qadrouh, Stefano Picotti, Jing Ba

Abstract


The shear (SH)-wave transfer function and the horizontal-to vertical (HV) spectral ratio are fast methods to estimate the S-wave velocity profile and thickness of surface layers overlying a bedrock on the basis of resonance frequencies. Understanding the behaviour of the respective frequency spectra is essential to obtain reliable results. In this work, we propose a full-wave numerical method, based on a pseudospectral spatial differentiation, to simulate SH and P-S waves generated by random sources distributed spatially and temporally (ambient noise). The modeling allows us to implement seismic attenuation, surface waves and casual source radiation patterns, based on random values of the angles of the moment tensor at each source location. We focus on the location of the resonance peaks, without considering the amplitudes, since it is known that the HV method cannot predict the amplitudes. First, we analyze Lamb's problem for which an analytical P-S solution exists. The modeling algorithm is verified for a Ricker time history, but the analysis can be performed by using spikes as sources. The experiments based on ambient noise are compared to those of a coherent line source as a reference spectrum (e.g., and earthquake event far away from the receivers). SH-wave resonance frequencies can be identified in the spectra only when the random sources are located below the bedrock. In the case of P-S waves, the SH-wave transfer function is a good approximation to the HV spectrum, mainly when the noise is generated in the bedrock. Finally, we have assumed a square basin and found that coherent (e.g., earthquake-type) sources may yield identiable peaks but ambient noise gives unreliable results.

Keywords


ambient noise; SH-wave transfer function; HV spectrum; full-wave modeling

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References


DOI: https://doi.org/10.4401/ag-7881

Published by INGV, Istituto Nazionale di Geofisica e Vulcanologia - ISSN: 2037-416X