Inversion of Rayleigh-wave dispersion curves using a finite-element eigenvalue/eigenvector solver, applied to the Alpine region and Italian peninsula

Questa tesi si pone come obiettivo lo studio del processo d'inversione nel caso di curve di dispersione delle onde di Rayleigh, con lo scopo di determinare il profilo della velocità delle onde S. Ho utilizzato due diversi metodi. Il primo tipo riguarda un approccio perturbativo basato sul metodo degli elementi -finiti; il secondo, è un metodo di calcolo diretto basato sulla formula di Dix e costituisce un metodo non perturbativo. Quest'ultimo fornisce un modello iniziale che potrà essere in seguito ottimizzato attraverso l'approccio perturbativo. Inizialmente, test basati su dati sintetici sono stati effettuati al fine di simulare la crosta oceanica. È stato notato che eventuali variazioni al modello iniziale o variazioni allo spessore degli elementi finiti sembrano essere di grande impatto per il modello risultante. Successivamente, l'inversione perturbativa è stata applicata con dati di velocità di fase, appartenenti al database risultante dall' analisi del rumore sismico effettuata da Molinari et al. . Tali dati sono stati registrati da stazioni presenti in tutto il territorio italiano e nelle Alpi. Il modello iniziale risulta da una combinazione del modello ottenuto dalla ricerca di Molinari et al. per la parte della crosta e dal modello PREM per la parte del mantello. Le mappe 3D di velocità di S risultanti da questo metodo vengono successivamente paragonate con le equivalenti mappe risultate dal lavoro di Molinari et al. . Infine, lo stesso procedimento è stato ripetuto per un numero limitato di celle appartenenti al medesimo database. In questo caso, il modello iniziale è il modello risultante dall' inversione di Dix e può essere considerato un modello di riferimento adeguato per determinare la struttura 1D delle celle prese singolarmente. Tuttavia, si riscontrerebbero alcune limitazioni nel caso in cui il metodo venisse impiegato per ottenere strutture 3D

Seismic surface wave focal spot imaging : numerical resolution experiments

Numerical experiments of seismic wave propagation in a laterally homogeneous layered medium explore subsurface imaging at subwavelength distances for dense seismic arrays. We choose a time-reversal approach to simulate fundamental mode Rayleigh surface wavefields that are equivalent to the cross-correlation results of three-component ambient seismic field records. We demonstrate that the synthesized 2-D spatial autocorrelation fields in the time domain support local or so-called focal spot imaging. Systematic tests involving clean isotropic surface wavefields but also interfering body wave components and anisotropic incidence assess the accuracy of the phase velocity and dispersion estimates obtained from focal spot properties. The results suggest that data collected within half a wavelength around the origin is usually sufficient to constrain the used Bessel functions models. Generally, the cleaner the surface wavefield the smaller the fitting distances that can be used to accurately estimate the local Rayleigh wave speed. Using models based on isotropic surface wave propagation we find that phase velocity estimates from vertical-radial component data are less biased by P-wave energy compared to estimates obtained from vertical-vertical component data, that even strong anisotropic surface wave incidence yields phase velocity estimates with an accuracy of 1 per cent or better, and that dispersion can be studied in the presence of noise. Estimates using a model to resolve potential medium anisotropy are significantly biased by anisotropic surface wave incidence. The overall accurate results obtained from near-field measurements using isotropic medium assumptions imply that dense array seismic Rayleigh wave focal spot imaging can increase the depth sensitivity compared to ambient noise surface wave tomography. The analogy to elastography focal spot medical imaging implies that a high station density and clean surface wavefields support subwavelength resolution of lateral medium variations.Peer reviewe

A seismic network to monitor the 2020 EGS stimulation in the Espoo/Helsinki area, southern Finland

Peer reviewe

Investigating the lateral resolution of the Rayleigh wave focal spot imaging technique using two-dimensional acoustic simulations

We investigate the lateral resolution power of the seismic Rayleigh wave focal spot imaging technique. We use two-dimensional acoustics simulations in a closed cavity for the passive Green’s function and focal spot reconstruction. Four different velocity distributions target different resolution aspects. The finite data range that is necessary to constrain the Bessel function model controls the lateral spreading of material contrasts, the distinction of two objects on sub-wavelength scales, and the image quality of complex random media. Good data quality from dense networks supports short range estimates and super-resolution.Peer reviewe

Long Period Rayleigh Wave Focal Spot Imaging Applied to USArray Data

International audienceAbstract We demonstrate the effectiveness of seismic dense array surface wave focal spot imaging using USArray data from the western‐central United States. We study dispersion in the 60–310 s period range and assess the image quality of fundamental mode Rayleigh wave phase velocity maps. We apply isotropic spatial autocorrelation models to the time domain zero lag noise correlation wavefield data at distances of about one wavelength. Local estimates of the phase velocity, its uncertainty, and the regression quality imply overall better ZZ relative to ZR or RZ results. The extension of the depth resolution compared to passive surface wave tomography is demonstrated by the inversion of three clustered dispersion curves from different tectonic units. We observe anisotropic surface wave energy flux and the influence of body wave energy, but sensitivity tests at 60 s targeting the data range, correlation component, and processing choices show that the ZZ focal spots yield consistent high‐quality images compared to regional tomography results in the 60–150 s period range. In contrast, at 200–300 s the comparatively small scales of the imaged structures and the imperfect agreement with low‐resolution global tomography results highlight the persistent challenge to reconcile imaging results based on different data sources, theories, and techniques. Our study shows that surface wave focal spot imaging is an accurate, robust, local imaging approach. Better control over clean autocorrelation fields can further improve applications of this seismic imaging tool for increased resolution of the elastic structure below dense seismic arrays