Ambient seismic noise is the ground motion that originates from processes occurring outside the solid earth or whose propagation in the solid earth is perturbed by modifications of external environmental solicitations or human activities. Though these vibrations are generally considered unwanted components in active seismic prospecting data, they are useful and offer the advantage of being a continuously available signal without any identifiable active source of seismic energy. For these grounds, in the last decade, passive seismic techniques have become an exciting research topic in various application fields, including seismology, ocean acoustics, ultrasound testing, structural engineering and many others. The project's focus concerns the development of advanced data processing algorithms for shallow subsurface characterisation to mitigate the hydrogeological risk associated with unstable slopes and to monitor groundwater variations. As far as hydrogeological risk mitigation is concerned, we developed slope monitoring systems that rely on the polarisation features variation of seismic noise. In more detail, seismic noise monitoring can support municipalities in managing the landslide risk. The polarisation analysis allows finding the preferential oscillation direction of seismic noise over time. The scientific literature indicates that the preferential direction of seismic noise could be used as a marker to track the directions of cracks due to their influence on geo-mechanical parameters. Differential movements of adjacent landslide areas lead to the formation of cracks that affect the seismic noise oscillation direction. In detail, when an area of a landslide moves, it typically creates tensile stress in the upward zone and compressive stress in the downward one. The tensile and compressive stresses produce normal tension and strike-slip cracks, respectively. The preferential oscillation direction of seismic noise is perpendicular to crack orientation, and it means that by identifying the variations of polarisation features, we can monitor the landslide activity. For what concerns groundwater, by knowing the water level’s depth, we can better use the water stored in the reservoir, avoid unnecessary exploitation, and protect the aquifer from pollution. We developed a new method that relies on seismic noise interferometry coupled with the stretching technique that allows retrieving the depth of the water level due to the density effect. The seismic noise interferometry retrieves the virtual seismic response that would be recorded at one receiver if the other acts like a source. Then, the stretching technique yields the relative velocity changes (dv/v) of the propagating waves thanks to cross correlation. By comparing the theoretical dv/v with the one derived from data processing, we can obtain the depth of the water level of the target period. Therefore, we can monitor the groundwater depth variation of a free aquifer by studying the dv/v variation over time. A minor topic of the research activities concerns subsurface geoelectrical monitoring of municipal solid waste landfill sites. In more detail, this project aims to mitigate the groundwater pollution risk by distinguishing intact from damaged impermeable liners, i.e., High-Density Polyethylene (HDPE) geomembranes placed under wastes to avoid leachate infiltration. If HDPE is damaged, leachate can infiltrate into the subsurface and pollute the groundwater. Since the liner is an electrical insulator and leachate is highly conductive, 3D geoelectrical resistivity tomography (ERT) can detect damages in the geomembrane. We compared ERT results obtained from a 3D survey with several modelling tests. We also developed a finite element algorithm to model the forward problem. Though geoelectrical investigations can indicate if the target liner is damaged or not, results suggest that accurately locating lacerations on the HDPE layers is still a challenging task.

Il progetto si focalizza sullo sviluppo di avanzati algoritmi di data processing per la caratterizzazione del sottosuolo superficiale con l’obiettivo di mitigare il rischio idrogeologico associato a versanti instabili e di monitorare le variazioni delle caratteristiche degli acquiferi utilizzati come risorse idriche. Per quanto riguarda la mitigazione del rischio idrogeologico, abbiamo sviluppato un sistema di monitoraggio che si basa sulla variazione delle proprietà di polarizzazione del rumore sismico. L’analisi di polarizzazione permette di trovare la direzione preferenziale di oscillazione del rumore sismico al variare del tempo. La letteratura scientifica indica che la direzione preferenziale di oscillazione del rumore sismico può essere utilizzata come marker per tracciare la direzione lungo cui si formano le fratture grazie al forte contrasto di impedenza. I movimenti differenziali di aree adiacenti di una frana portano alla formazione di fratture che influenzano la direzione di oscillazione del rumore sismico. Quando una delle aree della frana si muove produce uno stress tensionale nella sua parte sommitale, mentre uno stress compressivo in quella a valle. Lo stress estensionale porta alla formazione di normal tension cracks, mentre quello compressivo di strike-slip cracks. La direzione di oscillazione preferenziale del rumore sismico sarà perpendicolare all’orientazione delle fratture. Questo significa che identificando la variazione delle caratteristiche di polarizzazione possiamo monitorare l’attività della frana. Riguardo alla gestione delle risorse idriche, abbiamo sviluppato un nuovo metodo che si basa sull’accoppiamento delle tecniche di interferometria sismica e di stretching. Queste permettono di identificare la profondità del livello dell’acqua grazie all’effetto densità. La seismic noise interferometry restituisce il cosiddetto virtual seismic response che rappresenta il segnale sismico che varrebbe misurato da un ricevitore se un altro sensore si comportasse come una sorgente (Wapenaar et al., 2010). In seguito, la stretching technique permette di ottenere la variazione di velocità relativa (dv/v) delle onde che si propagano grazie alla cross correlazione. Mettendo a confronto il dv/v teorico con quello che si è ottenuto dalla stretching technique applicata a due virtual seismic response che si riferiscono a differenti periodi di monitoraggio, possiamo ottenere la profondità del livello dell’acqua del periodo target. Dunque possiamo monitorare la variazione del livello dell’acqua di un acquifero libero analizzando la variazione del dv/v nel tempo. Un argomento minore dell’attività di ricerca si basa sul monitoraggio di siti di discarica adibiti al contenimento di rifiuti municipali attraverso l’utilizzo di tecniche geoelettriche. In maggio dettaglio, il progetto ha l’obiettivo di mitigare il rischio di inquinare gli acquiferi riuscendo a distinguere le geomembrane intatte da quelle danneggiate. La geomembrana è usualmente costituita di High-Density Polyethylene (HDPE) e si posa al di sotto del cumula di rifiuti per evitare che il percolato si infiltri nel terreno. Se il HDPE è danneggiato, il percolato ha la possibilità di infiltrarsi nel terreno e di inquinare l’acquifero. Dal momento che la geomembrana è anche un ottimo isolante elettrico ed il percolato è fortemente conduttivo, la tomografia di resistività elettrica 3D (ERT) ha buone possibilità di individuare eventuali danni della geomembrana. Abbiamo messo a confronto i risultati della tecnica ERT ottenuti da sondaggi 3D con diversi test di modelling. Inoltre, abbiamo sviluppato un algoritmo ad elementi finiti per simulare il farward problem. Le indagini geoelettriche possono indicare se la membrana target sia danneggiata.

Sviluppo di tecniche basate sul rumore sismico ambientale e geoelettriche per applicazioni relative al sottosuolo superficiale / Alessandro Aguzzoli , 2023 May 18. 35. ciclo, Anno Accademico 2021/2022.

Sviluppo di tecniche basate sul rumore sismico ambientale e geoelettriche per applicazioni relative al sottosuolo superficiale.

AGUZZOLI, ALESSANDRO
2023

Abstract

Ambient seismic noise is the ground motion that originates from processes occurring outside the solid earth or whose propagation in the solid earth is perturbed by modifications of external environmental solicitations or human activities. Though these vibrations are generally considered unwanted components in active seismic prospecting data, they are useful and offer the advantage of being a continuously available signal without any identifiable active source of seismic energy. For these grounds, in the last decade, passive seismic techniques have become an exciting research topic in various application fields, including seismology, ocean acoustics, ultrasound testing, structural engineering and many others. The project's focus concerns the development of advanced data processing algorithms for shallow subsurface characterisation to mitigate the hydrogeological risk associated with unstable slopes and to monitor groundwater variations. As far as hydrogeological risk mitigation is concerned, we developed slope monitoring systems that rely on the polarisation features variation of seismic noise. In more detail, seismic noise monitoring can support municipalities in managing the landslide risk. The polarisation analysis allows finding the preferential oscillation direction of seismic noise over time. The scientific literature indicates that the preferential direction of seismic noise could be used as a marker to track the directions of cracks due to their influence on geo-mechanical parameters. Differential movements of adjacent landslide areas lead to the formation of cracks that affect the seismic noise oscillation direction. In detail, when an area of a landslide moves, it typically creates tensile stress in the upward zone and compressive stress in the downward one. The tensile and compressive stresses produce normal tension and strike-slip cracks, respectively. The preferential oscillation direction of seismic noise is perpendicular to crack orientation, and it means that by identifying the variations of polarisation features, we can monitor the landslide activity. For what concerns groundwater, by knowing the water level’s depth, we can better use the water stored in the reservoir, avoid unnecessary exploitation, and protect the aquifer from pollution. We developed a new method that relies on seismic noise interferometry coupled with the stretching technique that allows retrieving the depth of the water level due to the density effect. The seismic noise interferometry retrieves the virtual seismic response that would be recorded at one receiver if the other acts like a source. Then, the stretching technique yields the relative velocity changes (dv/v) of the propagating waves thanks to cross correlation. By comparing the theoretical dv/v with the one derived from data processing, we can obtain the depth of the water level of the target period. Therefore, we can monitor the groundwater depth variation of a free aquifer by studying the dv/v variation over time. A minor topic of the research activities concerns subsurface geoelectrical monitoring of municipal solid waste landfill sites. In more detail, this project aims to mitigate the groundwater pollution risk by distinguishing intact from damaged impermeable liners, i.e., High-Density Polyethylene (HDPE) geomembranes placed under wastes to avoid leachate infiltration. If HDPE is damaged, leachate can infiltrate into the subsurface and pollute the groundwater. Since the liner is an electrical insulator and leachate is highly conductive, 3D geoelectrical resistivity tomography (ERT) can detect damages in the geomembrane. We compared ERT results obtained from a 3D survey with several modelling tests. We also developed a finite element algorithm to model the forward problem. Though geoelectrical investigations can indicate if the target liner is damaged or not, results suggest that accurately locating lacerations on the HDPE layers is still a challenging task.
Development of ambient seismic noise and geoelectrical techniques for near-surface applications
18-mag-2023
AROSIO, Diego
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