Céline Gélis

There is a growing interest in applying ambient noise processing techniques to fiber-optic arrays, which are now a mainstream tool in seismology. Traditionally applied to geophone arrays, noise-based interferometry methods have been widely used for subsurface imaging and monitoring over the last two decades. In this study, we retrieve surface wave phase velocities by cross-correlation of DAS-recorded ambient noise data to monitor the subsurface of a quiet brackish marsh site of the Loire estuary. Seasonal recordings were obtained from a DAS array consisting of multiple linear and a spiral cable layout, capturing the ambient seismic wavefield mainly influenced by natural forcing, for example, tidal activity in the river and streams (below 15 Hz), but also by anthropogenic sources. We use time-frequency weighted Phase Cross-Correlation (PCC) technique, which in addition to being efficient, reduces sensitivity to amplitude variations and emphasizes phase coherence. We observe diurnal and seasonal variation in ground water level and in noise characteristics, like amplitude and directionality. Such temporal variations provide an opportunity to monitor both (a) the changes in the subsurface medium itself, and (b) the impact of noise characteristics on surface wave retrieval. We also observe a pronounced lateral contrast of surface wave phase velocity across the marsh site, highlighting the extent of spatial variability of the subsurface in complex natural environments.

Distributed Acoustic Sensing (DAS) is an emerging technology that transforms fiber-optic cables into dense arrays of vibration sensors, offering significant potential for subsurface exploration in urban environments. DAS enables the recording of broadband ground vibrations generated by human activity, including both high-frequency seismic wavefields (>1 Hz) and low-frequency quasi-static deformations (

In certain geological settings such as sedimentary basins, the ground motion induced by an earthquake may be amplified by local site conditions. Estimating these site effects is important for seismic hazard assessment but can be difficult to do empirically due to the scarcity of site-specific field data in time and space, especially in low-to-moderate seismicity regions where the earthquakes needed for measuring the site effects have long return periods. In this study, we try to overcome these limitations and investigate an alternative approach based on ambient seismic noise and numerical simulations. More specifically, we use a 3D numerical model of seismic properties derived from Ambient Noise Surface-Wave Tomography (ANSWT) for 3D numerical simulations of seismic wave propagation, and consequently for a numerical estimation of seismic amplification in the basin. We illustrate the approach on a target site located in the French Rhône valley, where the Messinian salinity crisis has dug a paleo-canyon which is now filled by soft sediments in direct contact with a harder substratum, thereby providing typical conditions for significant site effects, as also observed by previous studies in the area. This work makes use of two dedicated datasets. On one hand, we use earthquake recordings acquired by a network of broadband stations deployed over the target site over 8 months, in order to estimate seismic amplification in the basin with respect to a rock-site reference via Standard Spectral Ratios (SSR), which we consider as our reference for evaluating our numerical results. On the other hand, we exploit one-month-long ambient noise recordings acquired by a dense array of 400 3C sensors. Prior to this work, this noise data was used to build a 3D shear-wave velocity ( V _S ) model of the target site via ANSWT, and also to estimate seismic amplification via noise-based Standard Spectral Ratios (SSRn). The obtained ANSWT model well reproduces the main geological structures of the basin, with lateral variations of velocities at depth depicting the deeper parts of the basin. However, our simulation results also show that some of its limitations related to surface wave sensitivity and resolution capability have an impact on the numerical amplification predicted in the basin. In particular, this ANSWT model lacks clear basin edges in order to efficiently trap seismic waves in the basin and to generate significant 3D wave propagation effects (diffractions, reflections, and generation of laterally propagating surface waves at the edges of the basin). As a result, the numerical amplification predicted in the ANSWT model remains dominated by a 1D response and does not reproduce the broadband character of the observed amplification at locations affected by significant 3D propagation effects. On the other hand, the numerical amplification predicted in the ANSWT model shows a good agreement with the observations at locations that seem less affected by 3D propagation effects, including in complex regions of the model where lateral variations must be taken into account. Our results therefore contribute to identify and better understand the potential and limitations of using ANSWT models for numerical site effect estimation. This study allows us to propose perspectives for future work to improve the approach, which remains promising for site effect assessment in low- to moderate-seismicity contexts.

Superficial geological layers can strongly modify the surface ground motion induced by an earthquake. These so-called site effects are highly variable from one site to another and still difficult to quantify for complex geological configurations. That is why site-specific studies can greatly contribute to improve the hazard prediction at a specific site. However, site-specific studies have historically been considered difficult to carry out in low-to-moderate seismicity regions. We present here seismological datasets acquired in the framework of the French–German dense array for seismic site effect estimation project in the heavily industrialized area surrounding the French Tricastin Nuclear Site (TNS). TNS is located above an ancient canyon dug by the Rhône River during the Messinian period. The strong lithological contrast between the sedimentary fill of the canyon and the substratum, as well as its expected confined geometry make this canyon a good candidate for generating site effects that are variable on short spatial scales. To investigate the impact of this geological structure on the seismic motion, we conducted complementary seismic campaigns in the area. The first main campaign consisted of deploying 400 nodes over a 10 × 10 km area for one month and aimed at recording the seismic ambient noise. A second seismic campaign involved the deployment of 49 broadband stations over the same area for more than eight months. This complementary campaign aimed at recording the seismicity (including local, regional, and teleseismic events). These different designs allowed us to target a variety of seismic data at different spatial and temporal scales. Beyond the interest for local operational seismic hazard applications, these datasets may be valuable for studying seismic wave propagation within complex kilometer-scale sedimentary structures. In this article, we present the deployment designs as well as initial analyses to provide information on the characteristics and the overall quality of the data acquired to future users.

SUMMARY: It has long been recognized that the effects of superficial geological layers, or site effects, can play a major role on the seismic ground motion at the free surface. In this study, we compute wave propagation in a 2-D asymmetrical basin considering both soil non-linearity and pore-pressure effects. Equations of elastodynamics of wave propagation are solved using the spectral element method (SEM). The geometry of the basin gives rise to basin-edge generated waves, that are different for in-plane (P-SV) and out-of-plane (SH) wave propagation and resulting in different non-linear response. Moreover, the excess-pore pressure development in superficial liquefiable layers (effective stress analysis) brings larger deformation and loss of strength than the analysis without pore-pressure effects (total stress analysis). The coupling of vertically propagating waves and the waves specifically generated in 1-D model leads to waves whose amplitude and duration are higher than the 1-D case. This multidimensional effect increases material non-linearity. Such complex wavefield provokes larger deformation and higher pore-pressure rise that cannot be predicted by 1-D modelling. Therefore, our paper suggests the use of multidimensional modelling while studying seismic wave propagation in both linear and non-linear complex media.