Using glacier seismicity for phase velocity measurements and Green's function retrieval

High-melt areas of glaciers and ice sheets foster a rich spectrum of ambient seismicity. These signals not only shed light on source mechanisms (e.g. englacial fracturing, water flow, iceberg detachment, basal motion) but also carry information about seismic wave propagation within glacier ice. Here, we present two approaches to measure and potentially monitor phase velocities of high-frequency seismic waves (≥1 Hz) using naturally occurring glacier seismicity. These two approaches were developed for data recorded by on-ice seasonal seismic networks on the Greenland Ice Sheet and a Swiss Alpine glacier. The Greenland data set consists of continuous seismograms, dominated by long-term tremor-like signals of englacial water flow, whereas the Alpine data were collected in triggered mode producing 1-2 s long records that include fracture events within the ice (‘icequakes'). We use a matched-field processing technique to retrieve frequency-dependent phase velocity measurements for the Greenland data. In principle, this phase dispersion relationship can be inverted for ice sheet thickness and bed properties. For these Greenland data, inversion of the dispersion curve yields a bedrock depth of 541 m, which may be too small by as much as 35 per cent. We suggest that the discrepancy is due to lateral changes in ice sheet depth and bed properties beneath the network, which may cause unaccounted mixing of surface wave modes in the dispersion curve. The Swiss Alpine icequake records, on the other hand, allow for reconstruction of the impulse response between two seismometers. The direct and scattered wave fields from the vast numbers of icequake records (tens of thousands per month) can be used to measure small changes in englacial velocities and thus monitor structural changes within the ic

Variabilité Interannuelle à Décennale en Atlantique Nord et Mers Nordiques. Etude conjointe d'Observations, de Simulations Numériques et de Réanalyses

This work aims to characterise and quantify the differences between large-scale oceanic climatic indices simulated by a 9-member set of realistic numerical ocean models (five hindcasts and four reanalyses). The focus is on the North Atlantic region. The following climatic indices : meridional overturning circulation and meridional heat transport, are diagnosed in the various models. The models are run over a common period of nine years. The focus of this study is climatic variability at interannual scales. The influence of numerical parameters, such as spatial configuration and resolution, and the influence of various sequential data assimilation schemes, are evaluated. We evaluate them through comparison with observed large-scale climatic indices. The model solutions exhibit a strong diversity concerning the averaged values and the temporal variations of the meridional volume and heat transports. The ocean reanalyses considered in this study, although constrained towards an observed ocean, do not converge towards the coherent large-scale climatic indices. These reanalyses are not coherent, neither between each other, nor compared to available observational estimations, underlying the difficulty to simulate coherent large-scale climatic indices with local data assimilation.Ce travail de thèse a pour objectif de caractériser et quantifier les différences entre indices climatiques océaniques grande échelle simulés par une hiérarchie de neuf modèles numériques océaniques réalistes (cinq simulations libres et quatre réanalyses). Cette étude se focalise sur la région Atlantique Nord. Les indices climatiques suivants : transport méridien de volume (circulation méridienne d'overturning) et transport méridien de chaleur sont diagnostiqués dans les différents modèles. La période commune à huit des simulations s'étend sur neuf ans, l'étude de la variabilité de ces indices climatiques se focalise sur l'echelle interannuelle. L'influence de différents paramètres numériques : configuration et la résolution spatiales des modèles numérique libres, et l'influence de différents schémas d'assimilation de données séquentielle des réanalyses, sont évaluées au regard des observations et estimations disponibles de ces indices climatiques grande échelle. Il ressort de ce travail une importante diversité des solutions des modèles quant aux valeurs moyennes et aux variations de ces transports méridiens de volume et de chaleur. Les réanalyses océaniques étudiées ici, qui ont pourtant été contraintes vers un océan observé, peinent à converger vers des indices climatiques grande échelle cohérents, tant en moyenne qu'en variabilité interannuelle. Le manque de robustesse de ces réanalyses s'exprime par leur faible cohérence entre elles mais aussi au regard des estimations observationnelles disponibles, et illustre ainsi la difficulté de simuler des indices climatiques cohérents à grande échelle en assimilant avec des contraintes locales

Characteristics of planetary waves in the North Atlantic from altimetry and the CLIPPER 1/6° model: Surface validation and subsurface structure

International audienceSatellite altimetry has been recording the surface signature of planetary waves in the world's oceans since 1992. These observations have highlighted the limits of standard theories about planetary waves, and stimulated the development of new ones, both of which emphazize the importance of subsurface features, i.e. the impact of baroclinic shears and bottom topography. However, the subsurface structure of these waves is still poorly known, and realistic numerical simulations have a clear potential for such a 3D investigation. The present study focuses on the North Atlantic subtropics, and makes use of altimeter (Topex/Poseidon + ERS) sea-level anomalies (SLA) and of a 1/6° realistic Atlantic simulation performed during the French Clipper project. Westward-propagating surface structures are tracked over the period 1993-2000 from both observed and simulated SLAs. Our method, based on the Radon Transform, has been improved to extract the first baroclinic mode of the planetary waves. This surface validation of observed and simulated waves is done in terms of zonal phase speeds and amplitudes, and reveals the realism of modelled waves. The same analysis is thus extended below the surface. Our analysis highlights the complex structure of simulated waves in the vertical, the impact of the Mid-Atlantic Ridge, and might help support theoretical investigations

Dynamic Imaging of Glacier Structures at High‐Resolution Using Source Localization With a Dense Seismic Array

International audienceDense seismic array monitoring combined with advanced processing can help retrieve and locate a variety of seismic sources with unprecedented resolution and spatial coverage. We present a methodology that goes beyond classical localization algorithms through gathering various types of sources (impulsive or continuous) using a single scheme based on a gradient-descent optimization and evaluating different levels of phase coherence. We apply our methodology on an Alpine glacier and demonstrate that we can retrieve the dynamics of active crevasses with a metric resolution using sources associated with high phase coherence; the presence of diffracting materials (e.g., rocks) trapped in transverse crevasses using sources with moderate phase coherence; and the two-dimensional time evolution of the subglacial hydrology system using sources with low phase coherence. Our study highlights the strength of using an appropriate and systematic seismological approach to image a wide range of subsurface structures and phenomena in settings with complex wavefields

Observing the subglacial hydrology network and its dynamics with a dense seismic array

International audienceSubglacial water flow strongly modulates glacier basal motion, which itself strongly influences the contributions of glaciers and ice sheets to sea level rise. However, our understanding of when and where subglacial water flow enhances or impedes glacier flow is limited due to the paucity of direct observations of subglacial drainage characteristics. Here, we demonstrate that dense seismic array observations combined with an innovative systematic seismic source location technique allows the retrieval of a two-dimensional map of a subglacial drainage system, as well as its day-to-day temporal evolution. We observe with unprecedented detail when and where subglacial water flows through a cavity-like system that enhances glacier flow versus when and where water mainly flows through a channel-like system that impedes glacier flow. Most importantly, we are able to identify regions of high hydraulic connectivity within and across the cavity and channel systems, which have been identified as having a major impact on the long-term glacier response to climate warming. Applying a similar seismic monitoring strategy in other glacier settings, including for ice sheets, may help to diagnose the susceptibility of their dynamics to increased meltwater input due to climate warming

Observing the subglacial hydrology network and its dynamics with a dense seismic array

International audienc

Observing the subglacial hydrology network and its dynamics with a dense seismic array

International audienc

Observing the subglacial hydrology network and its dynamics with a dense seismic array

International audienc

Space-time monitoring of groundwater fluctuations with passive seismic interferometry

International audienceHistoric levels of drought, globally, call for sustainable freshwater management. Under pressing demand is a refined understanding of the structures and dynamics of groundwater systems. Here we present an unconventional, cost-effective approach to aquifer monitoring using seismograph arrays. Employing advanced seismic interferometry techniques, we calculate the space-time evolution of relative changes in seismic velocity, as a measure of hydrological properties. During 2000–2020 in basins near Los Angeles, seismic velocity variations match groundwater tables measured in wells and surface deformations inferred from satellite sensing, but the seismological approach adds temporal and depth resolutions for deep structures and processes. Maps of long-term seismic velocity changes reveal distinct patterns (decline or recovery) of groundwater storage in basins that are adjacent but adjudicated to water districts conducting different pumping practices. This pilot application bridges the gap between seismology and hydrology, and shows the promise of leveraging seismometers worldwide to provide 4D characterizations of groundwater and other near-surface systems