30 research outputs found
DĂ©couverte de mĂ©gadunes dans lâestuaire moyen du fleuve Saint-Laurent, QuĂ©bec, Canada
Des levĂ©s rĂ©gionaux de gĂ©ologie marine effectuĂ©s sur trois ans dans lâestuaire du Saint-Laurent permettent pour la premiĂšre fois de caractĂ©riser en dĂ©tail le relief sous-marin entre lâIle aux Coudres et Pointe-des-Monts, QuĂ©bec Canada. Les levĂ©s de bathymĂ©trie multifaisceaux, couplĂ©s Ă des levĂ©s gĂ©ophysiques Ă haute rĂ©solution (sismique et sonar Ă balayage latĂ©ral) et Ă lâĂ©chantillonnage du fond marin, ont permis dâimager et de documenter des champs de dunes dĂ©camĂ©triques pouvant atteindre une dizaine de kilomĂštres de longueur et quelques kilomĂštres de largeur. La prĂ©sence de ces mĂ©gadunes suggĂšre un environnement trĂšs dynamique ainsi quâune grande mobilitĂ© des sĂ©diments. Deux de ces champs, localisĂ©s Ă lâintĂ©rieur des limites du Parc marin SaguenayâSaintâLaurent, font lâobjet dâune description plus dĂ©taillĂ©e afin de comprendre lâenvironnement physique dans lequel ils ont Ă©tĂ© mis en place. Des ondes internes modernes dans lâestuaire moyen du SaintâLaurent, phĂ©nomĂšnes connus et documentĂ©s, sont peut-ĂȘtre un mĂ©canisme important de la mise en place et de la mobilitĂ© des mĂ©gadunes. Toutefois, les profils sismiques dans ces champs de mĂ©gadunes montrent que certaines pourraient ĂȘtre un vestige dâune pĂ©riode oĂč le rĂ©gime hydraulique de lâestuaire Ă©tait diffĂ©rent, avec un dĂ©bit beaucoup plus fort vers lâaval et des courants de marĂ©e plus importants. Des mĂ©gadunes imagĂ©es sur le fond marin, dâune amplitude de 10 m et dâune longueur dâonde moyenne de 200 m, sont en effet observĂ©es sur un des profils sismiques enâdessous de boues modernes. Les ondulations du fond marin sont identifiables Ă plusieurs Ă©chelles, les plus grosses Ă©tant surmontĂ©es de plus petites, qui elles pourraient reprĂ©senter le rĂ©gime hydrodynamique actuel. Il reste Ă Ă©valuer lâactivitĂ© moderne des mĂ©gadunes par le biais de levĂ©s multiâannuels afin de quantifier le taux de dĂ©placement, les changements morphologiques et dâĂ©tablir une relation directe avec les conditions hydrodynamiques modernes.Three years of regional marine geology surveys in the St. Lawrence Estuary allow for the first time the detailed characterization of the seabed topography between Ile aux Coudres and Pointe-des-Monts, QuĂ©bec, Canada. Multibeam bathymetric surveys complemented with high resolution geophysical surveys (seismic and side-scan sonar) and seabed sampling, are the basis for documenting megadune (10 m) fields up to 10 km in length and a few kilometers in width. The presence of these megadunes suggests a very dynamic environment as well as high sediment mobility. Two of these fields, located within the limits of the SaguenayâSt. Lawrence Marine Park, are described in detail in order to better understand the physical environment in which they were formed. The occurrence of internal waves in the upper St. Lawrence Estuary, a known and documented phenomenon, suggests that this may be an important factor in the development and mobility of the megadunes. However, seismic profiles over the megadune fields show that some of them may be a relict of a time when hydrodynamic conditions in the estuary were different, i.e. with a much stronger discharge and more important tidal currents. Some megadunes imaged on the seabed, with an amplitude of ten meters and an average wavelength of 200 m, have been observed on the seismic profiles under modern mud. Bedforms are observed at various scales, the largest being overridden by smaller ones, these ones perhaps representing modern hydrodynamic conditions. The modern activity of the megadunes needs to be evaluated with multi-year bathymetric surveys, which will allow quantifying their displacement characterization of changes in morphology associated with varying hydrodynamic conditions, and establish a direct relationship with modern hydrodynamic conditions
Late Pleistocene and Holocene transgression inferred from the sediments of the Gulf of San Jorge, central Patagonia, Argentina
This study presents the first detailed description of the upper sedimentary succession of the late Pleistocene and Holocene deposits in the Gulf of San Jorge (Patagonia) based on several hundred kilometers of high-resolution seismic (sparker) profiles and numerous sediment cores. High-resolution seismic stratigraphy confirms the existence of a paleo-fluvial network formed during sea-level lowstands and buried by central basin estuarine deposits during the last marine transgression. Analyses of lithostratigraphy and radiocarbon ages indicate the onset of subtidal sedimentation at ~14âcal ka bp. Before the onset of subtidal conditions, the first steps of marine incursion seem to have led to the development of lagoonal/windâtidal flat environments, advocating for a sea-level stillstand. An abrupt increase in the log(Ti/Ca) ratio in a distinct multi-centimeter-thick layer and the identification of a wave-ravinement surface suggest rapid sea-level rise in the gulf prior to ~14âcal ka bp, consistent with Meltwater Pulse 1A. Overall, this study highlights the significant impact of sea-level rise on sedimentation in the gulf from the onset of marine incursions to the mid-Holocene, as well as the reduced contribution, as currently observed, of riverine inputs due to the progressive diminution and withdrawal of glacial drainage starting before the Holocene. -- Keywords : elemental geochemistry ; incised valleys ; MWP-1A ; Patagonia ; postglacial sea-level changes
Imaging the PâWave Velocity Structure of Arctic Subsea Permafrost Using LaplaceâDomain FullâWaveform Inversion
Climate change in the Arctic has recently become a major scientific issue, and detailed information on the degradation of subsea permafrost on continental shelves in the Arctic is critical for understanding the major cause and effects of global warming, especially the release of greenhouse gases. The subsea permafrost at shallow depths beneath the Arctic continental shelves has significantly higher Pâwave velocities than the surrounding sediments. The distribution of subsea permafrost on Arctic continental shelves has been studied since the 1970s using seismic refraction methods. With seismic refraction data, the seismic velocity and the depth of the upper boundary of subsea permafrost can be determined. However, it is difficult to identify the lower boundary and the internal shape of permafrost. Here, we present twoâdimensional Pâwave velocity models of the continental shelf in the Beaufort Sea by applying the Laplaceâdomain fullâwaveform inversion method to acquired multichannel seismic reflection data. With the inverted Pâwave velocity model, we identify anomalous high seismic velocities that originated from the subsea permafrost. Information on the twoâdimensional distribution of subsea permafrost on the Arctic continental shelf area, including the upper and lower bounds of subsea permafrost, are presented. Also, the twoâdimensional Pâwave velocity model allows us to estimate the thawing pattern and the shape of subsea permafrost structures. Our proposed Pâwave velocity models were verified by comparison with the previous distribution map of subsea permafrost from seismic refraction analyses, geothermal modeling, and wellâlog data
Uniting statistical and individual-based approaches for animal movement modelling
<div><p>The dynamic nature of their internal states and the environment directly shape animals' spatial behaviours and give rise to emergent properties at broader scales in natural systems. However, integrating these dynamic features into habitat selection studies remains challenging, due to practically impossible field work to access internal states and the inability of current statistical models to produce dynamic outputs. To address these issues, we developed a robust method, which combines statistical and individual-based modelling. Using a statistical technique for forward modelling of the IBM has the advantage of being faster for parameterization than a pure inverse modelling technique and allows for robust selection of parameters. Using GPS locations from caribou monitored in Québec, caribou movements were modelled based on generative mechanisms accounting for dynamic variables at a low level of emergence. These variables were accessed by replicating real individuals' movements in parallel sub-models, and movement parameters were then empirically parameterized using Step Selection Functions. The final IBM model was validated using both k-fold cross-validation and emergent patterns validation and was tested for two different scenarios, with varying hardwood encroachment. Our results highlighted a functional response in habitat selection, which suggests that our method was able to capture the complexity of the natural system, and adequately provided projections on future possible states of the system in response to different management plans. This is especially relevant for testing the long-term impact of scenarios corresponding to environmental configurations that have yet to be observed in real systems.</p></div
Reactivity tests for supplementary cementitious materials: RILEM TC 267-TRM phase 1
A primary aim of RILEM TC 267-TRM: âTests for Reactivity of Supplementary Cementitious Materials (SCMs)â is to compare and evaluate the performance of conventional and novel SCM reactivity test methods across a wide range of SCMs. To this purpose, a round robin campaign was organized to investigate 10 different tests for reactivity and 11 SCMs covering the main classes of materials in use, such as granulated blast furnace slag, fly ash, natural pozzolan and calcined clays. The methods were evaluated based on the correlation to the 28 days relative compressive strength of standard mortar bars containing 30% of SCM as cement replacement and the interlaboratory reproducibility of the test results. It was found that only a few test methods showed acceptable correlation to the 28 days relative strength over the whole range of SCMs. The methods that showed the best reproducibility and gave good correlations used the R3 model system of the SCM and Ca(OH)2, supplemented with alkali sulfate/carbonate. The use of this simplified model system isolates the reaction of the SCM and the reactivity can be easily quantified from the heat release or bound water content. Later age (90 days) strength results also correlated well with the results of the IS 1727 (Indian standard) reactivity test, an accelerated strength test using an SCM/Ca(OH)2-based model system. The current standardized tests did not show acceptable correlations across all SCMs, although they performed better when latently hydraulic materials (blast furnace slag) were excluded. However, the Frattini test, Chapelle and modified Chapelle test showed poor interlaboratory reproducibility, demonstrating experimental difficulties. The TC 267-TRM will pursue the development of test protocols based on the R3 model systems. Acceleration and improvement of the reproducibility of the IS 1727 test will be attempted as well
Detection of near-surface hydrocarbon seeps using P- and S-wave reflections
The combined use of P- and S-wave seismic reflection data is appealing for providing insights into active petroleum systems because P-waves are sensitive to fluids and S-waves are not. The method presented herein relies on the simultaneous acquisition of P- and S-wave data using a vibratory source operated in the inline horizontal mode. The combined analysis of P- and S-wave reflections is tested on two potential hydrocarbon seeps located in a prospective area of the St. Lawrence Lowlands in Eastern Canada. For both sites, P-wave data indicate local changes in the reflection amplitude and slow velocities, whereas S-wave data present an anomalous amplitude at one site. Differences between P- and S-wave reflection morphology and amplitude and the abrupt decrease in P-velocity are indirect lines of evidence for hydrocarbon migration toward the surface through unconsolidated sediments. Surface-gas analysis made on samples taken at one potential seeping site reveals the occurrence of thermogenic gas that presumably vents from the underlying fractured Utica Shale forming the top of the bedrock. The 3C shear data suggest that fluid migration locally disturbs the elastic properties of the matrix. The comparative analysis of P- and S-wave data along with 3C recordings makes this method not only attractive for the remote detection of shallow hydrocarbons but also for the exploration of how fluid migration impacts unconsolidated geologic media.</jats:p
Detecting subsea permafrost layers on marine seismic data: An appraisal from forward modeling
ABSTRACT: Detecting the top and base subsea permafrost from 2D seismic reflection data in shallow marine settings is a non-trivial task due to the occurrence of strong free surface multiples. The potential to accurately detect permafrost layers on conventional 2D seismic reflection data is assessed through viscoelastic modelling. Reflection imaging of permafrost layers is examined through the evaluation of specific characteristics of the subsurface, acquisition parameters and their impact. Results show that limitations are related to the principles of the method, the intrinsic nature of the permafrost layers, and the acquisition geometry. The biggest challenge is the occurrence of free surface multiples that overprint the base permafrost reflection, with the worst-case scenario the case of a thin layer of ice-bonded sand. Wedge models suggest that if the base permafrost is dipping, it would intersect internal and free surface multiples of the seafloor and the top permafrost and be detected. Also, the amplitude ratio of the base permafrost reflection and the multiples decreases with the increasing thickness of permafrost. Therefore, the crosscutting relationship between the reflection at base permafrost reflection and the multiples might not be enough to detect the base permafrost for thicker permafrost layers. Finally, the experiment results show that, for partially ice-bonded layers, the attenuation combined with the low reflectivity of the basal interface limits the likelihood to resolve the base permafrost, especially for thick permafrost layers