| Titre | Nearbed current and suspended sediment concentration recorded by a seabed lander deployed in the deep water at Sackville Spur, Grand Banks, Newfoundland and Labrador |
| Télécharger | Téléchargements |
| Auteur | Li, M Z; Schillinger, D J; Robertson, A |
| Source | Commission géologique du Canada, Dossier public 8136, 2016, 27 pages, https://doi.org/10.4095/299384 (Accès ouvert) |
| Image |  |
| Année | 2016 |
| Éditeur | Ressources naturelles Canada |
| Document | dossier public |
| Lang. | anglais |
| DOI | https://doi.org/10.4095/299384 |
| Media | en ligne; numérique |
| Formats | pdf |
| Province | Région extracotière de l'est |
| Lat/Long OENS | -47.0000 -45.0000 49.0000 47.0000 |
| Sujets | talus continental; sedimentation; courants; courants de fond; transport des sediments; transport sous-marin; silts; argiles; sables; Miocène; Pliocène; discordances; caractéristiques structurales;
érosion; biostratigraphie; bioturbation; géologie marine; stratigraphie; paléontologie; sédimentologie; Quaternaire; Tertiaire; Cénozoïque |
| Illustrations | location maps; photographs; tables; profiles; plots |
| Programme | Géo-risques marins, Géoscience pour la sécurité publique |
| Diffusé | 2016 12 02 |
| Résumé | (Sommaire disponible en anglais seulement)
Powerful near-bed currents exist on the Grand Banks and Labrador slopes down to >1000 m. Field data is lacking in these areas for assessing near-bed
current strength and the frequency and magnitude of sediment transport in deep slope waters. The Geologic Survey of Canada-Atlantic (GSCA) deployed a redesigned deep-water seabed lander from August 9 - 21, 2011 in 1200 m water depth on the northern
flank of the Sackville Spur, Grand Banks, to collect near-bed currents and sediment transport data. This seabed lander was designed for free-fall type deployment and equipped with a deep-water rated AquaDopp Acoustic Doppler Current Profiler (ADCP)
with an analog Optical Backscatter Sensor (OBS) and a camera capable of burst recording both video and still photos of the seabed. The goal of this redesigned lander is to expand our seabed lander technology to make observations of near-bed currents
and sediment transport in deep water environments. Assessments of the recorded data indicate that the AquaDopp ADCP ran for the entire deployment and recorded velocity and backscatter profile data as well as suspended sediment concentration at 1.2 m
above bottom for ~13 days. Unfortunately, the camera system only recorded seabed images intermittently for the first four days of the deployment due to the premature depletion of the battery power. Analysis of the acoustic backscatter and velocity
data recorded by the AquaDopp ADCP shows acoustic interference, likely from the lander frame, contaminating the data of the range from 50 to 60 cm from the transducer. Side-lobe interference from the bottom likely corrupted the bottom three range
bins to render the velocity data in the bottom 30 cm unreliable.
The seabed photos show grey mud with possible bioturbation at the lander site.. A cursory evaluation of the seabed images, recorded intermittently from August 9th to 13th, suggests
that no apparent sediment transport could be observed during these four days. The preliminary analysis of the ADCP data shows several moderate current events, in which the peak bottom currents reached nearly 30 cm/s. The direction of these peak
currents was predominantly to the east. During these moderate current events, the ADCP backscatter intensity (a proxy of suspended sediment concentration) was increased by nearly 50% from the mean background values. A positive correlation between the
depth-averaged acoustic backscatter intensity and the depth-averaged current speed was observed. This would suggest that increased current speeds were responsible for the increase of suspended sediment concentration. Profiles of backscatter averaged
over the deployment duration show that suspended sediment concentration increases with height above seabed from 0.3 m to 1.3 m above bottom. This suggests that averaged over the deployment duration, the suspended sediments were not bottom intensified
and were probably advected from other areas. In the bottom 0.3 m, however, the backscatter amplitude increases rapidly towards the seabed. The profile of the averaged backscatter for current speeds > 0.2 m/s also shows that the backscatter intensity
was higher near the seabed and decreased with the increasing height above bottom. This is evidence that local erosion of bottom sediments probably occurred when current speeds exceeded ~0.2 m/s. Nevertheless, the analysis of the data collected is
inconclusive whether the increased sediment concentration under increased currents was due to local erosion or being advected to the deployment site from another region. Data from a two day period during the early part of the deployment shows
intriguing sinusoidal variations of temperature, current speed and current direction with elevated suspended sediment concentration. Further efforts are warranted to explore the driving processes causing this observed event. |
| Résumé | (Résumé en langage clair et simple, non publié)
Il existe de puissants courants près du fond sur les talus des Grands Bancs et du Labrador. Un module de descente en eau profonde complètement
modifié a été déployé en août 2011 sur des dépôts vaseux à 1 200 m de profondeur, sur le flanc nord de l'éperon de Sackville, Grands Bancs, pour recueillir des données sur les courants près du fond et le transport des sédiments. L'analyse des données
sur les courants montre l'occurrence de plusieurs épisodes de courants modérés. Les courants de fond les plus forts ont atteint ~30 cm/s et étaient principalement vers l'est. Durant ces épisodes de courants, l'intensité de rétrodiffusion acoustique
(un indicateur de la concentration de sédiments en suspension) augmentait de près de 50 % par rapport aux valeurs de référence. Il existe une corrélation positive entre l'intensité de rétrodiffusion par rapport à la profondeur et la vitesse du
courant. Cela pourrait indiquer que des courants plus rapides ont été responsables de l'augmentation de la concentration de sédiments en suspension. Des variations sinusoïdales intrigantes de la température, de la vitesse et de la direction des
courants avec une concentration élevée en sédiments ont aussi été observées sur une période de deux jours. |
| GEOSCAN ID | 299384 |
|
|