Time-lapse Acoustic Imaging of Oceanic Fronts and Eddies
Seismic reflection surveying is used to generate acoustic images of the water column. This technique employs conventional multi-channel equipment which is used to image the solid Earth. In the water column, acoustic impedance contrasts are produced by variations in temperature and, to some extent, s...
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Format: | Thesis |
Language: | English |
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Apollo - University of Cambridge Repository
2019
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Online Access: | https://dx.doi.org/10.17863/cam.31936 https://www.repository.cam.ac.uk/handle/1810/284560 |
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Open Polar |
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DataCite Metadata Store (German National Library of Science and Technology) |
op_collection_id |
ftdatacite |
language |
English |
topic |
Seismic Oceanography Oceanic Fronts and Eddies Inversion |
spellingShingle |
Seismic Oceanography Oceanic Fronts and Eddies Inversion Gunn, Kathryn Louise Time-lapse Acoustic Imaging of Oceanic Fronts and Eddies |
topic_facet |
Seismic Oceanography Oceanic Fronts and Eddies Inversion |
description |
Seismic reflection surveying is used to generate acoustic images of the water column. This technique employs conventional multi-channel equipment which is used to image the solid Earth. In the water column, acoustic impedance contrasts are produced by variations in temperature and, to some extent, salinity. Acoustic impulses generated by an array of airguns suspended behind a vessel are reflected from these contrasts and recorded on long cables of hydrophones that are towed below the sea-surface. In this way, two- and three-dimensional images of thermohaline circulation can be generated. Critically, these images have equal vertical and horizontal resolutions of \textit{O}(10)~m. Here, I describe, process, and analyse a calibrated two-dimensional seismic survey from the Bellingshausen Sea of the Southern Ocean and a three-dimensional seismic survey from the Brazil-Falkland Confluence located offshore Uruguay. First, the Bellingshausen survey was designed to image the thermohaline structure across the west Antarctic shelf where warm-core eddies are reported. Processed and calibrated seismic images reveal the detailed thermohaline structure of Circumpolar Deep Water. Many warm-core eddies are observed, which have diameters of 1--12~km and thicknesses of 100--200~m. Pre-stack analysis demonstrates that this eddy field is being advected onto the shelf at speeds of \textit{O}(0.1)~m~s$^{-1}$. An iterative inverse modelling procedure is used to convert reflectivity into temperature and salinity, which confirms that the eddies have anomalously warm centres (i.e. $\sim$1$^{\circ}$C). These results have significant implications for ice shelf melting. Secondly, the Uruguay survey is used to investigate a large-scale frontal system. Although this system has been studied using hydrographic methods, these studies either have limited spatial resolution or have restricted depth penetration. The three-dimensional seismic survey, which was acquired in a `racetrack' pattern, permits the volume to be interrogated. Since the frontal system migrates southwestwards at a speed of \textit{O}(10)~km~day$^{-1}$, this survey is time-lapse in nature. Processed images reveal a band of dipping reflections that extend to depths of $\sim$2000~m. These reflections represent the frontal interface between the Brazil and Falkland currents. Physical oceanographic properties are calculated for images that cross this front. On the warm side of the front, the water mass is characterised by flat and continuous reflectivity. On the cold side of the front, the water mass is characterised by deformed reflectivity on all scales. Pre-stack analysis suggests that near-surface flow at the frontal interface is convergent. Between 0.5 and 1~km depth, a substantial eddy that is 30~km long and 250~m thick is visible on the cold side of the front. Detailed mapping suggests that this eddy grew and decayed over a period of 6~days. Its observed scale and duration are inconsistent with analytical and numerical studies of intra-thermocline eddies. Nevertheless, its duration is consistent with scaling arguments of frictional spin-down. Spatial and temporal distributions of mixing rates (i.e. diapycnal diffusivities) are estimated by spectrally analysing vertical displacements of automatically tracked reflections. Both internal wave and turbulent regimes are identifiable. Recovered diapycnal diffusivities are of \textit{O}($10^{-6}$--$10^{-2.2}$)~m$^{2}$~s$^{-1}$, consistent with hydrographically determined estimates. Mixing is suppressed and enhanced on the warm and cold sides of the front, respectively. Seismic Oceanography has considerable potential to quantify aspects of thermohaline circulation on multiple scales. |
format |
Thesis |
author |
Gunn, Kathryn Louise |
author_facet |
Gunn, Kathryn Louise |
author_sort |
Gunn, Kathryn Louise |
title |
Time-lapse Acoustic Imaging of Oceanic Fronts and Eddies |
title_short |
Time-lapse Acoustic Imaging of Oceanic Fronts and Eddies |
title_full |
Time-lapse Acoustic Imaging of Oceanic Fronts and Eddies |
title_fullStr |
Time-lapse Acoustic Imaging of Oceanic Fronts and Eddies |
title_full_unstemmed |
Time-lapse Acoustic Imaging of Oceanic Fronts and Eddies |
title_sort |
time-lapse acoustic imaging of oceanic fronts and eddies |
publisher |
Apollo - University of Cambridge Repository |
publishDate |
2019 |
url |
https://dx.doi.org/10.17863/cam.31936 https://www.repository.cam.ac.uk/handle/1810/284560 |
geographic |
Antarctic Bellingshausen Sea Southern Ocean Uruguay |
geographic_facet |
Antarctic Bellingshausen Sea Southern Ocean Uruguay |
genre |
Antarc* Antarctic Bellingshausen Sea Ice Shelf Southern Ocean |
genre_facet |
Antarc* Antarctic Bellingshausen Sea Ice Shelf Southern Ocean |
op_rights |
https://www.rioxx.net/licenses/all-rights-reserved/ All Rights Reserved All rights reserved |
op_doi |
https://doi.org/10.17863/cam.31936 |
_version_ |
1766263282178457600 |
spelling |
ftdatacite:10.17863/cam.31936 2023-05-15T13:56:04+02:00 Time-lapse Acoustic Imaging of Oceanic Fronts and Eddies Gunn, Kathryn Louise 2019 https://dx.doi.org/10.17863/cam.31936 https://www.repository.cam.ac.uk/handle/1810/284560 en eng Apollo - University of Cambridge Repository https://www.rioxx.net/licenses/all-rights-reserved/ All Rights Reserved All rights reserved Seismic Oceanography Oceanic Fronts and Eddies Inversion Text Thesis article-journal ScholarlyArticle 2019 ftdatacite https://doi.org/10.17863/cam.31936 2021-11-05T12:55:41Z Seismic reflection surveying is used to generate acoustic images of the water column. This technique employs conventional multi-channel equipment which is used to image the solid Earth. In the water column, acoustic impedance contrasts are produced by variations in temperature and, to some extent, salinity. Acoustic impulses generated by an array of airguns suspended behind a vessel are reflected from these contrasts and recorded on long cables of hydrophones that are towed below the sea-surface. In this way, two- and three-dimensional images of thermohaline circulation can be generated. Critically, these images have equal vertical and horizontal resolutions of \textit{O}(10)~m. Here, I describe, process, and analyse a calibrated two-dimensional seismic survey from the Bellingshausen Sea of the Southern Ocean and a three-dimensional seismic survey from the Brazil-Falkland Confluence located offshore Uruguay. First, the Bellingshausen survey was designed to image the thermohaline structure across the west Antarctic shelf where warm-core eddies are reported. Processed and calibrated seismic images reveal the detailed thermohaline structure of Circumpolar Deep Water. Many warm-core eddies are observed, which have diameters of 1--12~km and thicknesses of 100--200~m. Pre-stack analysis demonstrates that this eddy field is being advected onto the shelf at speeds of \textit{O}(0.1)~m~s$^{-1}$. An iterative inverse modelling procedure is used to convert reflectivity into temperature and salinity, which confirms that the eddies have anomalously warm centres (i.e. $\sim$1$^{\circ}$C). These results have significant implications for ice shelf melting. Secondly, the Uruguay survey is used to investigate a large-scale frontal system. Although this system has been studied using hydrographic methods, these studies either have limited spatial resolution or have restricted depth penetration. The three-dimensional seismic survey, which was acquired in a `racetrack' pattern, permits the volume to be interrogated. Since the frontal system migrates southwestwards at a speed of \textit{O}(10)~km~day$^{-1}$, this survey is time-lapse in nature. Processed images reveal a band of dipping reflections that extend to depths of $\sim$2000~m. These reflections represent the frontal interface between the Brazil and Falkland currents. Physical oceanographic properties are calculated for images that cross this front. On the warm side of the front, the water mass is characterised by flat and continuous reflectivity. On the cold side of the front, the water mass is characterised by deformed reflectivity on all scales. Pre-stack analysis suggests that near-surface flow at the frontal interface is convergent. Between 0.5 and 1~km depth, a substantial eddy that is 30~km long and 250~m thick is visible on the cold side of the front. Detailed mapping suggests that this eddy grew and decayed over a period of 6~days. Its observed scale and duration are inconsistent with analytical and numerical studies of intra-thermocline eddies. Nevertheless, its duration is consistent with scaling arguments of frictional spin-down. Spatial and temporal distributions of mixing rates (i.e. diapycnal diffusivities) are estimated by spectrally analysing vertical displacements of automatically tracked reflections. Both internal wave and turbulent regimes are identifiable. Recovered diapycnal diffusivities are of \textit{O}($10^{-6}$--$10^{-2.2}$)~m$^{2}$~s$^{-1}$, consistent with hydrographically determined estimates. Mixing is suppressed and enhanced on the warm and cold sides of the front, respectively. Seismic Oceanography has considerable potential to quantify aspects of thermohaline circulation on multiple scales. Thesis Antarc* Antarctic Bellingshausen Sea Ice Shelf Southern Ocean DataCite Metadata Store (German National Library of Science and Technology) Antarctic Bellingshausen Sea Southern Ocean Uruguay |