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Analysis of Acoustic Scattering Layers in and Around Petermann Fjord, Northwest Greenland.

紀錄類型:	書目-電子資源 : Monograph/item
正題名/作者:	Analysis of Acoustic Scattering Layers in and Around Petermann Fjord, Northwest Greenland./
作者:	Heffron, Erin.
出版者:	Ann Arbor : ProQuest Dissertations & Theses, : 2022,
面頁冊數:	470 p.
附註:	Source: Masters Abstracts International, Volume: 83-12.
Contained By:	Masters Abstracts International83-12.
標題:	Remote sensing. -
電子資源:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=29167235
ISBN:	9798819397053

Analysis of Acoustic Scattering Layers in and Around Petermann Fjord, Northwest Greenland.
Heffron, Erin.

Analysis of Acoustic Scattering Layers in and Around Petermann Fjord, Northwest Greenland. - Ann Arbor : ProQuest Dissertations & Theses, 2022 - 470 p.

Source: Masters Abstracts International, Volume: 83-12.

Thesis (M.S.)--University of New Hampshire, 2022.

This item must not be sold to any third party vendors.

In 2015 a major international collaborative expedition took place focused on understanding the processes associated with the recent rapid decline of the Greenland Ice Sheet (GIS) and the impact that this decline could have on global sea-level rise. The Petermann Expedition collected a broad range of data designed to characterize the Petermann Glacier system, a marine-terminating glacier with a floating ice tongue that has undergone dramatic changes in the last decade. During the expedition, sonars were used to map the seafloor and the water column, generating a continuous dataset over 30 days. The water column mapping revealed extensive acoustic scattering layers, so called because the components of the layer - typically zooplankton and fish - scatter acoustic energy when concentrated in layers in the water column. The scattering layer was observed to change depth in a geospatially consistent manner and corresponded to our general, but limited understanding of the complex circulation patterns in the study area. This unexpected observation became the research question investigated in this thesis: Is the distribution of the acoustic scattering layer observed in and around Petermann Fjord a proxy for spatial and temporal changes in water mass structure and interactions? In order to answer this question, we focused on four objectives: determine the geospatial distribution of the scattering layer, determine if light influences the scattering layer depth distribution, determine if there is a consistent relationship to water column structure and circulation, and investigate the components of the scattering layer for clues as to its make-up and subsequently any potential reasoning for its distribution.Understanding the distribution of water masses and their circulation patterns in Arctic fjords are critical to understanding the fate of floating ice shelves and the glaciers they buttress, as the most pronounced change is occurring where ice sheets are grounded below sea level due to enhanced interaction with warming ocean waters. However, our ability to predict future sea level rise is hampered by our limited knowledge of these glacial systems, including the regional water mass distribution and circulation responsible for that enhanced ocean-ice interaction. Indeed, quantification of melting processes at marine terminating glaciers represents the largest source of uncertainty in predicting global sea level rise (Church et al., 2013). Traditional methods of oceanographic observation provide relatively sparse information at high cost, whereas acoustic records are continuous and, if the observed relationship between scattering layer depth and regional hydrography holds true, can potentially provide information about circulation, productivity, and ocean dynamics over large areas from underway platforms.Evaluation of the scattering layer distribution focused on the continuous Simrad EK80 18 kHz split-beam echosounder sonar records (section 3.1.1.1). The top of the scattering layer was manually picked on each echogram, providing the latitude, longitude, and depth for the top of each layer (section 3.2.1) that were then plotted to show the geospatial and depth distribution. The resulting distributions (section 4.1) showed a recognizable geospatial pattern that was consistent with our understanding of the distribution of water masses. Broadly, there was a scattering layer generally present in the fjord along the coast of Greenland (eastern Hall Basin) and ringing central Hall Basin, and absent in northern Hall Basin, along the coast of Ellesmere Island (northern Nares Strait and western Hall Basin), central Hall Basin, and southern Nares Strait. The top of the scattering layer was significantly shallower in the fjord and along the coast of Greenland, deepening in the central ring and western Hall Basin (when it was present). We evaluated whether there was a linear correlation between the scattering layer depth and the bathymetric depth and slope (sections 3.1.1.2, 3.1.1.5, 3.2.2), but no correlation was found (section 4.2.1).The second objective was to determine whether the scattering layer distribution was influenced by light rather than water mass distribution. This analysis was undertaken because of the typical association of scattering layers with daily migrations corresponding to daily light cycles as a means of predator avoidance (section 1.3.3). Though the expedition took place in Arctic summer during the 'midnight sun' regime of 24-hour light, there was enough daily change to discern a cycle in the ship-based radiation data collected by a Photosynthetically Active Radiation (PAR) Sensor mounted on the roof of the ship's bridge (section 3.1.2.2). The relationship between light levels and scattering layer depth was examined (section 3.2.3), finding no linear correlation (section 4.2.2). A second analysis was done to see if we could discern a difference in water clarity across the study area using satellite-derived Kd(490) data, the diffuse attenuation coefficient for downwelling irradiance at 490 nm (section 3.1.3), and evaluate its effect on the scattering layer depth. Though available data for this region was very limited and there was some evidence of higher attenuation in the fjord. (Abstract shortened by ProQuest).

ISBN: 9798819397053Subjects--Topical Terms:

535394
Remote sensing.
Subjects--Index Terms:

Bathymetry

Analysis of Acoustic Scattering Layers in and Around Petermann Fjord, Northwest Greenland.
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520 $a In 2015 a major international collaborative expedition took place focused on understanding the processes associated with the recent rapid decline of the Greenland Ice Sheet (GIS) and the impact that this decline could have on global sea-level rise. The Petermann Expedition collected a broad range of data designed to characterize the Petermann Glacier system, a marine-terminating glacier with a floating ice tongue that has undergone dramatic changes in the last decade. During the expedition, sonars were used to map the seafloor and the water column, generating a continuous dataset over 30 days. The water column mapping revealed extensive acoustic scattering layers, so called because the components of the layer - typically zooplankton and fish - scatter acoustic energy when concentrated in layers in the water column. The scattering layer was observed to change depth in a geospatially consistent manner and corresponded to our general, but limited understanding of the complex circulation patterns in the study area. This unexpected observation became the research question investigated in this thesis: Is the distribution of the acoustic scattering layer observed in and around Petermann Fjord a proxy for spatial and temporal changes in water mass structure and interactions? In order to answer this question, we focused on four objectives: determine the geospatial distribution of the scattering layer, determine if light influences the scattering layer depth distribution, determine if there is a consistent relationship to water column structure and circulation, and investigate the components of the scattering layer for clues as to its make-up and subsequently any potential reasoning for its distribution.Understanding the distribution of water masses and their circulation patterns in Arctic fjords are critical to understanding the fate of floating ice shelves and the glaciers they buttress, as the most pronounced change is occurring where ice sheets are grounded below sea level due to enhanced interaction with warming ocean waters. However, our ability to predict future sea level rise is hampered by our limited knowledge of these glacial systems, including the regional water mass distribution and circulation responsible for that enhanced ocean-ice interaction. Indeed, quantification of melting processes at marine terminating glaciers represents the largest source of uncertainty in predicting global sea level rise (Church et al., 2013). Traditional methods of oceanographic observation provide relatively sparse information at high cost, whereas acoustic records are continuous and, if the observed relationship between scattering layer depth and regional hydrography holds true, can potentially provide information about circulation, productivity, and ocean dynamics over large areas from underway platforms.Evaluation of the scattering layer distribution focused on the continuous Simrad EK80 18 kHz split-beam echosounder sonar records (section 3.1.1.1). The top of the scattering layer was manually picked on each echogram, providing the latitude, longitude, and depth for the top of each layer (section 3.2.1) that were then plotted to show the geospatial and depth distribution. The resulting distributions (section 4.1) showed a recognizable geospatial pattern that was consistent with our understanding of the distribution of water masses. Broadly, there was a scattering layer generally present in the fjord along the coast of Greenland (eastern Hall Basin) and ringing central Hall Basin, and absent in northern Hall Basin, along the coast of Ellesmere Island (northern Nares Strait and western Hall Basin), central Hall Basin, and southern Nares Strait. The top of the scattering layer was significantly shallower in the fjord and along the coast of Greenland, deepening in the central ring and western Hall Basin (when it was present). We evaluated whether there was a linear correlation between the scattering layer depth and the bathymetric depth and slope (sections 3.1.1.2, 3.1.1.5, 3.2.2), but no correlation was found (section 4.2.1).The second objective was to determine whether the scattering layer distribution was influenced by light rather than water mass distribution. This analysis was undertaken because of the typical association of scattering layers with daily migrations corresponding to daily light cycles as a means of predator avoidance (section 1.3.3). Though the expedition took place in Arctic summer during the 'midnight sun' regime of 24-hour light, there was enough daily change to discern a cycle in the ship-based radiation data collected by a Photosynthetically Active Radiation (PAR) Sensor mounted on the roof of the ship's bridge (section 3.1.2.2). The relationship between light levels and scattering layer depth was examined (section 3.2.3), finding no linear correlation (section 4.2.2). A second analysis was done to see if we could discern a difference in water clarity across the study area using satellite-derived Kd(490) data, the diffuse attenuation coefficient for downwelling irradiance at 490 nm (section 3.1.3), and evaluate its effect on the scattering layer depth. Though available data for this region was very limited and there was some evidence of higher attenuation in the fjord. (Abstract shortened by ProQuest).
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