Over the past 20 years, globally-averaged sea level has risen at a rate near the upper end of the sea level projections of the Intergovernmental Panel on Climate Change. With many coastal areas being located close to or below Mean Sea Level (MSL), there is a growing threat of them being engulfed. A series of tide gauges (TG) located at coastal sites measure the MSL. Tide gauge records of mean sea level are an important data set for the computation of long-term estimates of changes in sea level, for both regional and global analyses. These measurements, however, contain deformation due to sea level variation and vertical land motion (VLM). In order to fully understand long-term sea level changes and to thereby plan effectively for flood risk management, there is a need to identify the component of the change in mean sea level measured by a tide gauge that is due to changes in land level. Measurements obtained from precise levelling, continuous Global Positioning System (CGPS) and Absolute Gravity (AO) at, or close to, tide gauges are used to correct the tide gauge records for VLM. However, these techniques are only capable of providing a point-wise estimate for VLM, i.e. at the COPS antenna installation. Persistent Scanerer Interferometry (PSI) is a powerful technique for identifying long-term changes in vertical land motion with millimetric accuracy and it has been successfully applied to deformation monitoring in both urban and rural areas. The PSI technique takes advantage of a large number of multi-date imaging radar data to identify and analyse dense arrays of naturally occurring, consistently bright or coherent points in the images. Unlike precise levelling, COPS and AO, PSI is capable of providing estimates over a wide spatial extent. However, like precise levelling, these estimates are relative as opposed to the absolute VLM provided by CGPS and AG. This research in this thesis will investigate the application of PSI to aid monitoring of tide gauge sites in the United Kingdom. The primary research question is whether the Interferometric Synthetic Aperture Radar (lnSAR) time-series analysis can provide patterns of regional deformation of coastal and inland areas and whether this technique can be used to assess the stability of the tide gauge sites and its use in long term sea level monitoring. Four study areas with various settings were selected based on the extent of urbanisation, available imagery and the existence of other geodetic information at the site. These test sites were Newlyn and Sheerness (both characterised as rural areas with sparse urban regions), Liverpool and North Shields (both highly urban). In addition to the differences in population density, these four test sites also covered a range of geological conditions around the coast. This research considered PSI processing difficulties that arise from the application of this technique to coastal areas. These included the ambiguities arising from the phase unwrapping of the edge PS points (knovo11 as a "boundary problem"), the choice of an optimal reference point, and the effect of the ocean tide loading on the estimated PS velocities. It was shown that deformation can be properly detected, even when located on the coastline. Although the ocean tide loading can introduce displacement gradient of the order of a few centimetres per l00km in the interferogram, -this study has shown that the ocean tide loading has little effect on the PS points velocity estimates. This research has also investigated the possible ways of integrating the CGPS deformation with the PSI. By choosing the nearest PS point (on a ground considered stable) to the CGPS station as a reference, and adding on the difference between the PSI and CGPS deformation to all PS points, velocity estimates can be obtained that are similar to those reported by CGPS. The assessment of the application of PSI to the four coastal sites consisted of: a) considering the underlying regional deformation for better understanding of the deformation processes and their possible causes and b) studying the local deformation in close proximity to the tide gauge and the CGPS station to assess the stability of the site. As expected, the PSI results for the sites with sparse urban areas gave very poor coherence and too few PS points to fully understand the deformation pattern of the structure supporting the tide gauge and the CGPS station. However, for the highly urban areas, a good PS point density was obtained, revealing the deformation pattern in the area. By comparing the land motion present at the coast with that of a more inland area, it was shown that, for most sites, there is negligible differential motion, although, in few cases a more complex deformation pattern was revealed. It was also shown that the highly non-linear deformation pattern was not reflected in the linear velocity estimates, highlighting the importance of investigating the non-linear deformation profile of the PS point.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:594863 |
| Date | January 2012 |
| Creators | Adamska, Ligia Maria |
| Publisher | University of Nottingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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