1. module Using DInSAR to assess vertical ground motions at Tide Gauges
Daniel Raucoules1, Cyril Poitevin2, Guy Wöppelmann2, Gonéri Le Cozannet1, Laurent Testut1, Marcello de Michele2
1: French Geological Survey, BRGM, Orléans (France); 2: Université La Rochelle, LIENSs (France);
Before the altimetry era, tide gauges are a unique source of information to evaluate past sea-level changes. However, they can be affected by vertical ground motions acting at different space scales.
We use Synthetic Aperture Radar Interferometry techniques to assess these ground motions and their consequences for key geodetic instruments such as Tide Gauge, GPS, Doris stations. Summarizing results obtained at Alexandria (Egypt), Manila (Philippines), Dakar (Senegal), and Brest (France), we identify different situations in terms of data availability and ground motion context.
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Tide-gauges with time series longer than 100 years and more than 70% completeness.
Manila, Philipines: rapid non-linear ground motions
Alexandria, Egypt: Persistent Scatterers analysis
Figures: Line of Sight ground motion velocity maps obtained by the stacking of ERS and Envisat interferograms
Results:
Large and non-linear vertical ground motions, presumably due to groundwater pumping
Observed ground motions affect the location of several geodetic instruments (permanent GPS, tide-gauge and Doris station).
Deformations observed along the Marikina Valley fault could be related to tectonic strain or to subsidence related to water extraction, the fault acting as a barrier in the latter case
Study area of Alexandria (Egypt) showing the tide gauge (circle) and the continuous GPS station from the CEAlex (square) used in our analysis. Linear line of sight velocities over the period 2002–2010 from the PSI results are given with respect to the GPS station. Positive velocities indicate site uplift, whereas negative velocities indicate subsidence.
PSI and GPS time series projected along the satellite line of sight with (a) the persistent scatterer nearest to the continuous GPS antenna and (b) the persistent scatterer nearest to the tide gauge. The red line shows the best linear fit.
Results: consistence of two independent geodetic measurements in Alexandria:
altimetry-mareography using the Kuo et al. (2004) method
Permanent GPS and local ground motion characterization using PSI
This Tide Gauge results reliable for past sea-level assessment.
Dakar, Senegal: few ground motions; few SAR data
Brest, France: Persistent Scatterers analysis
Sea levels have been recorded in Brest since the 17th century (data available in the PSMSL database goes back to 1807).
The latest global GPS solution of the university of La Rochelle indicates that the permanent GPS is very sable (0.01+/ mm/year). However, the GPS is located 200m southwestward of the tide gauge.
The PSI results show that the city is mostly stable, except in the east embankments, where a subsidence of 1,5 to 3,5mm/year is observed.
The yearly sea level time series recorded in the PSMSL are presented below (forward-backward kalman filter and 1-sigma uncertainties), assuming that the tide gauge is affected by the same vertical ground motion than the GPS of the SONEL database (0.01+/ mm/year).
Data: 25 (resp. 10) valid ERS (resp. Envisat ASAR) SAR images
Method: stacking of the entire set of ERS and Envisat-ASAR interferograms
Estimation of errors: less than 1 mm/year
ERS: 49 images from 1992 to 2000
- average deformation: mm/yr
- PS density: 119 PS/km²
Envisat: 19 images from 2002 to 2008
- average deformation: -0,11 mm/yr
- PS density: 211 PS/km²
Results:
Considering the errors in each geodetic technique, the measurements of the different geodetic instruments (Permanent GPS and Doris station) are consistent
Surprisingly, despite a complex geology, land use and ground resources management, few ground motions are observed
Only the northern part of the harbour is affected by a slight subsidence (-4+/-1mm/year from 2002 to 2010), presumably due to construction works
The location of the historical tide gauge is unaffected by ground motions.
A suitable site to estimate past sea-levels along the coast of Africa, using the time series covering a timespan longer than 100 years.
yearly Sea level (mm)
Conclusions
These examples illustrate that:
when strong ground motions affect the tide gauge, the technique can easily help rejecting tide gauges records (case of Manila)
when no ground motions can be observed using InSAR in the vicinity of geodetic instruments (Case of Alexandria, Dakar and Brest), it remains challenging to confirm that tide gauge records are indeed suitable for monitoring sea level changes at the required accuracy
In the latter case, some assumptions must be made on the linearity of past ground motions and a large set of SAR data should be available.
For future science application of Sentinel 1 in the field of geodesy underpinning sea level science, it will be necessary to define appropriate background missions of key coastal sites.
InSAR and PSI Processing has been funded by BRGM (Project EVARISTE) with additional support from the ANR (CECILE project). SAR data have been provided by ESA (Cat. 1 projects).