MEASURING LAND SUBSIDENCE IN HA NOI CITY

7th FIG Regional Conference Spatial Data Serving People Land Governance and the Environment – Building the Capacity MEASURING LAND SUBSIDENCE IN HA NOI CITY BY MEANS OF RADAR INTERFEROMETRY TECHNIQUES Nguyen Ba Duy, Ho Tong Minh Dinh, Francesco Sarti, Steffen Dransfeld, Ramon Hanssen DEPARTMENT OF PHOTOGRAMMETRY AND REMOTESENSING HA NOI UNIVERSITY OF MINING AND GEOLOGY CONTENT Introduction Radar Interferometry technique Preliminary results in Ha Noi city Conclusion INTRODUCTION Subsidence in Ha Noi City is a long term problem • Convensional methods : precise levelling and GPS – they are costly and time consuming • Radar Interferometry – Differential InSAR - economical, fast, provides information on a large area INSAR – DInSAR – Pass SAR: P = a ⋅ sin j (θ +φ ) noise Phase component Amplitude component SAR sensors ERS - (1991 - 2000) ERS - (1995 – now) RadarSAT - (1995 – now) ENVISAT (2002 – now) INSAR – DInSAR – Pass InSAR: Interferometry SAR • Two satellite images of the same area at two different times • Data are processed into complex SAR images • The phase difference of the two images is processed to obtain height and also motion information of the Earth’s Surface INSAR – DInSAR – Pass Ỉ Differential InSAR InSAR Relation phase - height: ∆φ = − 4π Bn q λ R sin θ Phase change : ∆φd = 4π λ d d: the relative scatterer displacement projected on the slant range direction ==> ∆φ = − Altitude contribution 4π Bn q 4π + d λ R sin θ λ Motion contribution Suppose that some of the point scatterers on the ground slightly change their relative position in the time interval between two SAR observations (as, for example, in the event of subsidence, landslide, earthquake, etc.) In such cases the following additive phase term, independent of the baseline, appears in the interferometric phase: ∆φ d = 4π λ d where d is the relative scatterer displacement projected on the slant range direction • This means that after interferogram flattening, the interferometric phase contains both altitude and motion contributions: ∆φ = − Bn q 4π + d λ R sin θ λ 4π • If a digital elevation model (DEM) is available, the altitude contribution can be subtracted from the interferometric phase (generating the socalled differential interferogram) and the terrain motion component can be measured Pass Differential Two-pass differential interferometry (2pass DInSAR) is based on: - An interferometric image pair ; - A Digital Elevation Model (DEM) This method only employs two SAR images, thus producing just one interferogram To perform the differencing, another interferogram has to be created, or synthesised The synthesised interferogram is generated from an existing digital elevation model (DEM) of the area (and from precise knowledge as to the satellite position at the time of image acquisitions orbital state vectors) The synthesised interferogram is then subtracted from the original interferogram, thereby removing all fringes that relate to ground elevation, leaving only fringes that represent surface displacement 10 SAR SLC data Reference DEM Two Co-registered Interferogram Generation Topographic Interferogram Simulated Interferogram Differential Interferogram Measuring Displacement Enhanced Interferogram Unwrapped Interferogram 11 RESULT Study area • Regions in Ha Noi City 12 RESULT Data N0 Sensor Date Orbit ENVISAT_ASAR 13-Oct-2003 8460 ENVISAT_ASAR 17-Nov-2003 8961 ENVISAT_ASAR 19-Jul-2004 12468 ENVISAT_ASAR 05-Apr-2004 10965 ENVISAT_ASAR 22-Sep-2004 13406 ENVISAT_ASAR 22-Nov-2007 29502 Source: ESA CAT Project: C1F.5203 13 RESULT Data Baseline distribution for HaNoi City data set 14 RESULT Data Average backscatter intensity 15 RESULT Data SRTM DEM 16 RESULT Interferograms 12468 8460 Master Slave Master(y/m/d) Salve(y/m/d) Bperp(m) Btime(days) 12468 8460 2004/06/19 2003/11/13 190.5 -280 RESULT 17 Interferograms 12468 20965 Master Slave Master(y/m/d) Salve(y/m/d) Bperp(m) Btime(days) 12468 29502 2004/06/19 2007/11/22 -381.1 1190 18 RESULT Differential interferogram 19 RESULT DInSAR subsidence value 20 DInSAR subsidence value 21 CONCLUSION The subsidence at the area study has been studied using Pass DInSAR with ENVISAT imagery The DinSAR results shows the subsidence with the maximum magnitude of 64 mm during 190 days (from 2003/11/13 to 2004/06/19) According to the result above, it is easily realized that the subsidence in the investigated area is not the same, some area has the highest subsidence such as Thanh Cong (6cm), the lowest subsidence area is Mai Dich (3cm) This can be explained by the geographic structure such as the level of water exploition in the areas are different 22 • Moreover, at some areas where there are different Underwater Exploition Stations, the subsidence is different The area of which geographic structure is weak or in which there are different level of water exploition, will have a higer subsidence than the others where the structure is strong and there is less water exploition • Join GPS/InSar study of postseismic deformation • Time series analysis of the deformation along the Ha Noi City segments of fault 23 Thank you very much 24 Outline Map Ha Noi 25 The depression of surface (from 1998 to 2004) Place Ha Dinh Phap Van Ngoc Ha Thanh Cong Mai Dich The depression fo surface(mm) 123,45 138,40 13,65 283,93 18,28 26 Subsidence (from 1998 to 2004) 27 Subsidence (from 1998 to 2004) 28 DInSAR subsidence value interpretation 29 • • • Phase noise due to temporal change of the scatterers In the case of a water basin or densely vegetated areas, the scatterers change totally after a few milliseconds, whereas exposed rocks or urban areas remain stable even after years Of course, there are also the intermediate situations where the interferometric phase is still useful even if corrupted by change noise Phase noise due to different look angle Speckle will change due to the different combination of elementary echoes even if the scatterers not change in time The most important consequence of this effect is that there exists a critical baseline over which the interferometric phase is pure noise The critical baseline depends on the dimension of the ground range resolution cell (and thus also on the terrain slope), on the radar frequency, and on the sensor-target distance In the ERS case, the critical baseline for horizontal terrain is about 1150 metres It decreases for positive terrain slopes and increases for negative ones This phase noise term, however, can be removed from the interferogram by means of a pre-processing step of the two SAR images known as spectral shift or common band filtering This will be described in detail in the advanced sections of this manual (part B section 2.5 and part C section 2.2.1) Phase noise due to volume scattering The critical baseline reduces in the case of volume scattering when the elementary scatterers are not disposed on a plane surface but occupy a volume (e.g the branches of a tree) In this case the speckle change depends also on the depth of the volume occupied by the elementary scatterers 30 ... for example, in the event of subsidence, landslide, earthquake, etc.) In such cases the following additive phase term, independent of the baseline, appears in the interferometric phase: ∆φ d =... processed to obtain height and also motion information of the Earth’s Surface INSAR – DInSAR – Pass Ỉ Differential InSAR InSAR Relation phase - height: ∆φ = − 4π Bn q λ R sin θ Phase change : ∆φd...INTRODUCTION Subsidence in Ha Noi City is a long term problem • Convensional methods : precise levelling and GPS – they are costly and time consuming • Radar Interferometry – Differential InSAR