1. Perth subsidence monitoring project
Funded by
Perth subsidence monitoring project
Mick Filmer1, Will Featherstone1, Andreas Schenk2
& in conjunction with Landgate and ARC
1Western Australian Centre for Geodesy, Curtin University
2Geodetic Institute, Karlsruhe Institute of Technology, Germany

2. Vertical land motion (VLM)
Groundwater extraction results in VLM (usually subsidence)
Groundwater extraction in Perth increased ~1997
Evidence of subsidence up to ~6 mm/yr from CGNSS
Left panel: Subsidence of ~9 m between 1925 and 1977 in California. From Galloway & Burbey (2011)

3. Why is it necessary to monitor VLM?
Risk of damage to physical infrastructure
Houses/buildings
Transport infrastructure
Distortion and changes of AHD heights – Landgate is mandated to supply accurate heights
Benchmarks on vertical datum (AHD)
Tide gauges in Earth-centred reference frame

4. Heights change over time
AUSGeoid09 will not deliver AHD correctly, because benchmarks are moving vertically
HAHD = h – AG09fitted – Δht (φ,λ,t)

5. Aims of the project Quantify subsidence of the Perth Basin
including spatial and temporal variations
Combination of InSAR, CGNSS, repeat levelling
InSAR delivers high spatial resolution over large area
Develop vertical velocity model for Perth region
provide users with corrected transformation to AHD using GNSS
Correct tide gauge records for VLM
Fremantle is longest record in Southern Hemisphere

6. Few knew that Perth was sinking
Lack of dedicated geodetic monitoring system – we need data!
Two CGNSS stations (only since ~1993)
Limited repeat GNSS observations
Little repeat levelling
Few archived InSAR images
10 ERS
12 Envisat I2
11 Envisat I3
21 ALOS PALSAR images, but cost $$$

7. The status quo – a lot more required
Now three CGNSS stations
Extracted 1970s levelling (black)
Repeat levelling (red)

8. GNSS - Gnangara Subsidence correlates with increased water extraction
At this location for :
~-5 cm height change
~-25 m water level change
Top panel: GPS ellipsoidal height (m)
Bottom panel: Depth (m) of water in Yarragadee aquifer

9. GNSS - Hillarys Lots of discontinuities and noise in the time series
Complicates estimating VLM trends
Subsidence makes TG observed sea level rise appear faster
Top panel: GPS at Hillarys (antenna is above sign) Bottom panel: GPS ellipsoidal height (m)

10. InSAR introduction
Multiple radar images from repeat passes of the same satellite
Interferogram is formed by differencing images pixel-by-pixel
Multiple interferograms stacked to compute VLM time series
Figure is of ALOS (Japanese Aerospace Exploration Agency)

11. InSAR preliminary results
Envisat I3 deformation map
Contaminated by atmospheric signal
Short time-series (1 year) and only 11 InSAR images
PROBLEM: not enough data
Envisat data provided by European Space Agency (ESA)

12. InSAR preliminary results
TerraSAR-X deformation map Oct 2012-April 2013
12 images, but acquisitions continuing until end of 2013
up to 36 images
not enough yet…
TerraSAR-X data supplied by the German Space Centre (DLR) under GA-Curtin-Landgate-KIT science project

13. VLM by levelling Advantages Disadvantages
High precision – proven for detecting VLM
Repeatable along same traverse
Archived data can extend back over time of deformation – data available, but in hardcopy
Disadvantages
Low spatial resolution
Relatively slow – usually low temporal resolution

14. Repeat levelling
Compare 1970s ΔH to 2011 re-levelled ΔH between Hillarys and Fremantle; Hillarys to Gnangara
E-W profile will compare 2013 re-levelling with s ΔH
Re-levelling will be used to validate TerraSAR-X and CGNSS time-series

15. The outlook
Perth’s water shortage woes will continue, especially as our population increases
Monitoring of Perth’s subsidence needs serious and ongoing $$$ commitment over time
We need lots of good data over a long time period:
more CGNSS stations dedicated to VLM
more InSAR [long, well sampled time-series – large image stack]
Expand re-levelling program

16. The outlook
Current project is funded by the Australian Research Council, Landgate, and Curtin University
Project ends December 2013
Without additional funding, it will cease
Heights in Perth will degrade over time
Ongoing monitoring and solutions to the problem are dependent on the project continuing…

17. Acknowledgements We would like to thank
European Space Agency, German Space Agency, Landgate, Geoscience Australia and the WA Department of Water for supplying data
Australian Research Council, Landgate and Curtin University for project funding
Andy Hooper (StaMPS), TUDelft (Doris), JPL/Caltech (roi_pac) for freely available InSAR software
Nigel Penna (Newcastle University, UK)