Perth subsidence monitoring projectMick Filmer1, Will Featherstone1, Andreas

Schenk2

& in conjunction with Landgate and ARC

1Western Australian Centre for Geodesy, Curtin University2Geodetic Institute, Karlsruhe Institute of Technology, Germany

M.Filmer@curtin.edu.au; +61 8 9266 2582

Funded by

Vertical land motion (VLM)

Groundwater extraction results in VLM (usually subsidence)Groundwater extraction in Perth increased ~1997Evidence of subsidence up to ~6 mm/yr from CGNSS

2

Left panel: Subsidence of ~9 m between 1925 and 1977 in California. From Galloway & Burbey (2011)

Why is it necessary to monitor VLM?Risk of damage to physical infrastructure

Houses/buildingsTransport 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

3

Heights change over time

AUSGeoid09 will not deliver AHD correctly, because benchmarks are moving verticallyHAHD = h – AG09fitted – Δht (φ,λ,t)

4

Aims of the projectQuantify subsidence of the Perth Basin

including spatial and temporal variations

Combination of InSAR, CGNSS, repeat levellingInSAR delivers high spatial resolution over large area

Develop vertical velocity model for Perth regionprovide users with corrected transformation to AHD using GNSS

Correct tide gauge records for VLMFremantle is longest record in Southern Hemisphere

5

Few knew that Perth was sinking

Lack of dedicated geodetic monitoring system – we need data!

Two CGNSS stations (only since ~1993)Limited repeat GNSS observationsLittle repeat levellingFew archived InSAR images• 10 ERS• 12 Envisat I2• 11 Envisat I3• 21 ALOS PALSAR images, but cost $$$

6

The status quo – a lot more required

Now three CGNSS stationsExtracted 1970s levelling (black)Repeat levelling (red)

7

GNSS - GnangaraSubsidence correlates with increased water extractionAt this location for 1998-2012:

~-5 cm height change~-25 m water level change

Top panel: GPS ellipsoidal height (m)

Bottom panel: Depth (m) of water in Yarragadee aquifer

8

GNSS - HillarysLots of discontinuities and noise in the time seriesComplicates estimating VLM trendsSubsidence makes TG observed sea level rise appear faster

Top panel: GPS at Hillarys (antenna is above sign) Bottom panel: GPS ellipsoidal height (m)

9

InSAR introductionMultiple radar images from repeat passes of the same satellite Interferogram is formed by differencing images pixel-by-pixelMultiple interferograms stacked to compute VLM time series

10

Figure is of ALOS (Japanese Aerospace Exploration Agency) http://www.eorc.jaxa.jp/ALOS/en/about/palsar.htm

InSAR preliminary resultsEnvisat I3 deformation map 2008-2009Contaminated by atmospheric signalShort time-series (1 year) and only 11 InSAR imagesPROBLEM: not enough data

11

Envisat data provided by European Space Agency (ESA)

InSAR preliminary resultsTerraSAR-X deformation map Oct 2012-April 201312 images, but acquisitions continuing until end of 2013

up to 36 imagesnot enough yet…

12

TerraSAR-X data supplied by the German Space Centre (DLR) under GA-Curtin-Landgate-KIT science project

VLM by levellingAdvantages

High precision – proven for detecting VLMRepeatable along same traverseArchived data can extend back over time of deformation – data available, but in hardcopy

DisadvantagesLow spatial resolutionRelatively slow – usually low temporal resolution

13

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 1970s ΔH

Re-levelling will be used to validate TerraSAR-Xand CGNSS time-series

14

The outlookPerth’s water shortage woes will continue, especially as our population increasesMonitoring of Perth’s subsidence needs serious and ongoing $$$ commitment over timeWe need lots of good data over a long time period:

more CGNSS stations dedicated to VLMmore InSAR [long, well sampled time-series – large image stack]Expand re-levelling program

15

The outlookCurrent project is funded by the Australian Research Council, Landgate, and Curtin University

Project ends December 2013Without additional funding, it will ceaseHeights in Perth will degrade over time

Ongoing monitoring and solutions to the problem are dependent on the project continuing…

16

AcknowledgementsWe 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)17