School of Surveying and Spatial Information Systems
The University of New South Wales
Validating DInSAR with Ground Survey
By Peter Nguyen
Supervised by Dr. L Ge
Subsidence monitoring is a vital part of every underground coal mining in New South Wales. It is under statuary law that every colliery has a subsidence management plan to monitor the effects of underground mining – the Mining Act 1992. The objective of subsidence monitoring is to measure the potential impacts on landholders and the environment
Subsidence monitoring can be measured in a variety of ways. Traditionally it has been the role of the surveyor to measure and monitor the rate and pattern of ground subsidence. However with the evolution of technology, in particular remote sensing means that subsidence can now be measured using space and airborne sensor technologies – technologies such as DInSAR, GNSS and Lidar. However, each technology has its advantages and disadvantages. This thesis will explore, analyse and evaluate DInSAR.
The test site for this thesis has an extensive archive of subsidence monitoring data provided by the colliery. The data coincides with the years ALOS was in operation. The test site is an urban township, this favours using DInSAR because coherence with urban monuments is favourable compared to vegetated areas. It is with anticipation that this thesis along with the unique attributes of the test site and vast ground subsidence monitoring data can collectively validate ALOS abilities to measure subsidence monitoring sufficiently with an acceptable accuracy and precision.
Differential Interferometry Synthetic Aperture RADAR
This method essentially exploits two Synthetic Aperture RADAR (SAR) images at the same location over different times. This then allows it to generate a differential interferogram. A displacement can then be calculated from the differential interferogram. In other words it calculates the deformation of the area where the image was located.
In Synthetic aperture radar images are formed by recording the phase and amplitude of microwaves echoes returned from the earth’s surface (Engelbrecht, 2011). On board the remote sensing instrument is an antenna. It emits a wave on to the earth surface. It counts the phase, that is the how many wavelengths are required to make a round trip from the antenna to the earth’s surface and back. If the ground was under deformation, the phase difference between two images that were taken at different time can be calculated. The images are constructed from two Single Look Complex (SLC). This would give a displacement and the deformation is measured. This results in an interogram. From this the displacement information can be utilised. This is the principle of Synthetic Aperture Radar. DInSAR uses the technique as prescribed before, but there is one more added element. That is the addition of a DEM (Digital Elevation Model).
The ground survey data is used to compare observed and predicted subsidence, tilt and strain profiles along monitoring lines, and observes and predicts impacts on surface features. These surface features include houses, river banks, creeks, industrial buildings, roads, culverts etc. The survey work is carried out regularly along monitoring lines that were placed specifically along or near the longwalls. This is done intentionally to monitor and compare the predicted movements. With regular monitoring this allows the subsidence engineers to identify any anomaly surface movement that could potentially have an adverse impact.
Figure 1: GIS Location of Survey Monitoring Lines
• Filtered survey data to correlate with the images that are taken from ALOS on corresponding days to minimise temporal offset
•Produce ground deformation data with in-house software
•ArcGIS will be used to collate deformation data
•Compare DInSAR results with survey data
•Determine whether DInSAR can be used for subsidence monitoring for underground mining .
Figure 2: Interferogram generated for the period between 31/03/2008-16/05/2008.
Figure 3: Interferogram generated for the period between 01/01/2009-16/02/2009.
Discussion of Results
The survey line along Remembrance Dr is along a major road. The location of this survey line is directly above the longwall. The interferogram used to determine the DInSAR result was pair 5 as illustrated in figure 13. The survey data indicates that from mark RE1 to mark RE18 all experienced subsidence. However the DInSAR result showed otherwise. The DInSAR result indicates that this survey line also experienced upsidence.
For pair 1 (Rail Corridor 26/06/09-11/05/09) compared to the survey dates, the image was taken four days earlier then the ground survey. It should be noted that during these four days the longwall progressed 38.7 metres. The results for this were better than Remembrance Drive. The maximum difference was 40mm and the average difference between ground surveying and DInSAR was 10mm. both subsidence trends were very similar. This is a positive result.
The results for this were better than Remembrance Drive. The maximum difference was 40mm and the average difference between ground surveying and DInSAR was 10mm. both subsidence trends were very similar. This is a positive result.
Similarly to pair one, the results were promising. The average difference between ground based surveying and DInSAR was 34mm. and the maximum difference was 46mm. DInSAR clearly has the capacity to replicate the same trend has what surveying can offer. However the downfall is that it is still not accurate enough for subsidence monitoring in an urban area.
The trends over both techniques are similar. The average difference between DInSAR and ground survey was 0.016m and the maximum was 0.042m. Promising results however it still not accurate enough for subsidence monitoring in an urban township.
LW 24 Connection
This survey line yielded the best results. With an average difference between DInSAR and ground survey of 5.5mm. The maximum difference was 19mm. The reasons for this are because at the time of the survey, the face of the longwall had not impeded the survey line yet. It was still 160 metres away. In other words directly below the survey line had not been mined yet.
Hence the deformation of the survey line had not been affected by the longwall yet. With a slow velocity of subsidence, it was expected that the DInSAR result would be good.
· these results demonstrate that DInSAR has the ability to measure subsidence
· indicate if the velocity of the subsidence is slow then DInSAR is a definite alternative to traditional surveying
· where subsidence and strain are required to be measured it simply isn’t accurate it enough
From the results of the DInSAR measurements, it indicates that DInSAR is a suitable technique to measure subsidence however at the test site, where subsidence and strain are require to be measured it simply isn’t accurate enough. Millimetre is the sort of accuracy that is needed. Especially when monitoring dwellings where millimetres could mean the difference of the structural integrity of the dwelling, however if less accuracy is needed then DInSAR could be an appropriate technique to measure subsidence.