Linlin Ge

Mine subsidence monitoring using multi-source satellite SAR images

Ground subsidence due to underground mining has posed a constant threat to the safety of surface infrastructure such as motorways, railways, power lines, and telecommunications cables. Traditional monitoring techniques like using levels, total stations and GPS can only measure on a point-by-point basis and hence are costly and time-consuming. Differential interferometric synthetic aperture radar (DINSAR) together with GPS and GIS have been studied as a complementary alternative by exploiting multi-source satellite SAR images over a mining site southwest of Sydney. Digital elevation models (DEMs) derived from ERS-1 and ERS-2 tandem images, photogrammetry, airborne laser scanning, and the Shuttle Radar Topography Mission were assessed based on ground survey data using levelling as well as GPS-RTK. The identified high quality DEM was then used in the DINSAR analysis. Repeat-pass acquisitions by the ERS-1, ERS-2, JERS-1, RADARSAT-1 and ENVISAT satellites were used to monitor mine subsidence in the region with seven active mine collieries. Sub-centimeter accuracy has been demonstrated by comparing DINSAR results against ground survey profiles. The ERS tandem DINSAR results revealed mm-level resolution.

Multipath Mitigation of Continuous GPS Measurements Using an Adaptive Filter

Though state-of-the-art dual-frequency receivers are employed in the continuous Global Positioning System (CGPS) arrays, the CGPS coordinate time series are typically very noisy due to the effects of atmospheric biases, multipath, receiver noise, and so on, with multipath generally being considered the major noise contributor. An adaptive finite-duration impulse response filter, based on a least-mean-square algorithm, has been developed to derive a relatively noise-free time series from the CGPS results. Furthermore, this algorithm is suitable for real-time applications.Numerical simulation studies indicate that the adaptive filters is a powerful signal decomposer, which can significantly mitigate multipath effects. By applying the filter to both pseudorange and carrier phase multipath sequences derived from some experimental GPS data, multipath models have been reliably derived. It is found that the best multipath mitigation strategy is forward filtering using data on two adjacent days, which reduces the standard deviations of the pseudorange multipath time series to about one fourth its magnitude before correction and to about half in the case of carrier phase. The filter has been successfully applied to the pseudorange multipath sequences derived from CGPS data. The benefit of this techniques is that the affected observable sequences can be corrected, and then these corrected observables can be used to improve the quality of the GPS coordinate results.

GPS seismometers with up to 20 Hz sampling rate

The large near-field displacements before and during an earthquake are invaluable information for earthquake source study and for the detection of slow/silent quakes or pre-seismic crustal deformation events. However due to bandwidth limitations and saturation current seismometers cannot measure many of these displacements directly. In a joint experiment between the University of New South Wales (UNSW) and the Meteorological Research Institute (MRI), two Trimble MS750 GPS receivers were used in the Real-Time Kinematic (RTK) mode with a fast sampling rate of up to 20 Hz to test the feasibility of a “GPS seismometer” in measuring displacements directly. The GPS antenna, an accelerometer, and a velocimeter were installed on the roof of an earthquake shake-simulator truck. The simulated seismic waveforms resolved from the RTK time series are in very good agreement with the results from the accelerometer and the velocimeter, after integrating twice and once respectively. Moreover, more displacement information are revealed in the GPS RTK results although they are noisier.

Deformation mapping in three dimensions for underground mining using InSAR – Southern highland coalfield in New South Wales, Australia

This article presents 3D surface deformation mapping results derived from satellite synthetic aperture radar (SAR) data acquired over underground coal mines. Both ENVISAT Advanced Synthetic Aperture Radar (ASAR) and Advanced Land Observing Satellite (ALOS) Phased Array type L-band Synthetic Aperture Radar (PALSAR) data were used in this study. The quality of the 3D deformation mapping results due to underground mining is mainly limited by two factors. (1) Differential interferometric synthetic aperture radar (DInSAR) is less sensitive to displacement along the north–south direction in the case of the current SAR satellite configurations. (2) The mining-induced displacement is continuous and nonlinear; and the accuracy of the 3D DInSAR measurement is severely affected by the similar but non-identical temporal overlaps of the InSAR pairs. The simulation and real data analyzes have shown that it would be more practical to use DInSAR pairs with the assumption of negligible northing displacement to derive the displacements in the easting and vertical directions. The northing displacement could then be estimated from the residuals. This limitation could be overcome in the future with the launch of more radar satellites, which would provide better viewing geometry.

GPS MULTIPATH CHANGE DETECTION IN PERMANENT GPS STATIONS

Abstract We have proposed a technique based on an adaptive filter using the least-mean-square algorithm for detection of multipath change in permanent, continuously operating GPS (CGPS) stations. We have tested the technique on some experimental data, indicating that the multipath change smaller than the receiver noise level will go undetected. We have also conducted further tests with some CGPS data from the Japanese GEONET when there was a snowfall. The results show that when there is a change in the antenna environment it will indeed be detected in both the pseudo-range and carrier phase data. This technique provides an easy-to-implement, quality assurance tool for CGPS antenna environmental sensing after events such as typhoons, cyclones, snowfalls, volcano eruptions, earthquakes, etc. Other possible applications include the monitoring of slope stability and ground subsidence.

Low-cost densification of permanent GPS networks for natural hazard mitigation: First tests on GSI’s GEONET network

Researchers from The University of New South Wales (UNSW), Australia, and from the Geographical Survey Institute (GSI), Japan, have commenced a joint project to develop, deploy and test an innovative hardware/software system design for an automatic, continuously-operated ground deformation monitoring system based on low-cost GPS receiver technology. Conventional continuously-operated GPS (CGPS) networks, such as the one established in Japan by GSI to precisely measure earth surface movement, are very expensive. The high cost being primarily due to the fact that dual-frequency receivers are used. Japan’s nationwide GEONET network is the world’s largest, numbering nearly 1000 receiver stations, with an average station spacing of the order of 30 km. In order to densify such CGPS networks (important when high spatial resolution for the monitoring of the deformation phenomenon is required), and to promote the use of the CGPS technique in lesser developed countries, a significantly cheaper system architecture is needed. The proposed design is an integrated, dual-mode network consisting of low-cost, single-frequency GPS receivers across the area of interest, surrounded by a sparser network of dual-frequency GPS receivers. Initial tests of data collected at selected stations in the GEONET network have already shown that through enhanced data processing algorithms a CGPS network containing both single-frequency and dual-frequency receivers would be able to deliver better than centimetre level accuracies, at considerably lower cost than present systems based exclusively on dual-frequency instrumentation. This paper reports the results of the first field test of this new CGPS system design, in the Tsukuba area of Japan, in August 1999. The test network consisted of: (a) several stations of the GEONET network surrounding (b) an inner network of four single-frequency Canadian Marconi GPS receivers installed by UNSW researchers. The data from both the GEONET and the UNSW receivers were processed using a specially modified version of the Bernese GPS Software Package. The software first processes the GEONET GPS station data in order to generate empirical corrections which are then applied to the double-differenced data of the GPS baselines located within the test area enclosed by the dual-frequency CGPS stations. These corrections have the effect of improving baseline solution accuracy by up to an order of magnitude, even for baselines ranging up to 100 km in length. The baselines connecting the inner network to the surrounding GEONET stations are processed in a number of modes, including 24 hr files (as is the standard practice for geodynamic applications) and hourly data files (as in volcano deformation monitoring applications). The results indicate that single-frequency-with-correction processing can achieve accuracies of better than 5 mm in the horizontal components and 3 cm in height, while the dual-frequency results can achieve accuracies better than 2 mm in the horizontal components and 6 mm in height. In the authors’ opinion, for certain geodynamic applications there are no significant differences between the single-frequency-with-correction results and the dual-frequency results, especially for the horizontal components.

Detecting Zimbabwe's Decadal Economic Decline Using Nighttime Light Imagery

Zimbabwe’s economy declined between 2000 and 2009. This study detects the economic decline in different regions of Zimbabwe using nighttime light imagery from the Defense Meteorological Satellite Program’s Operational Linescan System (DMSP-OLS). We found a good correlation (coefficient = 0.7361) between Zimbabwe’s total nighttime light (TNL) and Gross Domestic Product (GDP) for the period 1992 to 2009. Therefore, TNL was used as an indicator of regional economic conditions in Zimbabwe. Nighttime light imagery from 2000 and 2008 was compared at both national and regional scales for four types of regions. At the national scale, we found that nighttime light in more than half of the lit area decreased between 2000 and 2008. Moreover, within the four region types (inland mining towns, inland agricultural towns, border towns and cities) we determined that the mining and agricultural sectors experienced the most severe economic decline. Some of these findings were validated by economic survey data, proving that the nighttime light data is a potential data source for detecting the economic decline in Zimbabwe.

The performance of RTK-GPS for surveying under challenging environmental conditions

The aim of this paper was to assess the availability of RTK GPS (Real-Time Kinematic Global Positioning Systems) under some challenging environmental conditions. Testing was performed under two of these conditions, i.e. a small tree-covered garden and a pylon for high-voltage electricity supply. The increase in the number of satellites had many benefits for the integer ambiguity resolution, but no significant contribution to the accuracy of RTK positions and PDOP (Precision Dilution of Position). The results of RTK testing were compared against results from Total Station surveying as a further quality check. The results indicate that integrating RTK GPS system with Total Station is favoured for surveying in urban environments.

3-D coseismic displacement field of the 2005 Kashmir earthquake inferred from satellite radar imagery

We use radar amplitude images acquired by the ENVISAT/ASAR sensor to measure the coseismic deformation of the 8 October 2005 Kashmir earthquake. We use the offset images to constrain the fault trace, which is in good agreement with field investigations and aftershock distribution. We infer a complete 3-D surface displacement field of the Kashmir earthquake using the offset measurements derived from both descending and ascending pairs of SAR images. The peak-to-peak offsets are up to (3.9, 3.6, 4.1) m in the east, north, and up directions respectively, i.e., 2.9 and 4.1 m along and across the fault assuming striking 325?. We model the coseismic displacements using a four-segment dislocation model in a homogeneous elastic half-space. We first estimate the source parameters using a uniform slip model. Then we fix the optimal geometric parameters and solve for the slip distribution using a bounded variable least-squares (BVLS) method. The resultant maximum slip is about 9.0 m at depth of 4–8 km beneath Muzaffarabad. We find a scalar moment of 2.34 × 1020 N m (Mw7.55), of which almost 82% is released in the uppermost 10 km.

Preliminary Results of Satellite Radar Differential Interferometry for the Co-seismic Deformation of the 12 May 2008 Ms8.0 Wenchuan Earthquake

Abstract Satellite differential SAR interferometry has been widely accepted as a powerful tool to map co-, post- and inter-seismic deformation since its successful application to the 1992 Landers Earthquake. As soon as the Ms8.0 Wenchuan Earthquake occurred on 12 May 2008 in the Sichuan Province of southwestern China, the Japan Aerospace Exploration Agency tasked its Advanced Land Observing Satellite (ALOS) to respond to the disaster by collecting images. This paper presents the preliminary DInSAR results of co-seismic deformation of the quake observed from two satellite paths of the onboard ALOS/PALSAR sensor with post-seismic images acquired on 19 and 24 May. Results from pixel offset analysis and difference of coherence will also be discussed. The radar mapping is still ongoing because the ruptured seismic fault is more than 300km in length. Each swath of the PALSAR fine beam covers only about a 75km segment of the fault, and it takes 46 days for ALOS to revisit the same site.