Figure: Mass variations for 1-10 September 2010 computed from Level-1B data of the GRACE mission using the ANU GRACE software.
Nature of Project(s):
Computational+fieldwork (analysis of satellite/remote-sensing data)
maths/physics courses at university level
The Earth system is complex and constantly changing. Satellite geodesy provides unique observations of the time-varying nature of Earth, which gives key insights into many geophysical processes occurring. For example, measurement of surface deformation allows us to quantify earthquake processes before, during and after earthquakes. Estimates of uplift and subsidence of the surface can tell us about isostatic adjustment after the Last Glacial Maximum as well as extraction of groundwater and changes in water resources. Space gravity missions “weigh” the continents and ocean, quantifying changes in continental hydrology and mass balance of polar glaciated regions. We know how Earth’s sea levels are varying globally through the use of satellite altimetry measurements of changes in the ocean surface. We know the spatial pattern of earthquake deformation and pre-seismic strain through the use of satellite imagery.
The research in the Geodesy and Geodynamics group involves the analysis of satellite data from a variety of satellite missions, including space gravity (GRACE, GRACE FO), satellite altimetry (Topex/Poseidon, Jason-1,2,3), navigation systems (GPS) and InSAR (ALOS, Sentinel-1 etc). Rather than just being a user of products, we develop and use software to analyse the high-level mission data in order to understand and improve the true capabilities and limitations of the missions. With more accurate estimates of changes on Earth, better insights into Earth processes become possible.
Possible Future Research Avenues:
List a range of possible research avenues suitable for a MES(A) student. Do not list specific projects. These should be designed in conjunction with the student. This is more to give a flavour of the type of things available.
- Crustal deformation. Plate tectonics causes collisions and separations at plate boundary zones, which result in seismic risks to communities. Through the use of several space geodetic techniques (notably GPS and InSAR), it is now possible to quantify the horizontal and vertical movement of the surface of the Earth with an accuracy of ~5 mm or less (which translates to < 1 mm/yr). With such capability, one can detect small changes in the deformation field that are related to the buildup of strain prior to the release through the next earthquake. Fundamental research that could be undertaken includes
- Crustal deformation in the Asia-Pacific region. What are the patterns of inter-seismic strain around the Australasian region and throughout Southeast Asia? What can we remote sensing data tell us about future seismic hazards? How is the Australian Plate deforming and what are the implications for Australian cities?
- Vertical land motion. We can detect uplift/subsidence through the analysis of GPS, InSAR and even GRACE data. This provides insights into processes that include earthquake-related deformation, glacial isostatic adjustment, variations in groundwater and soil moisture etc.
- Satellite orbit determination and data analysis. The fundamental basis of the analysis of remote sensing data for detecting changes on Earth is the highly accurate estimate of the position of satellites in their orbits around Earth. To achieve this, all the physics of gravitational attraction of Earth, the Sun, Moon and planets must be considered plus ocean tides, relativity and the propagation of signals through Earth’s atmosphere. Each of these steps require careful consideration and modelling in order to produce the most accurate estimates of change on and within Earth. Ongoing research includes:
- Improving aspects of the modelling of the GPS satellites in their orbits. Elements known to be deficient at present include solar radiation pressure modelling, orientation and behaviour of satellites when passing through the Earth’s shadow, propagation of transmitted signals through the troposphere, mitigating reflected signals at the ground-based tracking stations etc
- Analysis of the Level-1B space gravity data from the GRACE and GRACE Follow-On missions. The ANU GRACE software can be used to detect mass transport on and within Earth by detecting changes in the flight of satellites caused by minor variations in the gravity field. There are a number of modifications to the software that will further enhance the accuracy with which the mass variations can be estimated, including calibration of onboard accelerometer measurements, improvements in the stabilisation of the data inversions, handling of thrust effects caused by spacecraft manoeuvres etc.
- Integration of multiple mission data into a combined inversion. Different platforms currently used are analysed separately, yet there is only one Earth being observed. Combining observations into a single inversion will likely improve the results of the analyses of all platforms.
- Glacial Isostatic Adjustment. Earth has passed through many cycles of glaciation/deglaciation during which significant ice sheets build and subsequently melt. This causes large changes in global sea level as water is taken from/returned to the oceans. The continents subside under the weight of ice sheets, then rebound when the ice sheets melts, a process that takes thousands of years and is still occurring today in previously glaciated regions. We need to understand these processes in order to estimate present-day mass balance change and present-day increases in sea level. Research required includes:
- Constructing estimates of ice sheet history and Earth rheology. Evidence of past shorelines and present-day uplift patterns provide information that can be used to reconstruct the history of ice sheets in Antarctica, Greenland, North America and Fennoscandia. Can we invoke models of spatially variable rheology in order to create more accurate models?
- Correcting present-day sea level variations for GIA. Today’s seafloor is still subsiding because of the increased water load from ice that melted thousands of years ago. This subsidence affects present-day measurement of changes in the ocean surface and, therefore, must be accounted for in the analysis of satellite measurements of ocean height change. There are a number of global GIA models but they all predict different corrections. Why are they different, which model is the most accurate and what is the current rate of global sea level rise?
- Space gravity data. The GRACE and GRACE FO missions provide the unique capability to quantify mass transport on Earth that has permitted, for the first time, the quantification of many processes such as groundwater depletion, variations in water resources, mass balance changes of polar regions, increases in ocean mass etc. There are many studies that can be undertaken, involving the assessment and interpretation of GRACE and GRACE FO data, to understand geophysical processes that cannot be investigated through any other means.