Passive-ocean radial basis function approach to improve temporal gravity recovery from GRACE observations

We present a state-of-the-art approach of passive-ocean Modified Radial Basis Functions (MRBFs) that improves the recovery of time-variable gravity fields from GRACE. As is well known, spherical harmonics (SHs), which are commonly used to recover gravity fields, are orthogonal basis functions with global coverage. However, the chosen SH truncation involves a global compromise between data coverage and obtainable resolution, and strong localized signals may not be fully captured. Radial basis functions (RBFs) provide another representation, which has been proposed in earlier works to be better suited to retrieve regional gravity signals. In this paper, we propose a MRBF approach by embedding the known coastal geometries in the RBF parameterization and imposing global mass conservation and equilibrium behavior of the oceans. Our hypothesis is that, with this physically justified constraint, the GRACE-derived gravity signals can be more realistically partitioned into the land and ocean contributions along the coastlines. We test this new technique to invert monthly gravity fields from GRACE level-1b observations covering 2005-2010, for which the numerical results indicate that: (1) MRBF-based solutions reduce the number of parameters by approximately 10%, and allow for more flexible regularization when compared to ordinary RBF solutions; and (2) the MRBF-derived mass flux is better confined along coastal areas. The latter is particularly tested in the Southern Greenland, and our results indicate that the trend of mass loss from the MRBF solutions is approximately 11% larger than that from the SH solutions, and approximately 4% ∼ 6% larger than that of RBF solutions

The GRACE event calendar

GRACE mission is a joint venture of NASA and GFZ. This mission was launched to provide with unprecedented accuracy, estimates of the global high resolution models of the Earth’s gravity field. The study of time-variability of Earth’s gravity field is very helpful in climate sciences and earth’s sciences studies. People have done a lot of work to demonstrate the effect of many natural phenomenon on gravity. Gravity estimates from GRACE are used for estimating mass redistribution at continental scale. So, we can observe hydrology, seismology and glaciology potential areas where GRACE can be useful. This research work focuses on identifying the hydrological events such as floods and drought, seismic events such as earthquakes and volcanic activity and also the glacier melting in the GRACE time-series. The work includes the development of strategy for the analysis of these events keeping in mind their behaviour and GRACE limitations of spatial resolution and sensitivity. Further in this work we would produce a event calendar for such events stating whether gravity changes caused by such events are visible to GRACE. Calendars are generated for hydrological events, floods and droughts separately and also for earthquake events. For rest of the phenomenon we have not generated calendars since these events are very few in numbers. This work is a qualitative analysis, so we could observe whether GRACE signal is able to observe these events or not. Hydrological events are observed by searching outliers in the grace observed time-series. The large floods such as 2009 Amazon floods can be seen when we take whole catchment, but the small floods affecting smaller region such as Sao Paulo flood is not visible in catchment time-series, so we have to go for selected area time-series generation. The factors such as time period for floods and droughts are very important factors when we want to observe them by GRACE. Earthquakes visibility depends on range rate amplitude, and also the quality of ΔC20, we have discussed these aspects while analysing earthquakes occurred in last decade from GRACE. We have given the possible explanation for the events not visible, and those visible have helped in the development of a methodology for analysis of a particular event. The volcanic activity in Caldera and Bolivia are pushing earth upward so we can expect some signal, but the spatial extent of these areas is small with caldera area greater than that of Bolivia, only caldera showed a trend. We also did trend analysis for 2 Asian glaciers and a part of Greenland for observing the melting of these ice masses. The work finally produces a series of events which we were able to observe by GRACE and we also get the methodology suitable for analysis of an event

Modelling time-varying gravity fields from Level-1B GRACE data using mascons

PhD ThesisAs an alternative to spherical harmonics, mass concentration (mascon) parameters have been successfully applied to the recovery of time-varying gravity (TVG) fields from the GRACE satellite mission. However, before meaningful mass anomalies can be estimated, the noise and errors inherent in the solutions needs to be quantified and appropriate procedures adopted for mitigation. The uniqueness of the mascon methodology is the capability to mitigate noise and errors using spatial and temporal constraints, which can be adapted and tailored to any geophysical signal of interest. Therefore, in the first instance, this work was motivated by the need to improve the accuracy of GRACE TVG fields by understanding the effect of noise and errors. This study then aims to validate mascons for recovery of basin scale inter-annual mass variability at a 10 day temporal resolution. Newcastle University’s orbit determination software, Faust, was modified to allow for estimation of mascon parameters including: modelling of accelerometer bias values; mascon parameterisation; and processing based on short-arc gravity field recovery and KBRR data. Accuracy assessments were undertaken using simulations in the presence of realistic noise facilitating the comparison of mascons and spherical harmonic coefficients, including an assessment of potential limitations associated with each technique. Comparisons with time-series derived from CSR RL05 Level-2 data validated the mascon TVG field recovery, before estimation of the mass change of Antarctica, Greenland and Alaska. Several hydrological basins, including the Amazon and Indus were also assessed before the GRACE trends resulting from the Sumatra earthquake of 2004 were investigated. While only provided for validation, these comparisons provide confidence in the mascon mass estimates. Between January 2003 and December 2013 a mass change of -83 ± 12 Gt/year and -242 ± 7 Gt/year were estimated for Antarctica and Greenland respectively by linear regression using mascons with a 10 day temporal resolution. Overall, the work undertaken in this thesis provides evidence of the improved accuracy achievable when using mascon parameters to estimating TVG fields from Level-1B GRACE data. ii As part of this work a processing methodology to estimate mascon parameters from Level-1B GRACE data using Newcastle University’s orbit determination software Faust has been established and documented. This leaves the University well placed to continue processing mascon solutions from Level-1B GRACE data and to estimate mascon solutions from the GRACE-FO mission. Through simulations, mascon parameters were found to offer advantages over spherical harmonics for the mitigation of noise and for improving the temporal and spatial recovery of the TVG field from GRACE. The mascon constraint matrix allowed more signal to be preserved up to degree ~35. Using basin constraints, simulation revealed that the constraint matric can be tuned to recovery the gravity changes resulting from any geophysical phenomena of interest. Basin constraints were found to optimise the signal recovery of GLDAS and a known mass change signal over Antarctica and Greenland. A novel way to create realistic noise and errors in the KBRR measurement was also documented. Generating monthly and 10 day mascon solutions using real data revealed that the noise and errors in mascon solutions is comparably lower than in CRS RL05 solution while also validating the mascon methodology established here. Comparison to published mass trends to those estimated using mascon parameters showed that the estimation of mascon parameters has application in the study of mass change in the cryosphere, hydrological applications and for the study of the co-seismic mass changes resulting from earthquakes.NERC

Analysis of earthquake signals by spaceborne gravimetry

The Gravity Recovery And Climate Experiment (GRACE) mission was launched on Mar. 17, 2002 and has provided the scientists with the gravity data for nearly ten years. The time variable gravity field provided by the GRACE has improved our knowledge of the earth in many fields such as hydrology, oceanography and glaciology. But compared to those “hot” fields, the publications of GRACE in seismology is considerably less. However, GRACE can provide scientists with an independent observation of the earthquake process. Coincidentally, some of the largest earthquakes are within GRACE’s life span - Sumatra-Andaman Earthquake (Indonesia) 2004, Maule Earthquake (Chile) 2010 and Tohoku Earthquake (Japan) 2011. Furthermore, a smaller earthquake - Sichuan Earthquake (China) 2008 has also been examined to test whether the GRACE can detect earthquakes smaller than Mw = 8.0. Different from the traditional methods of the earthquake researches, the gravity method has its advantages: 1. Massive: global scale; 2. Insight: gravity changes can reveal the underground mass changes which do not cause so much motion on the earth surface; 3. Convenient: superior to the traditional methods, the spaceborne gravimetry can get the data from the ocean and glacier parts. The conditions of the data are different among these four earthquakes. The procedures to eliminate the GRACE observation errors and unwanted geophysical data are necessary. First, the C20 term should be replaced by the Satellite Laser Ranging (SLR) data. Second, the hydrology signal especially in the regions of Chile and Sichuan should be eliminated by the Global Land Data Assimilation System (GLDAS) model. Third, Fan filter or Gauss filter 350 km should be applied. Time series analysis by the two-phase changepoint detection and hypothesis testing are applied for each earthquake which is a point-wise analysis. Least squares adjustment is performed on each point to display the coseismic and postseismic signals. Meanwhile, the surface analysis is done by the Empirical Orthogonal Functions (EOF) as it has a flexible base which can suit the data automatically. Although the observation errors have been removed as much as possible, the limited spatial and time resolutions of the GRACE satellite and to retrieve relatively weak earthquake signal among the strong hydrological signals are still problems in the analysis. GRACE can detect some of the large earthquakes, but it depends on the earthquake type, area and the length of the time-series before and after the earthquake. Both coseismic signal and postseismic signal are detected in Sumatra-Andaman Earthquake. Meanwhile, there is no significant coseismic signal in the time series of Sichuan Earthquake, but the EOF detects suspicious earthquake signal in mode 2 with the magnitude less than 1 µGal. For Maule Earthquake, only the coseismic signal is detected. Due to the limited dataset, the detection of the coseismic signal is successful but the postseismic signal is not long enough to be detected in Tohoku Earthquake. However, the different filters will affect the magnitude of the gravity change, so the real gravity changes of those four areas are still under debate. Last, EOF can be used for the separation of the earthquake signals. Compared to other geodetic technics the gravity method can detect the signals underground and in the ocean areas. The coseismic and postseismic signals detected by GRACE show underground processes of the earthquakes which can help scientists better understand the earthquake mechanism and will contribute to the earthquake prediction in the future

Estimating Earth's temporal gravity field from GRACE observations: Mitigation of thermal errors and the interplay between orbital characteristics, basis functions and spatial resolution

The Gravity Recovery and Climate Experiment (GRACE) mission measured the combined effect of the Earth's static and time-variable gravity fields globally and near-continuously over 15 years at unprecedented accuracy. Launched in 2002, the GRACE mission used a unique low-Earth orbit satellite-to-satellite tracking mission design. The time-variable gravity field is influenced by the movement of masses within the hydrosphere and the solid Earth. By directly monitoring mass balance changes due to flood, drought, groundwater extraction, ocean circulation, ocean mass increase and ice mass loss, results from the GRACE mission have increased understanding of the impacts of human activity, natural variation and climate change. The accuracy of GRACE estimates of the time-variable gravity field and the associated mass anomaly time series is affected by several factors. These include orbital characteristics, quality of the observations and background forcing models and the processing strategies used for precise orbit determination and temporal gravity field estimation. This study aims to improve GRACE-based estimates of the time-variable gravity field to analyse mass anomalies by mitigating measurement errors and optimising the choices of processing strategies. The precise calculation of GRACE satellite orbits is reliant on knowledge of accurate non-gravitational forces acting on the spacecraft. Optimal performance of the accelerometers requires a highly stable thermally controlled environment which was not maintained throughout the mission. In this study, I developed pre-processing and calibration strategies to account for thermally-induced errors in the non-gravitational acceleration measurements. Accurate time-variable gravity models could then be estimated from GRACE data even in the presence of thermally-induced error. Some mathematical form, or basis function, must be assumed to parameterise the temporal gravity field on the surface of Earth. The choice of the inter-satellite observation and basis functions can also improve the recovery of the gravity field by better localising the mass variations. This study demonstrates how mass concentration (mascon) tiles can reduce signal leakage and intra-mascon variability (the variations of mass change signals within a mascon). I identified the optimal mascon parameterisation through simulation, subsequently used to generate the ANU GRACE mass anomaly time series. Improved localisation of the mass variation signals was achieved using the range acceleration as the inter-satellite observation rather than the conventional approach of using range rate observations. The GRACE processing strategies chosen to optimise the accuracy of the temporal gravity solutions tend to be used - without change - across the mission's duration. However, as the geometry of the orbits of the twin spacecraft vary throughout the mission, the ability of the observations to recover high-frequency spatial signals also varies. Through simulation, I assessed the impact of the changing orbital elements on the spatial resolution of the GRACE mascon solutions as a function of altitude and ground track density. With appropriate regularisation, mascons as small as $\sim$150 $\times$ 150 km yield the most accurate solutions even during periods of orbit resonance. Under realistic simulation conditions, the temporal gravity field solutions are significantly improved with decreased orbit altitude. Many components of my work have been implemented into the ANU GRACE software, including pre-processing and calibration strategies that account for thermally-induced errors in the accelerometer measurements, filtering to mitigate high-frequency inter-satellite range acceleration noise, protocols to create mascon grids and the iteration procedure used to generate the ANU GRACE mass anomaly estimates. The results show substantial seasonal variations, ice sheet mass loss and global mean sea level increase consistent with previous studies

Beyond 100: The Next Century in Geodesy

This open access book contains 30 peer-reviewed papers based on presentations at the 27th General Assembly of the International Union of Geodesy and Geophysics (IUGG). The meeting was held from July 8 to 18, 2019 in Montreal, Canada, with the theme being the celebration of the centennial of the establishment of the IUGG. The centennial was also a good opportunity to look forward to the next century, as reflected in the title of this volume. The papers in this volume represent a cross-section of present activity in geodesy, and highlight the future directions in the field as we begin the second century of the IUGG. During the meeting, the International Association of Geodesy (IAG) organized one Union Symposium, 6 IAG Symposia, 7 Joint Symposia with other associations, and 20 business meetings. In addition, IAG co-sponsored 8 Union Symposia and 15 Joint Symposia. In total, 3952 participants registered, 437 of them with IAG priority. In total, there were 234 symposia and 18 Workshops with 4580 presentations, of which 469 were in IAG-associated symposia. ; This volume will publish papers based on International Association of Geodesy (IAG) -related presentations made at the International Association of Geodesy at the 27th IUGG General Assembly, Montreal, July 2019. It will include papers associated with all of the IAG and joint symposia from the meeting, which span all aspects of modern geodesy, and linkages to earth and environmental sciences. It continues the long-running IAG Symposia Series

Beyond 100: The Next Century in Geodesy

This open access book contains 30 peer-reviewed papers based on presentations at the 27th General Assembly of the International Union of Geodesy and Geophysics (IUGG). The meeting was held from July 8 to 18, 2019 in Montreal, Canada, with the theme being the celebration of the centennial of the establishment of the IUGG. The centennial was also a good opportunity to look forward to the next century, as reflected in the title of this volume. The papers in this volume represent a cross-section of present activity in geodesy, and highlight the future directions in the field as we begin the second century of the IUGG. During the meeting, the International Association of Geodesy (IAG) organized one Union Symposium, 6 IAG Symposia, 7 Joint Symposia with other associations, and 20 business meetings. In addition, IAG co-sponsored 8 Union Symposia and 15 Joint Symposia. In total, 3952 participants registered, 437 of them with IAG priority. In total, there were 234 symposia and 18 Workshops with 4580 presentations, of which 469 were in IAG-associated symposia. ; This volume will publish papers based on International Association of Geodesy (IAG) -related presentations made at the International Association of Geodesy at the 27th IUGG General Assembly, Montreal, July 2019. It will include papers associated with all of the IAG and joint symposia from the meeting, which span all aspects of modern geodesy, and linkages to earth and environmental sciences. It continues the long-running IAG Symposia Series

Assessment of groundwater potential in data scarcity situation in southern Laos

University of Technology Sydney. Faculty of Engineering and Information Technology.Water is a vital natural resource that is needed for the sustainability of the hydro-environment and also for socio-economic development. In many developing nations, however, data necessary for assessing the available water resources and for planning the sustainable utilisation of these resources are lacking. Arising from increased population pressures in the Sukhuma District of Southern Laos, there is a need to assess the available water resources. Of particular concern is the interaction between surface water and groundwater together with the implications of this interaction on assessing sustainable water usage. The focus of the research presented herein is the utilisation of available data to assess the seasonal interaction of surface water and groundwater in the Sukhuma District. In addition, a water balance model was developed to enable the assessment of the sustainable water resources available for anthropogenic activities. These research activities can be expressed as data-mining of the available data for information. The available field data comprised short-term rainfall records, streamflow records, and groundwater levels collected over non-consistent time periods; in other words, no period contained data from all three sources. This limited field data was supplemented by remotely sensed data. However, the scales of the remotely sensed data and the field data differed, requiring down-scaling of the remotely sensed data. Data from the Gravity Recovery and Climate Experiment (GRACE) and from the Global Land Data Assimilation System (GLDAS) database were used for this purpose. It was found that the remotely sensed data could be down-scaled to be consistent with the region of interest. Additionally, it was found that there was good agreement between the characteristics of the interaction between the surface water and groundwater predicted by both the water balance model and the remotely sensed data. Arising from the analysis of this interaction, it was found that groundwater recharge in Sukhuma District was 3 – 4% of annual rainfall with a lag of two to six weeks (average 3 weeks) between the wet season start and a rise in groundwater levels. As shown in this research, non-traditional and traditional data sources can be combined in a manner leading to extraction of the available information in an efficient manner

Crude oil and oil brine seeps: sources, detection and environmental effects in soil and water, Kirkuk NE Iraq

Natural hydrocarbons have extensively contaminated both the hydro-lithospheres, damaging the environment and the health of the people living in the Kirkuk region of North-East Iraq, which is an area with a world´s significant crude oil reserves as well as various hydrocarbon seeps and brines. The study area is located in the Zagros fold-thrust belt, within the Low Folded Zone in the northeast of the Mesopotamia basin. Complex fracture systems and faults frequently cut across the Eocene, and middle Oligocene limestone reservoirs and the evaporates Miocene cap rock. High-density maps of the detected faults and lineaments within Fatha Formation have interpreted as potential seepage locations, even for seeps that are not exposed on the surface. The present thesis aims to investigate hydrocarbon seeps, and oil brine seeps contamination impact on the surface, groundwater as well as the soil’s physical and chemical properties. Therefore, various methods were used starting by identifying the origin of the seeps as a base to assess the source and ending by estimating the contamination level of hydrocarbons and related brines in water and soil directly or remotely. The essential concept of the present thesis is based on the known hydrocarbon seepage sits – which were recorded for hundreds of meters on the surface – and the sub-surface properties of the stratigraphy and hydrogeology conditions. In addition to the several reports and studies, the primary data source was based on the wide variation of the collected samples, i.e., crude oil and brine water samples from the selected oil reservoir to define the reservoir characterization and migration level. Moreover, the reservoir oil types were used to compare them with surface crude oil seeps samples. The surface and groundwater from the selected location and different aquifers beside soil and rock samples explained the aquifer's recharge and led to the appropriate speculation of the hydrodynamic and hydrogeological conditions. The measurements included: a) oil density, organic and inorganic components and biomarkers for crude oil samples, and polycyclic aromatic hydrocarbon PAHs; b) hydrochemistry, stable isotopes; and c) the spectral reflectance behavior of crude oil and different contaminated soil samples, organic components (organic carbon (OC) and total petroleum hydrocarbons (TPHs)) and x-ray diffraction to explain the chemical composition of the soil samples. The multiple data were transformed into one database, and the results were used to complete the final hypotheses in a conceptual model, which explains the mixing mechanism of crude oil and brine seeps with the surrounding environment. The strontium isotopes (87Sr/86Sr) showed the mixing processes between shallow groundwater resources, uprising oil field brines and differentiates it from the dissolution of gypsum and halite from the Fatha Formation. The final discussion and conclusions connect all of the results and try to simulate the sub-surface hydraulic conductivity and highlight the contamination zones that were explained in the final comprehensive conceptual model, enriching our knowledge of the petroleum and the hydrogeology systems of the selected fields within the Zagros fold-thrust belt. The obtained results mainly highlight the reasons behind the environmental consequences that can be a threat to the human health. The conclusion of this study opens the door to compare the findings with other locations within the study region, which contains similar complex stratigraphy and structures