06 Fakultät Luft- und Raumfahrttechnik und Geodäsie

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/7

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Open Access
Analysing normal modes of the earth from high-rate GNSS time series
(2017) Chen, Zhongyi
Normal modes of the Earth, or Earth’s free oscillations, correspond to a global deformation of the Earth that vibrates at different frequencies, like a bell, after a strong excitation, usually an earthquake of magnitude greater than 6.5. Normal modes of the Earth were first described by Lord Kelvin (Kelvin, 1863) with a computation of the lowest fundamental spheroidal mode 0S2 frequency for a homogeneous Earth model (Lognonné and Clévédé, 2002). With the theory and the deployment of the first long-period sensors in the late 1950s, day-scale Earth’s free oscillation after large earthquakes has been detected by underground instruments such as strainmeters, gravimeters and seismometers (Benioff et al., 1961) (Dziewonski and Gilbert, 1972) (Mendiguren, 1973). In the 1960s, since the U.S. military developed the first satellite navigation system, Transit, the era of Global Navigation Satellite System (GNSS) has arrived. Among all navigation satellite systems, Global Positioning System (GPS), operated by the U.S. Department of Defense (DOD), is currently the world’s most utilized satellite navigation system. With the developments of receiver technology and sampling capability, GPS becomes a powerful tool to study long-period Earth deformations such as plate tectonics and post-glacial rebound, or to monitoring short-period and short-duration motion such as waves generated by earthquakes (Bilich et al., 2008). In recent years, several studies have demonstrated the effective use of GPS in estimating coseismic displacement waveforms induced by an earthquake with accuracies ranging from a few millimeters to a few centimeters. In these studies, two well-known processing strategies, single Precise Point Positioning (PPP) and Different Positioning (DP), have been used to reduce the latency between earthquake occurrence and coseimic displacement waveforms estimation. In this thesis, a new approach named Variometric Approach for Displacements Analysis Standalone Engine (VADASE) is used to detect the normal modes of the Earth. Then the Welch’s PSD estimate is applied to transform the time series into frequency domain. Several simulations have been performed on synthetic time series to investigate the influence of noise level, sampling rate, time series length, window size and overlapping rate of Welch’s method, as well as the influence of stacking. The experiments on real data show the capability of VADASE time series for detecting normal modes of the Earth with the help of the stacking method. Some fundamental modes with small amplitude are not visible because the SNR is not sufficient to lift the signal out of the noise.
Open Access
Analysis of earthquake signals by spaceborne gravimetry
(2011) Cao, Zhou
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.
Open Access
Analysis of long term variations of ocean mass in the arctic region using satellite gravimetry
(2020) Daud Gisiri, Thomas
The Arctic region has experienced rapid sea ice melt in the past decade, which leads to the global sea level rise. The volume of the ice, area and extent have been declining in recent years. The inflow of the Atlantic Ocean water into Arctic Ocean have made changes including the weaker stratification. Moreover, shoaling of Atlantic water in Eurasian Basin reduced sea ice and accelerate the Atlantic water layers’ heat fluxes further Northeast into Eurasian Basin. The salty water (denser than fresh water), the melting of sea ice and warming of Arctic Ocean surface are causing the density of water to change as well as the variations in ocean mass. These huge amounts of the mass variations lead the variations of the Earth gravity field that are being monitored by satellite gravimetry. The main objective of this thesis is to estimate and analyse ocean mass variations in the Arctic Ocean and all seas around namely Laptev Sea, Kara Sea, Barents Sea, Green Land Sea, Baffin Bay, Hudson Bay, Baffin Bay, Beaufort Sea, Chukchi Sea and East Siberian Sea. For this purpose, the standard GRACE level-2 data product from GeoForschungsZentrum (GFZ), Potsdam and the Centre for Space Research (CSR) from University of Texas has been used to estimate the ocean masses. Moreover, the Sea Level Anomaly (SLA) data from satellite altimetry was used for further analysis of the trend in the Arctic region computed from GRACE data. The SLA data is provided by Technical University of Denmark (DTU). Results show that the Beaufort Sea has experienced the highest positive trend in ocean mass of 9.99 mm/year and SLA of approximately 10 mm/year. The positive trends are influenced by the presence of Beaufort Gyre which the anticyclone winds accumulate the fresh water and deeper in this region there are cold and salty water from Pacific Ocean. The Chukchi Sea has positive trend in ocean mass of 7.64 mm/year and Sea Level Anomaly trend range from −1 mm/year and 1 mm/year. The positive trends are influenced by Pacific Ocean water which are warmer and saltier. The Baffin Bay has the strongest negative trend in ocean mass of −59.59 mm/year and Sea Level anomaly trend of approximately −9 mm/year. These negative trends are caused by the Gravitation attraction of Greenland masses and the Arctic Ocean freshwater outflows through it on the way to North Atlantic Ocean. The Hudson Bay have positive trend in ocean mass of 0.35 mm/year and Sea Level Anomaly trend ranges from 1 to 3 mm/year. The positive trends are due to large input of fresh water from river and precipitation, ice melting influenced by air temperature and inflow of Arctic Ocean waters. The Greenland Sea has negative trend in ocean mass of −9.10 mm/year and Sea Level Anomaly trend ranges between −1 mm/year and 6 mm/year. The negative trend is influenced by Gravitational attraction of Greenland masses or due to freshwater outflow from the Arctic Ocean through western side of Fram Strait. Barents Sea has positive trend in ocean mass of 0.13 mm/year and Sea Level Anomaly trend ranging from −1 mm/year to 7 mm/year. Kara Sea has positive trend in ocean mass of 3.73 mm/year, Laptev Sea has positive trend in ocean mass of 6.83 mm/year, East Siberian Sea has a positive trend of 8.25 mm/year and Arctic Ocean has positive ocean mass trend of 5.83 mm/year. The positive trends in this region are influenced by the so-called Atlantification of Arctic Ocean. The Kara Sea, Laptev Sea and East Siberian Sea all have negative trends in Sea Level Anomaly ranging from −2 mm/year to −6 mm/year. The negative trends are influenced by the gain of fresh water, net precipitation, run off increased and more discharge from the land to the sea water.
Open Access
Analysis of seasonal loading-induced displacements from GPS and GRACE
(2014) Zhang, Jinwei
Mass transport within the Earth system over time (e.g., hydrological circulation) induces the mass redistribution on the surface. The temporal variation of mass load on the surface consequently leads to elastic deformation of Earth’s surface (van Dam et al., 2001; Ilk et al., 2005; De Linage et al., 2007). The surface deformation could be derived from GRACE through time-variable gravity field and also be observed by IGS stations in GPS 3D coordinates. The surface deformations derived from GRACE are spatially smoothed with about 350 km resolution. However, the deformations of IGS stations observed by GPS are discrete point measurements on the globe. Therefore, a validation of the consistency between the deformations from GRACE and GPS is necessary to be done, which would benefit the further research on mass transport and climate change. In this study, using the data from GFZ, the deformations from GRACE are theoretically calculated in vertical and horizontal directions (Wahr et al., 1998; Kusche and Schrama, 2005). To investigate the disagreement between GPS and GRACE, a number of IGS stations in three regions are selected (i.e., Tibetan plateau, Danube basin and Great Lakes area) with period of 8 years (2003 – 2011). For a proper comparison, the spatial and temporal reference of GRACE and GPS need to be unified. For validation, the correlation coefficient, the Nash-Sutcliffe efficiency, and WRMS reduction are estimated. After comparisons of deformation time series, almost all the stations in those regions show good consistency between GRACE and GPS in vertical component. There is distinct disagreement in horizontal component, probably due to the weak loading signals and strong local effects. Thus, several representative stations in those regions would be discussed and analysed in detail. Furthermore, to detect an optimal filter for GRACE, 40 IGS stations in Europe are involved to evaluate the filter performance. As a result, 52.5% stations filtered by the stochastic filter (i.e., Wiener filter) show better results, which indicates the optimal choice.
Open Access
Analysis of water volume change of the lakes and reservoirs in the Mississippi River basin using Landsat imagery and satellite altimetry
(2021) Wang, Lingke
In recent years, the demand for freshwater has been steadily increasing owing to population growth and economic expansion. Surface waters such as lakes and reservoirs function as a dominant factor in mankind's freshwater provision. Analysis of changes in their water storage is consequently vital for understanding of the global water cycle and water resources. However, the water volume changes in lakes or reservoirs cannot be measured directly from space, but can be inferred from lake areas and lake water levels. Lake area can be measured globally from space but lake water level is not easy to be obtained globally. Because the number of in situ stations is few, and in situ data are only accessible for some lakes with few measurement epochs, despite in situ stations can measure lake water level and provide high accuracy observations. Although the altimetry technique can generate the time series of the water level for the majority of lakes, they are not global coverage due to the distance between satellite tracks and the gap between different missions. Therefore, in situ data and satellite altimetry measurements of water levels of lakes and reservoirs are not always available. For example, there are only 22 lakes or reservoirs in this study covered by satellite altimetry or in situ stations out of 90 research cases in Mississippi River Basin. Then, in case of unavailable in situ data or altimetry measurements, this research proposes an alternative method to estimate the water level through Digital Elevation Model (DEM). Because satellite imagery offers global coverage and DEM is the global digital representation of the land surface elevation with respect to any reference datum, this study allows for the evaluation of global water volume changes by acquiring lake area data from space and lake height data from DEM. Therefore, the objective of this study is that changes in water volume in lakes or reservoirs can be successfully monitored even when in situ data and satellite altimetry measurements are not available for lakes or reservoirs. Hereby, we investigate 90 lakes and reservoirs in the Mississippi River Basin and develop an alternative remote sensing technique to monitor the water volume changes by combining the improved water mask with DEM. Meanwhile, we propose practical methods to detect the shoreline pixels of the water body from improved water mask. Given the assumption that all pixels in the shoreline should have the same height, four water level estimation models are developed, including water level estimation model based on statistical analysis, frequency maps, change pixels and pixel pair analysis. To this end, the study estimates the time series of lake height from water level estimation model and obtains the time series of lake surface area from HydroSat. Subsequently, this study builds the unique function between the lake water level and the lake surface area and then develops the function between the lake water volume change and the lake surface area. Finally, this study analyses the water volume changes of lakes and reservoirs in the Mississippi River Basin using this alternative remote sensing method. Four water level estimation models are proposed and evaluated. They are respectively based on statistical analysis, frequency maps, change pixels and pixel pair analysis. As a result of their actions, the first model based on statistical analysis, with an average correlation of 0.62 and an average RMSE of 0.91 meters, functions in the majority of situations and demonstrates excessive outlier removal in some cases. The second model based on frequency maps is more general than the first, with an average correlation of 0.66 and an average RMSE of 1.11 meters. The average correlation for the third model based on change pixels is 0.71, and the average RMSE is 0.99 meters. The resulting model based on pixel pair analysis obtains a mean correlation of 0.67 and a mean RMSE of 1.00 meters. Finally, these models behave differently in different seasons, so they exhibit distinct monthly behaviour. To conclude, the above validation results show that this alternative method can be used in different lakes and reservoirs in case of absence of water level observation data, and achieve to monitor the water volume changes during a long period.
Open Access
Analysis of waveforms in the satellite altimetry by using neural networks
(2019) Mattes, Dennis Frederic
The in situ data of inland water bodies is only limited and is declining lately. At the same time, it is more and more important to monitor the inland water bodies, since the climate is changing rapidly. To handle this problem, space born sensors are used more and more. One of the possibilities is to use satellite altimetry, which was previously designed for measurements over the oceans. Thereby, the satellite is transmitting a radar signal towards the earth surface at nadir. This signal is reflected by the ground back to the satellite. By doing so, it estimates the surface height with the runtime of the signal. However, caused by the fast changing terrain over the inland, more noise is included and lead to errors in the height estimation. To solve this, retracker are applied which analyse the received signal and estimate the correct runtime. In this thesis, a new approach will be presented which aims to use neural networks for the retracking purpose. The advantage is that neural networks can learn the characteristic pattern of the signals and then find this pattern during the retracking process. Thereby two approaches are developed, one which uses solely a neural network and a second one, which uses the results of the neural network as an input for an algorithm. They are then applied to different study areas to analyse their performance. It could be shown that the neural networks can estimate the water height well so that a reasonable water height time series can be created. Thereby, the neural network approach shows better results than the algorithm. At the end also the transferability of the neural networks could be shown. Thus, one can use a trained neural network also on other water bodies as which are used for training.
Open Access
Application of image segmentation techniques in satellite altimetry retracking : a feasibility study
(2020) Eitel, Maximilian
Im Bereich der Ozeanographie ist Satellitenaltimetrie die bevorzugte Methode, um Meereshöhen präzise zu erfassen. Wir möchten diese Technologie auch für Binnengewässer nutzen und mit einer neuen Signalverarbeitungsmethode, die auf Bilderverarbeitungsalgorithmen basiert, die Genauigkeit dieses Verfahrens verbessern. Unser Ansatz basiert auf einer zweidimensionale Datenstruktur, die wir erlangen indem benachbarte Signale miteinander kombiniert werden. Dadurch entsteht ein Graustufenbild, welches die Stärke des gemessenen Signals in Graustufen illustriert. In den so entstehenden Bildern können wir die Leading Edge als helle herausstehende Struktur, die nach einem sehr dunklen Bereich auftritt, identifizieren. Unser Ziel ist es mit Hilfe von Bildsegmentierungsverfahren die Retracking Line aufzulösen indem wir das Bild in einen Hintergrundbereich (vor der Leading Edge) und einen Objektbereich (die Leading Edge) teilen.
Open Access
Assessment of altimetric river water level time series densification methods
(2018) Xia, Zhuge
Nowadays, collecting and analysing water level time series recorded by gauging stations or by satellite altimetry is crucial for the geodetic and environmental purposes, such as modelling ocean circulation and monitoring climate change. Since the 1970s, a large number of gauging stations has been removed. This has made altimetry increasing more important. However, data collected by individual altimetric satellites are limited, i.e., the temporal resolution is limited to the repeat cycle of satellites, and the spatial resolution is constrained to the distribution of virtual stations. In order to overcome these limitations, methods have been developed to combine all available altimetric satellite missions along a river to construct a new densified time series. This is referred to as densification. To our knowledge, there are only two proven densification methods applied to the river for now. The first is a hydraulic statistic densification method developed by Tourian et al. (2016). The other is the kriging densification method published by Boergens et al. (2017). However, each of them is realized under different circumstances, which makes them incomparable with each other. In this work, we implement the two densification methods and apply them under similar conditions. The various densified water level time series are compared and analysed both visually and statistically. Results reveal different characteristics of the two densification methods.
Open Access
Assessment of change to gravity field due to underground railroad tunnel construction
(2021) Bloedau, Eric von
Tunnel Ober-/Untertürkheim is an underground railroad tunnel that was constructed between 2015 and 2019. The tunnel crosses obliquely below Nähterstraße in Wangen, Baden-Württemberg, at a depth of circa 22 m. The 2014 Diplomarbeit of Benjamin Efinger involved a gravimetric survey of a set of points situated along Nähterstraße, near the site of the at that time still unbuilt tunnel. In 2020 I have resurveyed those points, in addition to which I have surveyed several additional points situated along the same line, near where the survey line crosses above the tunnel. The coordinates of all gravimetric survey points are known via SAPOS augmented GNSS survey. The purpose of this thesis is to compare the pre- and postconstruction gravitation fields. The first task of survey data postprocessing is to apply corrections to each gravimetric observation; the applied corrections account for the centrifugal component as well as the component that is height of instrument times free-air gradient. Next a survey least squares adjustment is implemented in such a way that partitions every corrected gravimetric observation between a constant component common to all observations, time dependent bias of the gravimeter response variable, difference of magnitude of gravitation relative to a reference value, and random error. According to predictive modelling contained in the 2014 Diplomarbeit of Efinger, the endpoints of the survey line are sufficiently far from the tunnel that there is at most negligible change of gravitation there due to tunnel construction. Except for the two endpoints of the survey line: for all points observed both pre- and postconstruction, the least squares survey adjustment distinguishes preconstruction relative gravitation from postconstruction relative gravitation. The known, one-dimensional coordinates of the survey points together with their adjusted relative gravitation values constitutes a dataset that supports simultaneous least squares estimation of two Legendre series: one corresponding to preconstruction state of gravitation, the other corresponding to postconstruction state of gravitation. The difference between the two Legendre series is a stochastic function that is denoted as F; it is a model of the change of gravitation from preconstruction to postsconstruction. At the point where the survey line crosses above the underground railroad tunnel, Efinger’s predicted change of gravitation is approximately F - 1.618 √var(F).
Open Access
Better understanding of permafrost in Lena and Yenisei river basins
(2020) Liu, Mo
Permafrost covers over 60% of the territory in the central Siberia region. In recent years, there has been growing evidence to suggest that the runoff in central Siberia has shown an increase. This is suspected to be associated with the permafrost thawing in central Siberia. In the Arctic, terrestrial water storage plays a vital role in the annual water budget. The thawing of permafrost would compensate for groundwater in central Siberia, thus affecting local groundwater storage. In this study, an investigation was conducted into the variations in terrestrial water storage and runoff across the Yenisei and Lena basins located in central Siberia, for analyzing the variation of local permafrost during the period from 2003 to 2011. Over the past 40 years, the number of GRDC in-situ runoff observation stations has been reduced on a continued basis by as much as about 75%. Thus, the estimated runoff values were attempted as a replacement for the in-situ observed runoff values. Based on the hydrometeorological approach, the estimated runoff values were obtained by using the terrestrial water storage data collected from GRACE mascon solutions and the vertically integrated moisture flux divergence data sourced from ERA-Interim reanalysis model. The terrestrial water storage measured by GRACE represented a significant positive linear trend in both basins from 2003 to 2011. During this period, the annually measured runoff from GRDC also showed a moderate linear increasing trend in the Yenisei basin and a weak linear increasing trend in the Lena basin. The annually estimated runoff demonstrated a slightly positive linear trend in Yenisei basin but a weak negative trend in Lena basin. Moreover, a comparison was performed between the estimated runoff values and the in-situ observed runoff values to assess the performance of the estimated runoff. The validation indicates that the estimations would underestimate the runoff and have a considerable fluctuation and dispersion degree. Finally, we discussed the impact of changes in terrestrial water storage and runoff on changes in permafrost with the assistance of the permafrost map provided by the International Permafrost Association (IPA). Base on the characteristics of permafrost, we speculated that the changes of terrestrial water storage and runoff during the period from 2003 to 2011 might be attributed to the thawing of continuous permafrost and growth of new talik or expansion of existing talik within the discontinuous permafrost in the central Siberian region.
Open Access
Capability of pulse-limited satellite radar altimetry to monitor inland water bodies
(2015) Roohi, Shirzad
Pulse-limited satellite altimeters were originally designed for oceanographic observations but they have been extended to monitor inland water bodies. So far, studying water level variations of inland water bodies, e.g. lakes, has been a challenge for this type of altimetry in terms of data quality. The returned altimetry waveforms can be seriously contaminated by topography and environmental error sources. Retracking is an effective method against this contamination to improve the accuracy of range measurement and, consequently, to determine more accurate water level. In addition, the design of an optimal retracking algorithm appropriate for a specific inland water body is very important in this respect. In this study we processed 1 Hz Geophysical Data Record (GDR) of Envisat RA2 altimetry data by on-board tracker and retrackers. We also analyzed 18 Hz data of this mission, i.e. Sensor Geophysical Data record (SGDR), with respective different retrackers.First we processed GDR data to determine water level variations from ALL, MEDIAN and MEAN values of water level in each satellite pass over an inland water body. In this step we analyzed to find the best on-board tracker and retrackers. In the next step, the whole waveform, called full-waveform, was processed to estimate retracked water level variations using OCOG, Threshold and β-parameter retrackers. Then we assumed that the reflecting surface inside the radar foot print is a complex surface with different responses. Therefore a given waveform was considered as a combination of a number of small waveforms, called sub-waveform. Each sub-waveform was processed by all of the mentioned retrackers to determine water level variations. The largest salt lake in the Middle East, Urmia Lake, has been selected as a testing area in this study. We found out that between on-board tracker and on-board retrackers the MEDIAN values, processed by ice-1 retracker, provide the most accurate water level variations. Finally the result of different retracked water level were compared with ice-1 retrackers, and with available in-situ gauge data. Our analysis shows that retracking on the sub-waveform outperforms the retracking on the full-waveform. The minimum RMS, 18 cm, was achieved by sub-waveform, retracked by Threshold 50% algorithm. Therefore sub-waveform retacked by threshold 50% is the best retracking scenario to retrieve the water level variations of Urmia Lake.
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Characterization of runoff-storage relationships in boreal catchments
(2018) Xia, Ruiheng
Due to global warming, Arctic sea ice has been shrinking steadily in recent decades, which is a concern to scientists and society. Freshwater inflow is crucial for the formation and preservation of sea ice, therefore the understanding of its status is helpful to assess the possible impact of sea ice decline in the Arctic sea. Due to the continuous reduction of the gauging stations for river discharge, the scientists began to develop other methods like using spaceborne data to replace or supplement in situ measurements. This study aims at characterizing the runoff-storage (R-S) relationships in boreal catchments. GRACE observations of the time-dependent gravity field provide us with the measurements for the state of total water storage on land masses, which allow for a direct comparison of monthly runoff and water storage. Investigation of the R-S relationship in boreal catchments shows a distinct periodic behavior with a hysteresis. Motivated by the work of Riegger and Tourian (2014), the hypothesis of a R-S relationship characterized by the superposition of linear contributions from coupled/liquid storage and nonlinear contributions from uncoupled storage is investigated by means of remote sensing. MODIS snow coverage data is used to separate total water storage into liquid part and solid part. After a least-squares linear fit, the non-linear part of the RS relationship can be fully assigned to the solid storage while runoff and liquid storage can be characterized as a LTI system. This can then be used for direct determination of river runoff from GRACE mass and vice versa.
Open Access
Characterizing storage-based drought using satellite gravimetry
(2021) Saemian, Peyman
Drought is a complex phenomenon leading to a wide range of socio-economic, environmental, and political problems. The storage-based drought which represents the persistent lack of water in different levels of the Total Water Storage (TWS) from deep groundwater to surface water plays a vital role in proactive drought management. Despite its necessity, TWS could not be monitored due to the lack of consistent measurements from regional to continental scale. Since its launch in 2002, the Gravity Recovery and Climate Experiment (grace) mission and its successor GRACE Follow-On have provided unique observations of the TWS change at the global scale. In this study, we have investigated characterizing the storage-based drought at the global scale using GRACE measurements. To this end, the Equivalent Water Height (EWH) has been retrieved from GRACE level 02 solutions. We have addressed the short record of GRACE observations in capturing the full hydroclimate variations. Based on our analysis, regions with a considerable direct human intervention like overexploitation of groundwater in the Middle East, regions that were affected by climate change like ice-melting over the Mackenzie river basin in Canada, or extreme precipitation events over the Ob river basin in the boreal regions are more sensitive to the length of ewh time series. Due to the crucial need for a long (at least 30 years) record of EWH, we have extended GRACE observations back to 1980 using an ensemble of models. The extended dataset has been developed using a pixel-wise selection of best-performed models among global hydrological models, land surface models, and atmospheric reanalysis models. The extended dataset has been used in the study for drought characterization over the grac period. The proposed Storage-based Drought Index (SDI) successfully captured the documented drought events globally in terms of intensity and spatio-temporal distribution. Moreover, the analysis of SDI over the five classes of drought from D0 as abnormally dry to D4 as exceptional drought showed that most regions have suffered at least once from the storage-based drought over the GRACE period (2002–2016). Besides, the map of exceptional drought frequency highlights regions with significant groundwater extraction like California, the Middle East, and north of India and regions with exceptional shifts in the precipitation and temperature pattern and intensity like Amazon in South America and China. Finally, our comparison of SDI with three most widely used drought indices namely the Standardized Precipitation Index (SPI), the Standardized Precipitation Evapotranspiration Index (SPEI), and the Palmer Drought Severity Index (PDSI) reveals that despite their high correlation over climate-driven regions, these indices failed to characterize anthropogenic drought events, especially over regions with considerable groundwater withdraws. The study allows for a more informative storage-based drought with a more robust climatology as the reference, thus enabling a more realistic risk assessment.
Open Access
Coherency analysis between SGs at BFO and Strasbourg
(2013) Zhang, Ying
The twin satellite GRACE mission has provided high-precision, spaceborne measurements of the Earth time-varying gravity field. Independent validation of the GRACE derived gravity field models using superconducting gravimeters (SGs) has been discussed controversially in the literature, since SGs provide gravity observations at a stationary point with high-precision and low instrumental drift. To evaluate whether ground based gravity observations can be used to validate GRACE gravity field models we compare 3 years of continuous SG data from the Strasbourg Observatory (ST) and the Black Forest Observatory (BFO). These two stations are only 57.5 km apart which is twelve times smaller than the shortest resolved wavelength (about 700 km) in the weekly GRACE gravity field models. Thus, since GRACE derived models predict essentially the same temporal gravity field variations for both ST and BFO we require high correlation between ST and BFO at periods longer than one week. A lack of correlation at these long periods would point to more local sources of these gravity variations and would make a GRACE validation impossible, if BFO and ST are representatives of typical GGP stations. The coherence between the residuals of the two stations ranges from 0.65 to 0.9 at periods from 15 days to 30 days. We further investigate the local hydrology of the SG stations which may not be embodied in the GRACE predictions. Local hydrological signals are known to be a major signal in gravity residuals at period longer than 1 day. We inspect locally recorded precipitation and global hydrological models data in this context. The difference of residuals between the two stations is enlarged after correcting for the local hydrological effects. Correcting for the local hydrological effects using GLDAS model or ERA-interim does not improve the coherence from 15 days to 30 days significantly. The trends of SG residual variations before correcting for local hydrology agree well with GRACE predictions. We suggest more sophisticated measuring and modelling of local hydrology to better estimate the hydrological effect.
Open Access
Complex singular spectrum analysis of earth orientation time series
(2018) Li, Yang
The separation of geodetic time series into a sum of a small number of independent and interpretable components is of great importance. Approaches such as Singular Spectrum Analysis (SSA) and Multi-channel Singular Spectrum Analysis (MSSA) have shown their advantage in this field. The aim of this thesis is to develop theoretical aspects of Complex Singular Spectrum Analysis (CSSA) technique and demonstrate that CSSA can be considered as a powerful method of time series analysis. CSSA is a non-parametric method which extends SSA into bivariate time series analysis technique with complex values. Using EOP time series in the time period spanning from 1960 to 2009, Chandler wobble (CW), annual wobble (AW) as well as a non-linear trend are successfully separated in both x and y direction by SSA and MSSA. This paper provides evidence for CSSA when performing tasks of EOP time series decomposition. CSSA functions quasi-equivalent with SSA and MSSA of polar motion time series analysis.
Open Access
Computational simulation of fluid-structure interaction of soft kites
(Stuttgart : University of Stuttgart, Institute of Mechanics, Structural Analysis and Dynamics, 2018) Adam, Niklas Johannes
In order to aid the development and automation of airborne wind energy (AWE) systems, the foundation for fluid-structure interaction (FSI) simulations considering soft kites is developed. FSI simulations are used as a way to predict the deformation of highly flexible structures exposed to a fluid flow and the resulting interaction of solid and fluid. This is especially important for kites since the aeroelastic effects can not be neglected if a realistic approach is regarded. Therefore, the open-source structural multibody dynamics solver MBDyn is coupled to an extension of the open-source computational fluid dynamics (CFD) solver OpenFOAM, namely FOAM-FSI, via the coupling environment preCICE. Relevant modeling features of MBDyn for soft kites such as membrane elements and appropriate boundary conditions are evaluated by means of simple test cases. Furthermore, an adapter for the communication between preCICE and MBDyn is developed and assessed as well. Since an adapter for FOAM-FSI and preCICE already exists, no efforts considering this aspect had to be made. Using this approach, a simple FSI simulation on a ram-air kite section is performed. Due to convincing results regarding the test cases, MBDyn is considered to be a suitable solver for the simulation of soft kites. Moreover, the correct implementation of the adapter is verified by the coupled FSI simulation of a modified benchmark with respect to the aforementioned participating solvers. An approach to FSI simulations on soft kites is successfully developed and verified. However, no reliable final evaluation for the kite section can be made due to the lack of reference solutions.
Open Access
Controls on satellite altimetry over inland water surfaces for hydrological purposes
(2012) Tourian, Mohammad Javad
The global available and freely accessible in situ measurements of hydrological cycles is unsatisfactory, limited and has been on the decline, lately. This together with large modeling error for hydrological cycles, support the efforts to seek for alternative measuring techniques. In the recent past, satellite altimetry has been used to measure non-ocean water level variations for hydrological purposes. Due to the effect of topography and heterogeneity of reflecting surface and atmospheric propagation, the expected echo shape for altimeter returns over land differs from that over ocean surfaces. As a result, altimetry measurements over inland waters are erroneous and include missing data. In the present study, we have developed an algorithm to improve the quality of water level time series over non-ocean surfaces. This algorithm contains an outlier identification and elimination process, an algorithm for excluding the noisy waveforms, an unsupervised classification of the satellite waveforms and finally a retracking procedure. The two preliminary steps of outlier identification and noisy waveforms exclusion allow to achieve better results for further classification and retracking steps. We have employed data snooping algorithm to identify and eliminate outliers in the water level time series. Further, an algorithm based on comparing each waveform with fitted waveform from 5β algorithm is developed to identify the noisy waveforms. An unsupervised classification algorithm is implemented to classify the waveforms into consistent groups, for which the appropriate retracking algorithms are performed. The classification algorithm is based on computing the heterogeneity of data sets, which is computed through the difference between median and modal waveforms. We have employed the algorithm to improve the water level time series in Balaton (Hungary) and Urmia (Iran) lakes. After then, we validated the results of proposed algorithm against the available in situ measurements.
Open Access
Converted Total Least Squares method and Gauss-Helmert model with applications to transformation among ITRF realizations
(2019) Dong, Dalu
This thesis is an extension and improvement of the theory and applications of Converted Total Least Squares method (CTLS). Converted Total Least Squares (CTLS) dealing with the errors-in-variables (EIV) model take the stochastic design matrix elements as virtual observations, and the TLS problem can be transformed into a LS problem. In the coordinate transformation, the transformation model is always used after centering like it is published in most papers. This thesis directly uses the transformation model to generate a new design matrix with CTLS method. The result will present the consistency of the transformation model with and without centering in coordinates transformation. Then the 3D Helmert-transformation in Gauss-Helmert and Gauss-Markoff model is introduced (Koch 2002). The study is to find that, the connections between CTLS and the Gauss-Helmert model. To prove their similarity is a strong support for the theory of the CTLS method. After that, this thesis gives a brief introduction to the International Terrestrial Reference System (ITRF). The CTLS has been proved itself with coordinate transformation in Baden-Württemberg with equal weight and large scale. The new application with more parameters and smaller scale together with the weight information in ITRF is presented. The comparison and accuracy assessment of the published parameters and the parameters estimated by CTLS are discussed in detail with the applications.
Open Access
Copula-based analysis of correlation structures in case of GRACE coefficients
(2015) Modiri, Sadegh
Data from the Gravity Recovery and Climate Experiment (GRACE) has significantly improved our knowledge of the terrestrial water cycle. With the availability of GRACE data from 2002, we are now able to perform even climate change studies with respect to water storage variations. However, as GRACE is already after its expected lifetime, we have to find methods for filling the missing months in the past data and to possibly bridge the gap until GRACE Follow On. In this study, we, therefore, analyze the potential of Copula-based methods for simulating GRACE coefficients data from other hydrological data sources. The method exploits linear and non-linear relationships between two or more variables by fitting a theoretical Copula function into an empirical bivariate or multivariate distribution function. Finally, new data, which is then consistent with the previously derived dependence structure, can be simulated by evaluating the conditional distribution function given by the theoretical Copula. First, we want to analyze the applicability of the proposed method to spherical harmonic coefficients data from GRACE. As the approach involves several drawings of random data, we are interested if this random nature has any impact on the results. We therefore generate filtered out of unfiltered GRACE coefficients, based on the previously derived dependence structure. The comparison between the simulated and filtered data shows a very good agreement with negligible differences in both of the spatial and spectral domain. We also want to evaluate if Copula-based methods are able to estimate reliable water storage changes from the independent hydrological data. Therefore, we derive the dependence structure between filtered water storage changes from GRACE and global gridded precipitation data from the Global Precipitation Climatology Center GPCC. Based on the fitted theoretical Copula, we then simulate water storage changes from precipitation data. The Copula-based estimates are compared with filtered GRACE coefficients data in both of the spectral and spatial domain. We also perform a catchment-based analysis between area-aggregated time-series of simulated and GRACE-derived water storage change. The analysis shows that our estimates and the original filtered GRACE coefficients data are in very good agreement. Thus, we conclude that the proposed method is indeed able to fill the missing months in the GRACE-dataset and to extend even the time-series until the launch of GRACE Follow On.
Open Access
Determination of surface water area using multitemporal SAR imagery
(2015) Gahlaut, Shakti
Inland water and freshwater constitute a valuable natural resource in economic, cultural, scientific and educational terms. Their conservation and management are critical to the interests of all humans, nations and governments. In many regions these precious heritages are in crisis. The main focus of this research is to investigate the capability of time variable ENVISAT ASAR imagery to extract water surface and assess the water surface area variations of lake Poyang in the basin of Yangtze river, the largest freshwater lake in China. Nevertheless, the lake has been in a critical situation in recent years due to a decrease of surface water caused by climate change and human activities. In order to classify water and land areas and to achieve the temporal changes of water surface area from ASAR images during the period 2006-2011, the image segmentation technique was implemented. For this purpose, a thorough analysis of the SAR system and its properties is first discussed. Indeed, some impairments can affect the SAR imaging signals. These impairments such as different types of scattering, surface roughness, dielectric property of water, speckle and geometric distortions can reduce SAR image quality. To avoid these distortions or to reduce their impact, it is therefore important to pre-process SAR images effectively and accurately. All the images were pre-processed using NEST software provided by ESA. To calculate the water surface area, each image was tiled into 9 parts and then it is segmented using two different methods. Firstly histogram for each tile is observed. Using a local adaptive thresholding technique, two local maxima were determined on the histogram and then in between these local maxima, a local minimum is determined which can be considered as the threshold. In the second technique a Gaussian curve was fitted using Levenberg-Marquardt method (1944 and 1963) to obtain a threshold. These thresholds are used to segment the image into homogeneous land and water regions. Later, the time series for both methods is derived from the estimated water surface areas. The results indicate an intense decreasing trend in Poyang Lake surface area during the period 2006-2011. Especially between 2010 and 2011, the lake significantly lost its surface area as compared to the year 2006. Finally, the results are presented for both locally adaptive thresholding and Levenberg-Marquardt methods. These results illustrate the effectiveness of the locally adaptive thresholding method to detect water surface change. A continuous monitoring of water surface change would lead to a long term time series, which is definitely beneficial for water management purposes.