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Chen P.,Southwest University | Chen P.,State Key Laboratory of Information Engineering in Surveying | Chen X.,Southwest University | Chen X.,Key Laboratory of Poyang Lake Wetland and Watershed Research
Nongye Gongcheng Xuebao/Transactions of the Chinese Society of Agricultural Engineering

Drought is one of the principal pertubations in Poyang Lake Ecological Economic Zone(PLEEZ). It is important to investigate the mechanism of droughts in order to evaluate potential losses of droughts and provide a scientific basis for policymaking. We selected fourteen indicators representing exposure, sensitivity and adaptive capacity to droughts respectively. The weights of the indicators were calculated by analytic hierarchy process. Then using the composite index approach these indicators were combined into a drought vulnerability index of agricultural system by a linear aggregation. Maps of drought vulnerability and its elements in PLEEZ were drawn by applying the software of ArcGIS9.3. The results showed that the drought vulnerability of the agricultural system in PLEEZ were mainly at middle rank. Spatial differentiation of the drought vulnerablity existed that the rank of the drought vulnerability in the northern areas of PLEEZ were higher than that in the south. On the basis of the relationship between the drought vulnerability and exposure, sensitivity and adaptive capability, some suggestions for the drought hazard management in the future were proposed. Source

Ding H.,Wuhan University | Shen W.-B.,Wuhan University | Shen W.-B.,State Key Laboratory of Information Engineering in Surveying
Journal of Geophysical Research: Solid Earth

The inner core translational mode (1S1), known as the Slichter triplet, can provide important information particularly on the Earth's inner core and its dynamics. A time domain spectral analysis method, the optimal sequence estimation (OSE), is devised and applied to search for the Slichter modes. Applications of the OSE and the multistation experiment (MSE) technique for detection of the singlets of 3S1 and 2S1 show that both OSE and MSE can isolate their singlets, but OSE provides a higher signal-to-noise ratio. OSE also can completely isolate the modes 0S2 and 1S2. Using two superconducting gravimeter data sets from nine Global Geodynamic Project stations before and after the 2004 Sumatra earthquake, we search for the possible Slichter triplet via OSE and provide a simple validation using the product spectrum. We set two preliminary criteria in order to identify possible signals; results show three signals as candidates for the Slichter triplet, more likely in the data after the 2004 Sumatra earthquake. While choosing the most reasonable criteria needs further studies, we emphasize here the effectiveness of the OSE in searching for the Slichter modes rather than claiming actual detections. Key Points Optimal sequence estimation (OSE) for search for Slichter modes is put forwardValidity of OSE is confirmed by isolating all singlets of 2S1, 3S1, 0S2 and 1S2Three possible singlets of Slichter modes (m equals -1,0 and +1) were suggested © 2013. American Geophysical Union. All Rights Reserved. Source

Ding H.,Wuhan University | Shen W.,Wuhan University | Shen W.,State Key Laboratory of Information Engineering in Surveying
Annals of Geophysics

Based upon SG (superconducting gravimeter) records, the autoregressive method proposed by Chao and Gilbert [1980] is used to determine the frequencies of the singlets of seven spheroidal modes (0S2, 2S1, 0S3, 0S4, 1S2, 0S0, and 3S1) and the degenerate frequencies of three toroidal modes (0T2, 0T3, and 0T4) below 1 mHz after two recent huge earthquakes, the 2010 Mw8.8 Maule earthquake and the 2011 Mw9.1 Tohoku earthquake. The corresponding quality factors Qs are also determined for those modes, of which the Qs of the five singlets of 1S2 and the five singlets (m=0, m=±2, and m=±3) of 0S4 are estimated for the first time using the SG observations. The singlet m=0 of 3S1 is clearly observed from the power spectra of the SG time series without using other special spectral analysis methods or special time series of polar station records. In addition, the splitting width ratio R of 3S1 is 0.99, and consequently we conclude that 3S1 is normally split. The frequencies and Qs of the modes below 1mHz can contribute to refining the 3D density and attenuation models of the Earth. © 2013 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved. Source

Shen W.-B.,Wuhan University | Shen W.-B.,State Key Laboratory of Information Engineering in Surveying | Ding H.,Wuhan University
Geophysical Journal International

Superconducting gravimeter (SG) records after the 2004 Sumatra Mw 9.0, 2010 Maule Mw 8.8 and 2011 Tohoku Mw 9.1 earthquakes are selected to observe the singlets of six spheroidal normal mode multiplets below 1 mHz (0S2, 2S1, 0S3, 0S4, 1S2 and 3S1). To clearly observe their spectral splitting, the ensemble empirical mode decomposition (EEMD) is applied to SG records as a dyadic filter bank. Comparisons of the product spectra obtained from the direct residual gravity records and those obtained after using EEMD clearly confirm the validity of EEMD. After using EEMD, all of the singlets of the six multiplets, particularly those of 0S4 and 1S2, are completely observed with high signal-to-noise ratio, whereas some of singlets could not be well resolved without the application of EEMD. This study demonstrates that EEMD may be important in the detection of the splitting of some weak and low-frequency seismic modes. The relevant observation results may improve the Earth's density models. © The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society. Source

Shi C.,Wuhan University | Shi C.,State Key Laboratory of Information Engineering in Surveying | Gu S.,Wuhan University | Lou Y.,Wuhan University | Ge M.,German Research Center for Geosciences
Advances in Space Research

PPP with low-cost, single-frequency receivers has been receiving increasing interest in recent years because of its large amount of possible users. One crucial issue in single-frequency PPP is the mitigation of ionospheric delays which cannot be removed by combining observations on different frequencies. For this purpose, several approaches have been developed, such as, the approach using ionospheric model corrections with proper weight, the GRAPHIC (Group and Phase Ionosphere Calibration) approach, and the method to model ionospheric delays over a station with a low polynomial or stochastic process. From our investigation on the stochastic characteristics of the ionospheric delay over a station, it cannot be precisely represented by either a deterministic model in the form of a low-order polynomial or a stochastic process for each satellite, because of its strong irregular spatial and temporal variations. Therefore, a novel approach is developed accordingly in which the deterministic representation is further refined by a stochastic process for each satellite with an empirical model for its power density. Furthermore, ionospheric delay corrections from a constructed model using GNSS data are also included as pseudo-observations for a better solution. A large data set collected from about 200 IGS stations over one month in 2010 is processed with the new approach and several commonly adopted approaches for validation. The results show its significant improvements in terms of positioning accuracy and convergence time with a negligible extra processing time, which is also demonstrated by data collected with a low-cost, handheld, single-frequency receiver. © 2012 COSPAR. Published by Elsevier Ltd. All rights reserved. Source

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