Single SAR image direct positioning is to determine the ground coordinate for each pixel in the SAR image assisted with a reference DEM. During this procedure, an iterative procedure is essentially needed to solve the uncertainty in elevation of each pixel in the SAR image. However, such an iterative procedure may suffer from the problem of divergence in shaded and serious layover areas. To investigate this problem, we performed a theoretical analysis on the convergence conditions that has not been intensively studied till now. The Range-Doppler (RD) model was simplified and then the general surface is degenerated into a planar surface. Mathematical deduction was then carried out to derive the convergence conditions and the impact factors for the convergence speed were evaluated. The theoretical findings were validated by experiments for both simulated and real scenarios. 相似文献
TerraSAR-X (TSX) can acquire high-resolution SAR images and due to its high orbit precision as well as its ability to acquire data from different off-nadir viewing angles, the high-precision stereo geolocation can be obtained. In this study, we investigate the absolute geolocation accuracy of TSX high-resolution images in Wuhan, China. We present a direct stereo SAR geolocation method and analyze the 2D and 3D geoposition accuracy of two corner reflectors. The sub-meter localization accuracy was achieved using only atmospheric correction information available in the TSX metadata. 相似文献
The streamflow on the Tibetan Plateau (TP) plays an important role in the water supply of Asia's main river basins. To enhance understanding of hydrologic cycle under the pronounced warming over the TP, this study comprehensively investigates the streamflow changes at the upstream of six major rivers (Yellow River, Yalong River, Jinsha River, Lancang River, Nu River, and Yarlung Zangbo River) originating from the TP, and then diagnoses their possible causes by analysing the impacts of climate variability and human activities. Results indicate that these six major rivers studied have generally insignificant increasing trends in annual streamflow during the last half century, except for two stations. The significant increase appears at the Tuotuohe station in the headwater area of Jinsha River, while the dramatic decrease occurs at the Yunjinghong station in the downstream of Lancang River. In terms of climate factors, the six river basins show a distinct warming trend, along with a noticeable increase in precipitation over the central and northern regions. Pan evaporation, wind speed, sunshine duration, and relative humidity have been found to gradually decrease in most areas. As for the Tuotuohe station, both warming-induced meltwater and increasing precipitation might jointly contribute to the increasing streamflow. But for the Yunjinghong station, the results simulated by the Variable Infiltration Capacity (VIC) model indicate that human activities, especially for the impoundment processes of Xiaowan and Nuozhadu dams, significantly influenced the streamflow, contributing to approximately 69% of the streamflow reduction during 2009–2013. In the context of accelerated global warming, greater attention should be paid to hydrometeorological changes on the TP to offer further insights for the water resources management of the ‘Asian Water Tower’. 相似文献
Although hydraulic fracturing has been massively studied and applied as a key technique to enhance the gas production from tight formations, some problems and uncertainties exist to accurately predict and analyze the fracture behavior in complex reservoirs, especially in the naturally fractured reservoirs like shale reservoirs. This paper presents a full 3D numerical model (FLAC3D) to study hydraulic fracturing behavior under the impact of preexisting orthogonal natural fractures. In this numerical model, the hydraulic fracture propagation direction is assumed perpendicular to the minimum principal stress and activated only by tensile failure, whereas the preexisting natural fractures can be activated by tensile or shear failure or a combination of them, and only tensile failure can open the natural fracture as well. The newly developed model was used to study the impact of preexisting orthogonal natural fractures on hydraulic fracturing behavior, based on a multistage hydraulic fracturing operation in a naturally fractured reservoir from the Barnett Shale formation, northwest of Texas in USA. In this multistage operation, two more representative stages, i.e., stage 1 with a relatively large horizontal stress anisotropy of 3.3 MPa and stage 4 with a comparatively small one of 1.3 MPa, were selected to conduct the simulation. Based on the numerical results, one can observe that the interaction between hydraulic and natural fracture is driven mainly by induced stress around fracture tip. Besides, the horizontal stress anisotropy plays a key role in opening the natural fracture. Thus, no significant opened fracture is activated on natural fracture in stage 1, while in stage 4 an opened fracture invades to about 90 m into the first natural fracture. Conversely, the hydraulic fracture length in stage 1 is much longer than in stage 4, as some fluid volume is stored in the opened natural fracture in stage 4. In this work, the shear failure on natural fractures is treated as the main factor for inducing the seismic events. And the simulated seismic events, i.e., shear failure on natural fractures, are very comparable with the measured seismic events.
