Historical forest biomass dynamics modelled with Landsat spectral trajectories

Estimation of forest aboveground biomass (AGB) is informative of the role of forest ecosystems in local and global carbon budgets. There is a need to retrospectively estimate biomass in order to establish a historical baseline and enable reporting of change. In this research, we used temporal spectral trajectories to inform on forest successional development status in support of modelling and mapping of historic AGB for Mediterranean pines in central Spain. AGB generated with ground plot data from the Spanish National Forest Inventory (NFI), representing two collection periods (1990 and 2000), are linked with static and dynamic spectral data as captured by Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper Plus (ETM+) sensors over a 25 year period (1984–2009). The importance of forest structural complexity on the relationship between AGB and spectral vegetation indices is revealed by the analysis of wavelet transforms. Two-dimensional (2D) wavelet transforms support the identification of spectral trajectory patterns of forest stands that in turn, are associated with traits of individual NFI plots, using a flexible algorithm sensitive to capturing time series similarity. Single-date spectral indices, temporal trajectories, and temporal derivatives associated with succession are used as input variables to non-parametric decision trees for modelling, estimation, and mapping of AGB and carbon sinks over the entire study area. Results indicate that patterns of change found in Normalized Difference Vegetation Index (NDVI) values are associated and relate well to classes of forest AGB. The Tasseled Cap Angle (TCA) index was found to be strongly related with forest density, although the related patterns of change had little relation with variability in historic AGB. By scaling biomass models through small (∼2.5 ha) spatial objects defined by spectral homogeneity, the AGB dynamics in the period 1990–2000 are mapped (70% accuracy when validated with plot values of change), revealing an increase of 18% in AGB irregularly distributed over 814 km² of pines. The accumulation of C calculated in AGB was on average 0.65 t ha⁻¹ y⁻¹, equivalent to a fixation of 2.38 t ha⁻¹ y⁻¹ of carbon dioxide.     

SAR影像时间序列支持下的互花米草盐沼提取——以江苏盐城东部沿海为例

利用江苏盐城东部沿海8幅ERS-2 SAR影像构建像素级时间序列,根据训练样本得出互花米草盐沼的标准时间序列曲线;基于动态时间弯曲DTW距离进行时间序列相似性分析后提取互花米草盐沼,用同年Landsat TM影像、CBERS-02B CCD影像目视解译的真实地面数据进行精度评价。结果表明:1)互花米草盐沼的时间序列曲线呈波动性变化,与研究区其他4类典型地物的时间序列曲线差异较大;2)互花米草盐沼提取结果的精度较高,基于Landsat TM影像的精度评价结果——正确率86.81%,完整率84.63%,基于CBERS-02B CCD影像的精度评价结果——正确率87.84%,完整率83.87%。     

A STUDY ON THE WARPING NEAR IRRIGATION HEADWORK OF THE LOWER REACH OF YELLOW RIVER

IINTRODUCTIONTheYellowRiverisfamousforitsttemendousamountofsedimenttransport.Basedonastatisticaldataanalysis,along-timeaverageannualof1.6billiontonsofsedimententerstheLowerYellowRiver,inwhich400milliontonsaredepositedintherivercharmel.Thesedimentdepositioncausesavapidriseofthebedelevation.Withahistoricalevollltionoftheriverchanges,aso-called"suspendedriver"graduallyformed.UPtonow,thegroundleveloftheareaolltsidetheriverleveesismuchlowerthanthewaterlevelintheriver.Thissituationbringsanex…     

基于散射理论与动态时间规整的时移地震波动方程差异反演

时移地震勘探作为储层动态监测的有效手段, 对获取地下介质的弹性或物性参数变化起到了至关重要的作用.由于多期地震勘探之间介质的传播速度存在差异, 同时环境条件、采集方式及处理手段等方面的不同, 都会使地震记录产生走时差异.而这种走时差异对时移地震反演存在着严重的影响, 尤其是基于波动方程的反演方法.为了消除时移差异, 我们引入动态时间规整(DTW)方法实现角道集优化匹配, 以获取地震记录的有效变化信息.此外, 本文提出根据散射理论, 通过弹性波动方程进行时移地震差异反演, 得到地层的差异弹性参数, 实现储层差异定量表征.该方法采用WKBJ近似模拟非均匀背景, 具有更高的反演精度, 尤其是在复杂地质构造条件下优势更加明显.本文对所提出的方法进行了数值模拟实验与实际数据应用, 证明该方法得到的反演结果优于传统方法, 可以实现对地层时移差异的准确刻画.

     

Springing analysis of a seagoing vessel using fully coupled BEM-FEM in the time domain

The quasi-steady resonant vibration of a flexible seagoing vessel under resonant wave excitation force, called springing, is studied in this paper. A higher-order B-spline Rankine panel method is used to represent the effects of the fluid motion surrounding this flexible seagoing vessel, and a finite element formulation based on Vlasov beam is employed for structural response. The boundary integral equation and finite element equation, both for fluid and structural domains, are fully coupled with each other using an iterative implicit method in the time domain. Coupling between the two field equations is achieved by relying on fixed-point iteration with relaxation aided by Aitken's δ² process to maximize convergence speed. The steady-unsteady coupling term or m-term in the linearized body boundary condition derived by Timman and Newman is taken into account for accurate prediction of flexible body motion when forward speed is present. The 2nd derivative of basis potential in the m-term is obtained by modifying Nakos approach, which was originally developed using the Stokes theorem for rigid body ship motion problem. For the solution of the FE equation, instead of conventionally used modal superposition method, a direct integration scheme based on Newmark method is employed. It is believed that this technique is more attractive in the sense that it allows us free from the selection of optimum number of mode-shapes in the computation.