Wave refocusing for linear diffractor using the time-reversal principle

One of the problems encountered in a variety of near-surface investigations is detecting and mapping localized heterogeneities. The heterogeneities may be classified under two kinds of objects: (1) a point diffractor that can be considered as an approximation of a small quasi-isometric, such as small karstic cavities and caves; (2) a linear diffractor roughly approximating an elongated object, such as a tube or fault plane. The point and linear diffractors generate two types of seismic diffraction: tip and edge waves, respectively. During the last few decades, different methods were proposed by many researchers for detecting these heterogeneities utilizing seismic waves diffracted by them. An alternative method for detecting point diffractors using a time-reversal principle combined with focusing analysis is proposed in this study: we present an extension of the time-reversal method for linear diffractors. It consists of a coherent summation of seismic energy along edge-diffraction traveltimes. Real data examples show the feasibility and efficiency of the proposed method.     

Pre-stack Kirchhoff depth migration local space-shift imaging condition: synthetic and data examples

Extracting accurate common image angle gathers from pre-stack depth migrations is important in the generation of any incremental uplift to the amplitude versus angle attributes and seismic inversions that can lead to significant impacts in exploration and development success. The commonly used Kirchhoff migration outputs surface common offset image gathers that require a transformation to angle gathers for amplitude versus angle analysis. The accuracy of this transformation is one of the factors that determine the robustness of the amplitude versus angle measurements. Here, we investigate the possibility of implementing an extended imaging condition, focusing on the space-lag condition, for generating subsurface reflection angle gathers within a Kirchhoff migration. The objective is to determine if exploiting the spatial local shift imaging condition can provide any increase in angle gather fidelity relative to the common offset image gathers. The same restrictions with a ray-based approach will apply using the extended imaging condition as both the offset and extended imaging condition method use travel times derived from solutions to an Eikonal equation. The aims are to offer an alternative ray-based method to generate subsurface angle gathers and to understand the impact on the amplitude versus angle response. To this end, the implementation of the space-shift imaging condition is discussed and results of three different data sets are presented. A layered three-dimensional model and a complex two-dimensional model are used to assess the space shift image gathers output from such a migration scheme and to evaluate the seismic attributes relative to the traditional surface offset common image gathers. The synthetic results show that the extended imaging condition clearly provides an uplift in the measured amplitude versus angle over the surface offset migration. The noise profile post-migration is also improved for the space-lag migration due to the double summation inside the migration. Finally, we show an example of a space-lag gather from deep marine data and compare the resultant angle gathers with those generated from an offset migration and a time-shift imaging condition Kirchhoff migration. The comparison of the real data with a well log shows that the space-lag result is a better match to the well compared to the time-lag extended imaging condition and the common offset Kirchhoff migration. Overall, the results from the synthetics and real data show that a Kirchhoff migration with an extended imaging condition is capable of generating subsurface angle gathers with an incremental improvement in amplitude versus angle fidelity and lower noise but comes at a higher computational cost.     

利用高光谱对生物变量进行估计

该文分析了非光合背景物质—土壤光谱信号的特点 ,认为利用对光谱信号进行一阶导数的运算就能对混合光谱中的土壤光谱信号进行压缩 ,由此而得到的生物指数如LAI、SApar等的估计值更能客观地反映实际 ,这对于利用成象光谱进行环境应用有很大的实用价值。     

三维随钻反射声波成像测井的数值模拟

采用有限差分方法对基于圆弧片状声源和接收器的三维随钻反射声波成像测井进行模拟,研究了反射信号的幅度、相位等参数随方位和源距的变化规律,着重分析了利用该方法在水平井中对地层界面进行探测的可行性.研究结果表明,钻铤的存在使得圆弧片状声源能够向固定方位辐射声场,其主瓣三分贝角宽窄,旁瓣级低,向目的方位辐射的纵波场的幅度约为传统环状声源的0.6倍,适用于三维随钻反射声波成像测井;对于本文计算的井孔模型,反射波信号约为井孔导波信号的1/100;随着源距的增加,反射纵波幅度逐渐减小,转换波(P-SV、SV-P)的幅度先增加后减小,反射SV波的幅度增加,建议在实际应用中,选择合适源距的波形进行处理,并对其他非目的波动进行压制,以期获得更好的成像效果.本文模拟了在水平井中对地层上下界面进行探测的例子,结果显示,反射波纵波信号有较好的方位分辨率,能够准确获得井外波阻抗不连续界面的方位,而且对上、下界面的成像互不影响.     

中国3个典型城市气溶胶光学厚度地基观测及其MODIS气溶胶产品精度分析

利用中国太阳分光观测网的观测资料结合MODIS(中分辨率成像光谱仪)的气溶胶产品分析了北京、兰州、上海3个典型区域城市的气溶胶光学特性。结果表明:北京AOD(气溶胶光学厚度)年平均为0.41±0.35,春夏高,秋冬低,Angstrm波长指数α年平均为1.40±0.85表现为细模态粒子,MODIS的光学厚度为0.52±0.39与地面观测相关系数为0.91,存在系统性高估;兰州AOD年平均为0.55±0.21,夏季最低,秋冬较高,α年平均为0.95±0.20表现为粗模态粒子,MODIS光学厚度为0.43±0.21与地面观测相关系数仅为0.07,存在系统性低估;上海AOD年平均为0.55±0.21,无明显季节变化,α平均为1.03±0.25,MODIS光学厚度为0.74±0.30与地面观测相关系数为0.75,存在系统性高估。城市地理位置和复杂地表等原因造成反照率的不确定,MODIS气溶胶产品在这3个城市的反演效果仍有很大提升空间。     

Evapotranspiration Estimation Based on MODIS Products and Surface Energy Balance Algorithms for Land （SEBAL） Model in Sanjiang Plain, Northeast China

In this study,the Surface Energy Balance Algorithms for Land(SEBAL) model and Moderate Resolution Imaging Spectroradiometer(MODIS) products from Terra satellite were combined with meteorological data to estimate evapotranspiration(ET) over the Sanjiang Plain,Northeast China.Land cover/land use was classified by using a recursive partitioning and regression tree with MODIS Normalized Difference Vegetation Index(NDVI) time series data,which were reconstructed based on the Savitzky-Golay filtering approach.The MODIS product Quality Assessment Science Data Sets(QA-SDS) was analyzed and all scenes with valid data covering more than 75% of the Sanjiang Plain were selected for the SEBAL modeling.This provided 12 overpasses during 184-day growing season from May 1st to October 31st,2006.Daily ET estimated by the SEBAL model was misestimaed at the range of-11.29% to 27.57% compared with that measured by Eddy Covariance system(10.52% on average).The validation results show that seasonal ET from the SEBAL model is comparable to that from ground observation within 8.86% of deviation.Our results reveal that the time series daily ET of different land cover/use increases from vegetation on-going until June or July and then decreases as vegetation senesced.Seasonal ET is lower in dry farmland(average(Ave):491 mm) and paddy field(Ave:522 mm) and increases in wetlands to more than 586 mm.As expected,higher seasonal ET values are observed for the Xingkai Lake in the southeastern part of the Sanjiang Plain(Ave:823 mm),broadleaf forest(Ave:666 mm) and mixed wood(Ave:622 mm) in the southern/western Sanjiang Plain.The ET estimation with SEBAL using MODIS products can provide decision support for operational water management issues.     

An inexpensive system for continuous lake core photography

Representative images of split sediment cores document geological records, serve as visual aids for multimedia, and provide samples for image analysis. Existing core imaging systems, often containing integrated hardware and software, are capable of providing excellent images; unfortunately these systems are also bulky and expensive, and are therefore limited to labs that process large volumes of core and those actively involved in image analysis. For most research groups such systems are unfeasible to buy and maintain. Producing good quality core images with a hand-held digital camera is very difficult without a consistent lighting source, and changes in camera angle (in all three spatial dimensions) between photographs may prevent accurate image compositing. Tripods allow for camera stability, but typically do not accommodate downward-facing photography. Presented here is an easily constructed core-imaging system that minimizes many of the drawbacks of personal digital cameras. Software necessary for using this system is readily available and can be run from a personal computer.     

瞬变电磁虚拟波场的三维曲面延拓成像

根据瞬变电磁场扩散方程与波动方程间存在的数学对应形式,将已知瞬变电磁场数据转换为虚拟波场数据,在此基础上,提出了用克希霍夫积分法将所转换的波场从地面向地下反向延拓的方法.在延拓的数值计算中,为提高计算效率,采用三维边界元技术,把边界积分分解为诸三角单元积分的积分之和, 建立了曲面延拓方程式;通过地面测点的波场值,求出地下某一点的波场值,实现了地表为曲面的向下延拓成像计算.通过对理论模型计算和实际资料处理,证明了该方法可以增强瞬变电磁法识别地下电性分界面的能力,使瞬变电磁法对地下目标体的三维精细探测成为可能.     

Soil Organic Carbon mapping of partially vegetated agricultural fields with imaging spectroscopy

Soil Organic Carbon (SOC) is one of the key soil properties, but the large spatial variation makes continuous mapping a complex task. Imaging spectroscopy has proven to be an useful technique for mapping of soil properties, but the applicability decreases rapidly when fields are partially covered with vegetation. In this paper we show that with only a few percent fractional maize cover the accuracy of a Partial Least Square Regression (PLSR) based SOC prediction model drops dramatically. However, this problem can be solved with the use of spectral unmixing techniques. First, the fractional maize cover is determined with linear spectral unmixing, taking the illumination and observation angles into account. In a next step the influence of maize is filtered out from the spectral signal by a new procedure termed Residual Spectral Unmixing (RSU). The residual soil spectra resulting from this procedure are used for mapping of SOC using PLSR, which could be done with accuracies comparable to studies performed on bare soil surfaces (Root Mean Standard Error of Calibration = 1.34 g/kg and Root Mean Standard Error of Prediction = 1.65 g/kg). With the presented RSU approach it is possible to filter out the influence of maize from the mixed spectra, and the residual soil spectra contain enough information for mapping of the SOC distribution within agricultural fields. This can improve the applicability of airborne imaging spectroscopy for soil studies in temperate climates, since the use of the RSU approach can extend the flight-window which is often constrained by the presence of vegetation.