Strategies for stable attenuation compensation in reverse‐time migration

Attenuation in seismic wave propagation is a common cause for poor illumination of subsurface structures. Attempts to compensate for amplitude loss in seismic images by amplifying the wavefield may boost high‐frequency components, such as noise, and create undesirable imaging artefacts. In this paper, rather than amplifying the wavefield directly, we develop a stable compensation operator using stable division. The operator relies on a constant‐Q wave equation with decoupled fractional Laplacians and compensates for the full attenuation phenomena by performing wave extrapolation twice. This leads to two new imaging conditions to compensate for attenuation in reverse‐time migration. A time‐dependent imaging condition is derived by applying Q‐compensation in the frequency domain, whereas a time‐independent imaging condition is formed in the image space by calculating image normalisation weights. We demonstrate the feasibility and robustness of the proposed methods using three synthetic examples. We found that the proposed methods are capable of properly compensating for attenuation without amplifying high‐frequency noise in the data.     

基于谱比法的江苏强震动台站场地特征分析

选取江苏省强震动台网近年来获得的强震动记录,基于水平竖向谱比法(HVSR),计算了22个强震动台站的速度反应谱平均谱比曲线,采用谱比形状一致法进行场地分类,并结合典型台站场地的工程地质资料进行对比分析。研究结果表明,平均谱比曲线的卓越周期、场地放大等特征与台站场地地质资料相吻合,与传统场地分类方法相比,基于谱比法的场地特征分类方法能够突出不同场地的地震动反应特征,较好地体现了台站的场地反应特征。     

九寨沟M7.0地震仪器烈度计算比较分析

地震发生后,强震动观测台网可以获取灾区分布式台站位置的强震动记录,通过基于这些强震动记录得到的地震动参数可以快速地评估地震烈度的空间分布,以迅速判定不同地区的受灾程度,尤其是地震极震区的分布范围,为政府开展应急救援并合理地分配救援力量、物资等提供依据,以保证救援人员及时、准确地到达极震区展开搜救工作,减少人民群众的生命财产损失。本文介绍了国内外7种地震仪器的烈度计算方法,基于四川九寨沟M7.0级地震获取的强震动记录,对这7种方法的计算烈度值进行了对比分析。结果表明,各方法计算的仪器烈度与宏观烈度的差值均在1度误差范围以内,均显示了良好的实用性,且行业标准法和综合判别法两者的计算结果较为一致。     

Multispectral radiometric characterization of coffee rust epidemic in different irrigation management systems

Coffee rust (Hemileia vastatrix Berk. & Br.) is one of the most prominent diseases in coffee (Coffea arabica L.) and causes serious damage to the crop. The pathogen incubation period may be long for about 30 days and 10% incidence of rust may result in 3 times more disease few days after the signals of rust appear in the leaves, even in the absence of new infections. The objective of this study was to evaluate the applicability of coffee crop monitoring under different irrigation systems by orbital radiometry, exploring the spectral signature and spectral, spatial and temporal pattern of rust incidence in the coffee field. The study was carried out in four areas of coffee plantations in Carmo do Rio Claro, Minas Geris, Brazil, between August 2012 and December 2014, under self-propelled, drip, center pivot irrigation systems and rainfed system. Fifteen Landsat-7/ETM + and Landsat-8/OLI-TIRS images were used, trying to establish a better sequence of images between in situ data of rust incidence in the coffee leaves and coffee leaf growth evaluated by sample meshes in the field along the time. Space-time disease incidence distribution maps, Pearson correlation and reflectance spectral signatures were used to evaluate coffee rust progress in the different irrigation fields. The highest coffee rust incidence occurred in August and corresponded to the values of lower NIR reflectance for all evaluated areas, independently of the irrigation management system. In the visible, SWIR-1 and SWIR-2 spectral regions, there were higher reflectance values in the rainfed area when compared to irrigated areas in rainy periods. There was a greater spectral and temporal variation of rust in the center pivot irrigation system when compared to the other irrigation management systems, presenting high values ​​of average incidence of rust above 30% in the periods close to the harvest period, from June to August 2013 and 2014. The high incidence of rust associated with coffee fruit harvest probably led to a reduction in plant leaf growth in center pivot and rainfed fields. There was a negative correlation between near infrared and rust in the self-propelled and center pivot management systems. High coffee rust incidence values mainly in center pivot and rainfed coffee fields determined reduction in the average reflectance of NIR and green and increase reflectance in red, SWIR-1 and SWIR-2 when compared to periods with lower rust in the coffee fields.     

Rogue waves during Typhoon Trami in the East China Sea

As concluded from physical theory and laboratory experiment,it is widely accepted that nonlinearities of sea state play an important role in the formation of rogue waves;however,the sea states and corresponding nonlinearities of real-world rogue wave events remain poorly understood.Three rogue waves were recorded by a directional buoy located in the East China Sea during Typhoon Trami in August 2013.This study used the WAVEWATCHⅢmodel to simulate the sea state conditions pertaining to when and where those rogue waves were observed,based on which a comprehensive and full-scale analysis was performed.From the perspectives of wind and wave fields,wave system tracking,High-Order Spectral method simulation,and some characteristic sea state parameters,we concluded that the rogue waves occurred in sea states dominated by second-order nonlinearities.Moreover,third-order modulational instabilities were suppressed in these events because of the developed or fully developed sea state determined by the typhoon wave system.The method adopted in this study can provide comprehensive and full-scale analysis of rogue waves in the real world.The case studied in this paper is not considered unique,and rules could be found and confirmed in relation to other typhoon sea states through the application of our proposed method.     

线性光谱混合模型的适用观测尺度分析

线性光谱混合模型是目前应用最广泛的光谱混合模型,但由于遥感观测多分辨率的特点,模型的适用性会受到尺度效应的影响。为探索该模型在不同观测尺度下的适用程度,本文从地物辐射原理出发,通过理论推导微面元辐射通量表达式,得出地物辐射通量除了与端元反射率和面积比有关外,也与天顶角存在显著的非线性关系。因此,在线性光谱混合模型和微面元辐射通量的基础上,推导了更具普适性的积分线性光谱混合模型的表达式,再采用数值模拟的方法,计算了两模型的相对差值Δρ,结果表明Δρ的大小仅与探测单元的半瞬时视场角β有关,并通过实测光谱实验对上述推论进行了验证。研究表明,当β13°时,Δρ较小,线性光谱混合模型是积分线性光谱混合模型的一种近似表达形式,β完全可作为确定线性光谱混合模型适用观测尺度的关键依据,并且该模型的适用程度随β的增大而降低。     

Evaluation of the wave transformation in an open bay with two spectral models

An evaluation of two state of the art phase averaged wave models for the transformation scale, SWAN and STWAVE, is carried out in the present work. The target area is the Obidos Bay located in the central part of the Portuguese continental nearshore. The wave input for the two models is provided by an offshore buoy. In order to compare the nearshore outputs of the wave models against in-situ measurements, a directional buoy and an ADCP, operating in intermediate water depth, are used. The wave parameters considered for comparisons are significant wave height, peak period and wave direction. Sensitivity analyses studies and evaluations in the spectral and geographical spaces concerning the results of the two models are also carried out in both intermediate and shallow water. The present study provides some information on the performances of the two wave models in different forcing conditions as well as on their sensitivity in relationship with various input parameters and some physical processes. STWAVE appears to be faster and more robust than SWAN, which on the other hand has more options and flexibility. In statistical terms the results are comparable.     

印度洋海—气某些特征及其海温与北半球大气环流的联系

本文根据印度洋海-气热交换和海温信息区分布特征,得出阿拉伯海是印度洋海-气相互作用的一个重要区域。并讨论了阿拉伯海海温与北半球500hPa高度场和西太平洋副热带高压的一些联系。     

The spatial distribution patterns of biological soil crusts in the Gurbantunggut Desert, Northern Xinjiang, China

The Gurbantunggut Desert, the largest fixed and semi-fixed desert in China, is characterized by a predominant coverage of lichen-dominated biological soil crusts, which serve an indispensable role in sand fixation. Two findings of biological soil crusts have been disclosed from previous field observations: first, distribution of biological soil crusts is selective upon locations; second, species composition varies significantly for the biological soil crusts that are at different developing stages. In this study, a strategy was developed to investigate the spatial distribution of biological soil crusts by coupling remote sensing data and field measurements. A crust index for the Landsat ETM+ data has been developed and applied to detect the lichen-dominated biological soil crusts in the Gurbantunggut Desert. The results indicated the South of the desert encompassed the most abundant biological soil crusts. Besides, biological soil crusts were distributed in uniform density in the South of the desert whereas their distribution patterns become patchier in the rest of the desert. Finally, statistics from the classification revealed that biological soil crusts covered 28.7% of the land in the whole study area. However, it is worth mentioning that the crusts coverage may be underestimated given the fact that detection of crusts from the Landsat ETM+ imagery is viable only if crusts constitute more than 33% of the instantaneous field of view (IFOV) of the Landsat ETM+ sensor.     

ArcGIS空间分析模块在选址中的应用——以哈大齐工业走廊大庆地区为例

在哈大齐工业走廊用地总体规划中,大庆市共开发340.9 km2,其中,盐碱地达320 km2.利用ArcGIS的空间分析模块建立选址模型,以地图代数为分析语言,在栅格数据的环境下用层次分析法来确定权重,对大庆地区的工业用地选址情况进行模拟;当适宜度value≥8.38时,已获得的土地面积达到362.07km2.结果表明:在栅格数据的环境下,以GIS空间分析功能进行用地选址,科学性强,易于在计算机上实现,具有良好的效果.