基于改进MP的地震信号去噪

地震资料中的噪声分为相干噪声和随机噪声。针对随机噪声，采用匹配追踪（MP）算法进行去噪，能取得一定的去噪效果，但是MP算法过大的计算量严重影响去噪的效率。为解决这个问题，本文采用遗传算法（GA）寻找最优原子，大大减少了算法的运算量，提高了算法的运算速度。本文对Ricker子波进行改进，加入尺度参数、位移参数和相位参数，利用改进后的Ricker子波构建的过完备原字库进行去噪可以取得更好的效果。本文采用相邻残差比阈值作为迭代终止条件，与采用硬门限作为迭代终止条件相比增强了算法的鲁棒性。实验仿真时，首先对加有随机噪声的人工合成地震信号进行去噪，仿真结果表明：本文改进的去噪算法与基础MP算法相比，在信噪比、均方差和计算速度上具有明显优越性，然后将本文改进的方法应用于实际地震信号去噪，并取得很好的去噪效果。     

利用PIV系统对防护林不同配置模式防护效益的研究

农田土壤风蚀沙化是北方农牧交错区农业生产发展的主要限制因子,为了研究防护林带防沙机理和防护效益,利用PI V(particle image veloci metry)系统,在风洞中对陕北靖边县3种不同的防护林模式进行了模拟实验研究。试验结果表明,从起动风速、风蚀强度和流场特征三方面来看,两行乔木中间夹两带灌木的防护林模式能达到最佳的防护效益,是3种防护林模式中最佳的。乔灌结合的防护林模式防沙效果较好的原因是能显著降低近地表层风速。     

珠海市城市生态环境质量评价及问题分析

城市生态环境问题近年来越来越受到人们的关注,对城市生态环境现状进行评价研究也显得尤为重要。对珠海运用碳氧平衡法,生态阈值法,氧气需求法等城市生态指标的计算方法对珠海市的生态绿化现状进行分析研究,并提出珠海城市生态规划建议,以不断改善城市生态环境,提高城市生态效益。     

Past and future trajectories of forest loss in New Zealand

Historically, New Zealand was dominated by forest below the alpine treeline, but about 1000 years of Polynesian and European colonisation has resulted in the destruction of nearly three-quarters of the indigenous forest cover. In this study, the historical patterns of deforestation and forest fragmentation were assessed in relation to major topographical, climatic and anthropogenic variables that may drive forest loss. Deforestation has occurred almost equally on the two main islands, the North and South Islands, although the remaining indigenous forest is more fragmented in the North Island. Most deforestation has occurred in regions with a high-density of road networks, although gradients in climatic water availability and soil fertility also had weak effects. Deforestation rates over the period 1997-2002 were very low (nationwide deforestation rate of just −0.01% p.a.), but varied widely among political districts. Expansion of plantation forestry was the single most important driver of recent deforestation. Only 10 of 73 political districts are afforded long-term protection of native forest cover (having more than 30% forest cover that is managed by the Department of Conservation). Forest cover in the majority of New Zealand landscapes has been reduced below the level of an expected ‘extinction threshold’ (circa 30% native habitat cover) in 55 political districts, and long-term trajectories predict that ongoing deforestation threatens to force another five districts below the critical threshold within the next 45 years. Except for the most heavily deforested regions, relatively modest annual rates of habitat restoration could bring forest cover back above the extinction threshold by the year 2050.     

Habitat loss and the habitat fragmentation threshold: an experimental evaluation of impacts on richness and total abundances using grassland invertebrates

Based on empirical analyses and computer-based modelling, it has been suggested that the impact of habitat loss is essentially independent of habitat fragmentation when >10-30% of the original habitat is left and that habitat fragmentation is influential only when less that this amount remains. This is the threshold effect of habitat fragmentation. In many systems, effects of habitat loss cannot be distinguished from those of habitat fragmentation in a way needed to critically evaluate the existence of the threshold effect. Therefore, we used an experimental model system (EMS) that was constructed to produce multiple micro-landscapes in which the habitat-loss and habitat-fragmentation impacts were potentially distinguishable. We used responses of terrestrial invertebrates to measure the impacts. We did not find an interaction between habitat-loss and habitat-fragmentation effects in the predicted fashion, although it is possible the threshold of habitat loss we used for the experiments (90%) may have still been above a critical level for the invertebrates. The only significant components were a strong ‘edge-centre’ difference in both richness and abundance, and a temporal change in both variables. Thus, in this EMS, there was little support for the threshold phenomenon or for general effects of habitat loss and fragmentation although this conclusion needs to be tempered by the limited duration of the experiment.     

Effects of Vibration History on Modulus and Damping of Clay

Aiming at evaluating the effects of vibration history on dynamic characteristics of normally consolidated clay, a series of multi-cycled cyclic shear tests were conducted on 16 soil samples under consolidation pressure of 50 kPa, 100 kPa, 300 kPa and 500 kPa respectively. The experimental results reveal that, on the stage of drainage and consolidation, with the development of duration shear modulus curve ascends while damping curve descends; this trend became significant as consolidation pressure increased. The variation tends to be stable in 25 hours. When subjected to cyclic loading controlled by constant strain amplitude, both the modulus and damping of soil samples change significantly during the first 50 cycles, and then the differences become moderate afterwards, in which the damping curve acts in hyperbolical type. It is also found that there exists a clear sign of pre-straining that affects damping more significantly than modulus.     

玉米芯液化产物的固沙性能与微观机理研究

以玉米芯为原料,乙二醇为液化剂,浓硫酸为催化剂,考察了不同的液化温度、液化时间、固液比以及催化量对玉米芯液化产物起沙风速的影响规律.采用扫描电子显微镜对玉米芯液化产物处理前后的固结层进行了测试,推断玉米芯液化产物的固沙机理.结果表明,玉米芯液化产物主要通过物理胶粘作用增加沙粒的抗风蚀性能.     

Quantifying imperfect detection in an invasive pest fish and the implications for conservation management

In managing non-native species, surveillance programmes aim to minimise the opportunity for invasions to develop from initial introductions through early detection. However, this is dependent on surveillance methods being able to detect species at low levels of abundance to avoid false-negative recordings through imperfect detection. We investigated through field experimentation the ability to detect Pseudorasbora parva, a highly invasive pest fish in Europe, in relation to their known density and sampling method. Secure pond mesocosms of area 100 m² contained P. parva densities from 0.02 to 5.0 m⁻²; each density was in triplicate. These were searched using point sampling electric fishing and deployment of fish traps (non-baited and baited). No fish were captured at densities <0.5 m⁻² using any method and this was considered their detection threshold. Point sample electric fishing was the least effective detection method, producing high proportions of false-negative data even at high fish densities. Baited traps were the most effective detection method. Probability of detection of P. parva was 1.0 for baited traps at all densities >0.5 m⁻², whereas for electric fishing it only exceeded 0.95 at 5.0 m⁻² using high searching effort. These data reveal that small pest fishes such as P. parva may be prone to imperfect detection when at low densities and this is consistent with a number of other invasive species. This indicates the importance of designing surveillance programmes using methods of known statistical power to optimise conservation resource expenditure and enhance management outcomes.     

萝卜蚜和桃蚜最大发育速率温度的研究

本文研究了温度对为害蔬菜的萝卜蚜和桃蚜发育速率的影响，通过温度与这两种蚜虫发育速率的关系以模拟Ｓ型模型，并通过统计控制原理，估计了萝卜蚜和桃蚜的发育下限和最大发育速率温度，前者为６．２２℃和２８．５３℃，后者为２．７５℃和２８．１０℃；萝卜蚜从若蚜出生到下代若蚜出现期的有效积温为１３１．５８日度，桃蚜的为１４０．５８日度。     

早稻穗期赤斑黑沫蝉危害损失及防治指标研究

笼罩接虫试验表明，水稻穗期受赤斑黑沫蝉危害后造成产量损失的主要原因是水稻受害叶面积和秕谷率增加，并得出了早稻穗期产量损失（ｙ）与每丛禾赫斑黑沫蝉数量（ｘ）的回归方程，根据目前稻谷价格、防治费用等，导出赤斑黑沫蝉防治指为每丛禾０．２３头，经大田验证，与实际基本相符。