基于Douglas双侧多叉树的曲线综合算法研究

在对D-P算法存在问题进行全面分析的基础上,提出一种基于Douglas双侧多叉树的结构化曲线综合方法,在确保结果拓扑结构正确的基础上保持了较高的图形特征相似性和化简效率.     

地铁隧道施工引起地层位移规律的探讨

目前对城市隧道开挖所引起的地表以下地层的位移规律的认识还很不充分，长期以来只有定性地认识。英国学者Mair等人（1993年）根据在黏土中的有限实测资料，基于预测地表位移的Peck公式，考虑了沉降槽宽度随深度变化，从而提出了地表以下的位移计算公式。他们的研究成果被广泛引用，但是由于其仅适用于黏性土，且假定地表处沉降槽宽度参数为0.5，因此在应用上受到很大的限制。在Mair公式的基础上，着重讨论了沉降槽宽度参数随地层深度的变化趋势，并提出了修正计算公式，不仅可以考虑地表沉降槽宽度参数与0.5相差较大的情况，还适用于砂类土地层，因此比Mair公式具有更广泛的适用性。     

胶州湾沉积物中氮的地球化学特征及其环境意义*

通过分级浸取的方法获得了胶州湾沉积物中不同形态的氮,将可转化态氮分为离子交换态(IEF-N)、弱酸浸取态 (WAEF-N)、强碱浸取态(SAEF-N)和强氧化剂浸取态(SOEF-N),讨论了其地球化学特征和控制因素,并结合²¹⁰ Pb测年计算了不同年代氮的埋藏通量。在此基础上,分析了胶州湾沉积物中氮给出的环境信息。结果表明,强氧化剂浸取态氮是胶州湾沉积物中可转化态氮主要赋存形态,NO₃-N是胶州湾沉积物中氮的优势形态,氮的含量及埋藏通量的分布受到pH、盐度、温度、生物扰动、沉积物粒度、沉积速率等多种因素的影响。受人为活动的影响,目前每年排入胶州湾的固体垃圾和污水中的悬浮颗粒物的量已超过河流来沙,成为胶州湾主要的沉积物来源。在20世纪初由于污染情况较轻,并且受人为活动的影响较小,胶州湾沉积物各形态氮的埋藏通量较低,但自从20世纪80年代开始,环胶州湾地区的工农业迅猛发展,氮的埋藏通量急剧增加,胶州湾地区的环境一度恶化,但由于近年来采取的较为得力的治污与排放措施,胶州湾的环境状况已有明显改观。     

纵横波初至的相位关系及全波测并的意义(英文)

本文描述了在全波实验中所见到的三种现象,即:1.纵横波初至的振幅(绝对值)具有相同的变化规律;2.纵横波初至的相位彼此总是反向的;3.纵横波初至的振幅变化周期为2π。经过充分的分析和解释指出这些现象应该出现在全波测井条件下,从而为利用相位差从全波中提取有用的信息奠定了基础。     

等电位连接在现代建筑物防雷中的重要性

简述了LEMP的危害方式进而分析等电位连接在现代建筑物防雷中的重要作用,并对现在的防雷图纸有关等电位连接的审核提出建议。     

Polychlorinated biphenyls and chlorinated pesticides in harbor seals (Phoca vitulina concolor) from the northwestern Atlantic coast

Concentrations of polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethane (DDT) and its metabolites, chlordane-related compounds (CHLs), mirex, hexachlorocyclohexane isomers (HCHs), dieldrin, and hexachlorobenzene (HCB) were determined in blubber of harbor seals (Phoca vitulina concolor) from the northwestern Atlantic coast. PCBs, DDTs, and CHLs were the major persistent organochlorines in harbor seal blubber, while mirex, HCHs, dieldrin, and HCB were minor contaminants. Highest concentrations were found in the adult males, followed by the pups, yearlings, adult females, and fetuses. DDT and PCB concentrations have declined from the high levels reported in the early 1970s, but no declines were observed in our samples over the ten-year period 1991-2001. DDT/PCB ratios were indicative of a more rapid decline of DDTs than PCBs, while ratios of p,p'-DDE/DDT were indicative of a metabolic "weathering" of DDT. The population appears to be susceptible to disease outbreaks, as evidenced by a recurrence of viral epizootics since the late 1970s. The PCB burdens in these seals are similar to levels reported in seals from polluted regions of Europe and Asia, and exceed the estimated threshold levels for adverse reproductive and immune system effects in the species.     

Clionid sponge surveys on the Florida Reef Tract suggest land-based nutrient inputs

Bioerosion by Cliona delitrix and Cliona lampa was assessed at 43 sites along the Florida Reef Tract, USA, in the summer of 2001. Sponge abundances were estimated using rapid visual assessment. Tissue samples of sponges were taken for analysis of delta15N. Comparison samples were taken from Belize. Annual trends in sponge abundance were estimated from archived videos covering the period from 1996 to 2001. Sites with the greatest boring sponge size and cover were in the Backcountry and Lower Keys, where total nitrogen, ammonium, and delta15N levels were highest. The sites with the largest relative increase of C. delitrix and C. lampa over the 5 year period were in the Upper Keys, where the greatest relative decline in stony coral cover has occurred. Florida sponge delta15N values were 5.2(+/-0.1)%, suggesting the influence of human waste; in comparison, offshore Belize samples were 2.1(+/-0.1)%. These results suggest sewage contamination of the Florida Reef Tract, shifting the carbonate balance from construction to destruction.     

Spatial and temporal variation in seagrass coverage in Southwest Florida: assessing the relative effects of anthropogenic nutrient load reductions and rainfall in four contiguous estuaries

The estuaries of Tampa Bay, Sarasota Bay, Lemon Bay, and Upper Charlotte Harbor are contiguous waterbodies located within the subtropical environment of Southwest Florida. Based on an examination of rainfall data over the period of record (1916-2001) within the watersheds of these estuaries, there is no evidence for spatial differences (at the watershed level) or monotonic trends in annual rainfall. During the 1980s, nitrogen loads into Tampa Bay and Sarasota Bay (generated primarily by domestic wastewater treatment facilities) were reduced by 57% and 46%, respectively. In response, both Tampa Bay and Sarasota Bay have lower phytoplankton concentrations, greater water clarity and more extensive seagrass coverage in 2002 than in the early 1980s. As there is no evidence of a concurrent trend in rainfall during the period of 1982-2001, it is unlikely that variation in rainfall can account for the observed increase in seagrass coverage in these two bays. In contrast, seagrass coverage has remained relatively constant since the mid 1980s in Lemon Bay and Charlotte Harbor. Domestic wastewater treatment facilities are minor sources of nitrogen to Lemon Bay, and water clarity in Charlotte Harbor varies mostly as a function of dissolved organic matter and non-chlorophyll associated turbidity, not phytoplankton levels. Even in estuaries that share boundaries and are within 100 km of each other, varied responses to anthropogenic changes and natural phenomena were observed in water quality and associated seagrass extent. Resource management strategies must take into account system-specific factors-not all strategies will result in similar results in different systems.