什么引起了最近北磁极的加速漂移?

引言 北磁极位于地磁场竖直向下的地球表面.虽然地核通过内部对流的地球发电机过程维持了地球的主磁场,但却引起了北磁极随时间的不断漂移.北磁极的漂移速度从20世纪90年代初的15 km/a突然增加到90年代末的55 km/a.     

精密单点定位整周模糊度快速固定

提出利用GPS参考网估计电离层延迟、卫星相位偏差的算法,用于实现区域内精密单点定位(Precise Point Positioning, PPP)的整周模糊度快速固定.利用站间距约为100~200 km的参考网进行实验,结果表明:电离层延迟的内插和外推精度均优于1 dm,卫星相位偏差估值的日内变化不超过0.2周;此外,单天内不同时刻始,固定PPP整周模糊度所需时长最多不超过10 min,且当模糊度成功固定后,三维位置解较之相应浮点解的精度改善优于80%.新算法可望解决PPP普遍存在的收敛时间过长问题,增强了PPP技术的实用性.     

Shape factors of cylindrical piezometers in uniform soil

This article presents the analytical solution of Laplace equation for the steady flow around open-ended cylindrical piezometers located in an infinite, isotropic, and incompressible saturated soil. Shape factors are obtained for piezometers with varying length-to-diameter ratios. Comparisons are made with published factors obtained by means of approximate analytical solutions, numerical approaches, and measurements in electric analog models. It is shown that some expressions that are currently used in practice are inadequate and should be abandoned.     

Restoration of wadi aquifers by artificial recharge with treated waste water

Fresh water resources within the Kingdom of Saudi Arabia are a rare and precious commodity that must be managed within a context of integrated water management. Wadi aquifers contain a high percentage of the naturally occurring fresh groundwater in the Kingdom. This resource is currently overused and has become depleted or contaminated at many locations. One resource that could be used to restore or enhance the fresh water resources within wadi aquifers is treated municipal waste water (reclaimed water). Each year about 80 percent of the country's treated municipal waste water is discharged to waste without any beneficial use. These discharges not only represent a lost water resource, but also create a number of adverse environmental impacts, such as damage to sensitive nearshore marine environments and creation of high-salinity interior surface water areas. An investigation of the hydrogeology of wadi aquifers in Saudi Arabia revealed that these aquifers can be used to develop aquifer recharge and recovery (ARR) systems that will be able to treat the impaired-quality water, store it until needed, and allow recovery of the water for transmittal to areas in demand. Full-engineered ARR systems can be designed at high capacities within wadi aquifer systems that can operate in concert with the natural role of wadis, while providing the required functions of additional treatment, storage and recovery of reclaimed water, while reducing the need to develop additional, energy-intensive desalination to meet new water supply demands.     

MACROPORE FLOW AND MASS WASTING OF GULLIES IN THE LOESS PLATEAU, CHINA

Due to the extensive gullying from historically excessive erosion in the loess plateau of China, much of this region is being converted to native grass and shrub vegetation. Tunnel scour and mass wasting are important gully erosion processes resulting from preferential flow through macropores （ pores 〉 1 mm diameter）. The objective of this study is to assess the changes with time in macropore flow characteristics of soils on the Loess Plateau following conversion to grass vegetation and the associated degree of mass wasting of gully faces. Ridge areas that had been revegetated for 1 year, 6 years, and 〉 15 years following tilling, and for 6 years following contour-ditching and the adjacent gully faces were characterized for their macropore and soil matrix properties on a 50 cm by 50 cm area. The total number of macropores increased from 11.6/m^2 to 39.6/m2 from 1 to 6 years and to 51.6/m2 after 15 years of revegetation following tillage. The macroporosity increased from 0.0008 m^3/m^3 to 0.0018 m^3/m^3 from 1 to 6 years of revegetation following tillage but the lowest macroporosity （0.0005 m3/m3） was 6 years of revegetation following contour-ditching. The contourditched area had the lowest infiltration rate （95 m/d） through the soil matrix （areas without macropores） with the tilled areas having similar infiltration rates regardless of the number of years of revegetation （averaged 146 m/d）. Due to tunnel scour erosion of macropores during infiltration into the area revegetated for 1 year, pore diameters enlarged by more than 200% resulting in this condition having the highest individual macropore infiltration rates （7967 m/d）. Macropores in all other areas were stable with no tunnel scour erosion of macropores. The total capacity for infiltration through macropores increased significantly with time following revegetation. The number of macropores on the gully faces was triple （92.8/m2） and the macroporosity quadruple （0.004 m3/m3） that of the ridge surfaces. The upper gully faces exhibited 1.1 slumps m^-1 for a total soil loss of 48622 kg per ha.     

与苏门答腊特大地震有关的地壳内地震波速时间变化

特大地震的同震及震后滑动能引起地壳介质的时间变化,这是因为强地面震动破坏了近地表的沉积层,或者应力扰动改变了裂隙密度和/或深部流体的移动。这种随时间变化的地壳属性可引起地震波速度出现部分变化,并可根据互相关波形和测量重复地震序列内它们的时间滞后探测到。本文根据2004年苏门答腊—安达曼特大地震和2005年尼亚斯—锡默卢特大地震的重复余震序列,分析了高频（0.5~2.0Hz）P尾波和S尾波以及长周期（0.03~0.1Hz）面波的滞后时间。观测到的滞后时间揭示出几个主要特征：（1）2004年地震序列的S尾波滞后时间系列!（t）基本上在0值附近波动,有时随消逝时间而变为负值,而2005年地震序列的S尾波的!（t）则表现为随消逝时间而单调增长;（2）S尾波的平均速度降低（-δV S）是P尾波平均速度降低（-δV P）的2倍,瑞利波的平均速度降低（-δV LR）比勒夫波的平均速度降低（-δV LQ）大3~4倍;（3）δV S和δV LR显示了速度随时间的恢复,特别是2005年地震的序列。2005年地震序列δV S速度随时间恢复的形状与附近大地测量台站得到的位移时间系列类似。尽管我们讨论了可能的人为结果,如震源间距、噪声场的时间变化和仪器响应,但观测结果均表明地壳内的非均匀波速时间降低是由2004年和2005年的特大地震的同震和/或震后滑动引起的。     

Persisting challenges for performance-based building assessment

Intense research and refinement of the tools used in performance-based seismic engineering have been made,but the maturity and accuracy of these methods have not been adequately confirmed with actual data from the field. The gap between the assumed characteristics of actual building systems and their idealized counterparts used for analysis is wide. When the randomly distributed flaws in buildings as they exist in urban areas and the extreme variability of ground motion patterns combine,the conventional procedures used for pushover or dynamic response history analyses seem to fall short of reconciling the differences between calculated and observed damage. For emergency planning and loss modeling purposes,such discrepancies are factors that must be borne in mind. Two relevant examples are provided herein. These examples demonstrate that consensus-based analytical guidelines also require well-idealized building models that do not lend themselves to reasonably manageable representations from field data. As a corollary,loss modeling techniques,e.g.,used for insurance purposes,must undergo further development and improvement.     

Tropical cyclones and polar lows: Velocity,size, and energy scales,and relation to the 26°C cyclone origin criteria

The goal of this paper is to quantitatively formulate some necessary conditions for the development of intense atmospheric vortices. Specifically, these criteria are discussed for tropical cyclones (TC) and polar lows (PL) by using bulk formulas for fluxes of momentum, sensible heating, and latent heating between the ocean and the atmosphere. The velocity scale is used in two forms: (1) as expressed through the buoyancy flux b and the Coriolis parameter l_c for rotating fluids convection, and (2) as expressed with the cube of velocity times the drag coefficient through the formula for total kinetic energy dissipation in the atmospheric boundary layer. In the quasistationary case the dissipation equals the generation of the energy. In both cases the velocity scale can be expressed through temperature and humidity differences between the ocean and the atmosphere in terms of the reduced gravity, and both forms produce quite comparable velocity scales. Using parameters b and l_c, we can form scales of the area and, by adding the mass of a unit air column, a scale of the total kinetic energy as well. These scales nicely explain the much smaller size of a PL, as compared to a TC, and the total kinetic energy of a TC is of the order 10¹⁸-10¹⁹ J. It will be shown that wind of 33 m s^-1 is produced when the total enthalpy fluxes between the ocean and the atmosphere are about 700 W m^-2 for a TC and 1700 W m^-2 for a PL, in association with the much larger role of the latent heat in the first case and the stricter geostrophic constraints and larger static stability in the second case. This replaces the mystical role of 26^oC as a criterion for TC origin. The buoyancy flux, a product of the reduced gravity and the wind speed, together with the atmospheric static stability, determines the rate of the penetrating convection. It is known from the observations that the formation time for a PL reaching an altitude of 5--6 km can be only a few hours, and a day, or even half a day, for a TC reaching 15--18 km. These two facts allow us to construct curves on the plane of T^s and ΔT=T_s-T_a to determine possibilities for forming an intense vortex. Here, T_a is the atmospheric temperature at the height z=10 m. A PL should have ΔT>20^oC in accordance with the observations and numerical simulations. The conditions for a TC are not so straightforward but our diagram shows that the temperature difference of a few degrees, or possibly even a fraction of a degree, might be sufficient for TC development for a range of static stabilities and development times.     

Estimates of carbon stored in harvested wood products from the United States forest service northern region, 1906-2010

Background  

Global forests capture and store significant amounts of CO₂ through photosynthesis. When carbon is removed from forests through harvest, a portion of the harvested carbon is stored in wood products, often for many decades. The United States Forest Service (USFS) and other agencies are interested in accurately accounting for carbon flux associated with harvested wood products (HWP) to meet greenhouse gas monitoring commitments and climate change adaptation and mitigation objectives. This paper uses the Intergovernmental Panel on Climate Change (IPCC) production accounting approach and the California Forest Project Protocol (CFPP) to estimate HWP carbon storage from 1906 to 2010 for the USFS Northern Region, which includes forests in northern Idaho, Montana, South Dakota, and eastern Washington.     

Mineralogy, Chemical Characteristics and Upgrading of Beach Ilmenite of the Top Meter of Black Sand Deposits of the Kafr Al-Sheikh Governorate, Northern Egypt

Egyptian beach ilmenite occurs in a relatively high content in the naturally highly concentrated superficial black sand deposits at specific beach zones in the northern parts of the Nile Delta at Rosetta. Microscopic study shows that the ilmenite occurs as fresh homogeneous black or heterogeneous multicoloured altered grains and exhibits three types (homogeneous, exsolved and altered) of ilmenite varieties. XRD data of ilmenite indicates their association with minor hematite and quartz, whereas leucoxene shows its association with Nb‐rutile, pseudorutile and hematite. Grain size distribution suggests a very fine sand size of >89% and 80% and a fine sand size of 10.5% and 18.3% for fresh and altered ilmenites, respectively. The density of fresh, altered ilmenite and leucoxene concentrates varies from 2.70, 2.50 to 2.40 ton/m³, suggesting a gradual decrease from high grade fresh to leucoxene and consistent with variation in magnetic susceptibility as a consequence of the leaching of iron. Mass magnetic susceptibility reveals 97.6% of ilmenite and 92% of the altered form are obtained at 0.20 and 0.48 ampere. Fresh ilmenite exhibits variable TiO₂ (47.18%) and Fe₂O₃^T (46.10%) with minor MnO, MgO and Cr₂O₃ (1.22, 1.10 and 0.51%). The altered ilmenite is higher in TiO₂ (76.16%) and SiO₂ (4.68%) and lower in Fe₂O₃^T (14.45%), MnO, MgO and Cr₂O₃ (0.39, 0.52 and 0.11%) compared with the fresh form. Three concentrates of ilmenites (G1, G2 and G3) were prepared from crude ore using a Reading cross belt magnetic separator under different conditions, revealing a gradual increase of TiO₂, SiO₂, Al₂O₃ and CaO accompanied by a decrease of Fe₂O₃^T, MgO and Cr₂O₃ with repetition of the separation processes. Several ore dressing techniques were carried out to upgrade the ilmenite concentrate.