Spatial and temporal study of nitrate concentration in groundwater by means of coregionalization

 Spatial and temporal behavior of hydrochemical parameters in groundwater can be studied using tools provided by geostatistics. The cross-variogram can be used to measure the spatial increments between observations at two given times as a function of distance (spatial structure). Taking into account the existence of such a spatial structure, two different data sets (sampled at two different times), representing concentrations of the same hydrochemical parameter, can be analyzed by cokriging in order to reduce the uncertainty of the estimation. In particular, if one of the two data sets is a subset of the other (that is, an undersampled set), cokriging allows us to study the spatial distribution of the hydrochemical parameter at that time, while also considering the statistical characteristics of the full data set established at a different time. This paper presents an application of cokriging by using temporal subsets to study the spatial distribution of nitrate concentration in the aquifer of the Lucca Plain, central Italy. Three data sets of nitrate concentration in groundwater were collected during three different periods in 1991. The first set was from 47 wells, but the second and the third are undersampled and represent 28 and 27 wells, respectively. Comparing the result of cokriging with ordinary kriging showed an improvement of the uncertainty in terms of reducing the estimation variance. The application of cokriging to the undersampled data sets reduced the uncertainty in estimating nitrate concentration and at the same time decreased the cost of the field sampling and laboratory analysis. Received: 23 July 1997 · Accepted: 31 March 1998     

空间VLBI与天文地球动力学

介绍了空间VLBI的精密定轨及其在天文地球动力学应用研究中的最新进展．指出因为空间VLBI的时延和时延率观测量同时涉及到射电参考系、动力学参考系和地固参考系,所以特别适合于参考系的直接连接工作．对为评价参考系连接的精度而发展的协方差分析理论也作了介绍,还分析了将时延和时延率资料用于精密定轨时遇到的困难及其解决办法．     

An analysis of the Hubble Space Telescope Fine Guidance Sensor Coarse Track mode

There are two guiding modes of the Hubble Space Telescope used for the acquisition of astronomical data by one of its six scientific instruments. The nominally more precise one is called Fine Lock and the other is known as Coarse Track. Two of the three Fine Guidance Sensors are locked onto Guide Stars, in one of these two modes, thus maintaining the spacecraft's line of sight to a few milli-arc seconds (nominally 7 milli-arc seconds for Fine Lock and 17 milli-arc seconds for Coarse Track). In this paper I report on the most realistic and extensive simulations to date of the Coarse Track guidance mode. These simulations clearly show that the precision of Coarse Track is influenced by one adjustable parameter and that for default operating conditions its precision is 12 milli-arc seconds. A factor of two improvement should be possible by modifying the key parameter which governs the precision of Coarse Track. The importance of this for the scientific mission on the Hubble Space Telescope is that the use of Coarse Track in place of Fine Lock will result in a 1/3 reduction of engineering overhead per scientific target acquisition.See text for a full explanation.     

华北强震时-空变化某些特点与近期强震活动趋势

本文利用华北丰富的震史资料,分析该区强震活动时-空变化某些特点。并以此为依据,对近期强震活动趋势作出判断。文章最后部分还对华北地区最近两个活动期的强震活动水平的巨大差异的可能原因,从构造和介质条件等方面作些初浅讨论     

Galileo long-term dust monitoring in the jovian magnetosphere

The Galileo spacecraft was launched in 1989, and—between 1995 and 2003—was the first spacecraft in orbit about Jupiter. The in-situ dust instrument on board was a highly sensitive impact-ionisation dust detector which measured the speed, mass and impact direction of dust particles hitting a metal target. It provided a unique 12-year record of cosmic dust in interplanetary and circumjovian space. Degradation of the instrument electronics caused by the harsh radiation environment in the inner jovian magnetosphere was recognised in various ways: the sensitivity for dust detection dropped by a factor of 7.5 between 1996 and 2003 while the noise sensitivity decreased by up to a factor of 100. Shifts in the parameters measured during dust impacts and noise events (charge amplitudes and signal rise times, etc.) required a time-dependent algorithm for noise identification. After noise removal a total of 21 224 complete data sets for dust impacts (i.e. impact charges, signal rise times, impact direction, etc.) is available from the entire Galileo mission between 1989 and 2003 (18 340 data sets from the Jupiter mission after 1996). This homogeneous data set has been used in many investigations of jovian dust published already or ongoing. Electronics degradation prevents the application of the mass and speed calibration to data obtained after 2000. Only in cases where the impact speed of grains is known by other means can grain masses be derived for later measurements. The drop of the detection sensitivity also required a time-dependent correction for fluxes of jovian dust streams, reaching a factor of 20 in 2002. We use the derived homogeneous noise-removed data set for long-term monitoring of the jovian dust streams with Galileo. The derived fluxes of dust stream particles were highly variable by about five orders of magnitude, between 3×10^-3 and and exhibited strong orbit-to-orbit variability. This extensive and valuable data set is available for further detailed investigations.     

Unusual comets (?) as observed from the Hubble Space Telescope

In separate projects, the Hubble Space Telescope has been used to assess the nature of 3 unusual objects: Chiron, Pholus and P/Shoemaker-Levy 9. This paper will compare these objects and discuss how the unique capabilities of the HST may be used to address the issue of cometary activity in each. Chiron, which has exhibited obvious cometary characteristics for several years, might have a bound dust coma that is unresolvable from the ground. In an attempt to directly observe this bound coma, we have obtained a series of images of Chiron with the HST Planetary Camera. Inner coma structure out to 0.2 has been detected. From these observations we infer a low bulk nucleus density for Chiron. Both HST and ground-based images of 5145 Pholus have been obtained to search for evidence of activity. The ground-based data give the most sensitive limits; however, it is shown that the WFPC-2 on HST can give limits 2–3 orders of magnitude more sensitive than conventional ground-based limits. Finally, as part of a collaborative effort, we have been obtaining HST observations of SL9 in order to determine the fragment sizes and to assess their nature (i.e., cometary vs. asteroid). Both ground-based observations from the UH 2.2m telescope on Mauna Kea and HST observations show that the near-nucleus dust is redder than the sun. While FOS spectra did not detect OH emission, the WFPC-2 HST data show that the inner coma remained very circular from July 1993 up until 2 weeks prior to impact, implying continued production of dust.     

Evaluation of a 256×256 Si:As IBC detector array for astronomy

256×256-element IBC Si:As arrays have been evaluated for applications in IR astronomy from space-borne platforms. Basic figures of merit were measured at IR flux levels simulating those expected in space-based astronomical observations. Results include dark current <20e ^-/s, G as high as 3.8, / of 20%, and read noise below 100 rms e^-.     

The infrared space observatory camera

At the focal plane of the ISO 60 cm telescope, ISOCAM will take images of the sky in the wavelength range 2.5 to 17 m. It features two independent channels, containing each a 32×32 array detector. The long wavelength (4–17 m) detector, developed by LIR-LETI specifically for ISOCAM, is a Si:Ga array bonded with indium bumps to a DV readout circuit. The short wavelength (2.5–5.5 m) detector, developed by SAT, is an InSb array with a CID readout.Only one channel operates at a time. The channel selection is obtained with two field mirrors supported by a wheel. The field mirror in operation is in the focal plane of the telescope, covering the 3 arcmin field of view, and reflecting the light into one or into the other channel.In each channel, 10 or 12 discrete band pass filters and CVFs with resolution 45 are mounted on a wheel. A second wheel supports a set of lenses, which reimage the focal plane of the telescope on the array, with a pixel field of view of 1.5, 3, 6 or 12 arcsec.Two integrating spheres, illuminated through small holes by black body sources, are mounted on the selection wheel, for flat fielding and calibration purposes.The flight model of ISOCAM, fully tested and calibrated, has been, delivered to ESA and is ready to be integrated in the satellite, whose expected launch date is September 1995.     

Coffeehouses And The Art Of Social Engagement: An Analysis Of Portland Coffeehouses

Coffeehouses have been a part of America's cultural landscape since the seventeenth century. Their fortunes have risen and fallen with the changing demand for coffee and consumer preferences. This paper examines their historic function of promoting social engagement as so‐called third places among independently owned coffeehouses in the city of Portland, Oregon. Individual coffeehouses were evaluated to determine the extent to which they provided a supportive physical environment for social engagement in which a space is transformed into a place and detailed patron behavior was recorded over a three‐week period. Few coffeehouses provided a supportive physical environment. The majority of coffeehouse patrons during the weekday and weekend ordered drinks ‘to go.’ Among those who stayed during the weekday most sat alone and worked. Coffeehouse are spaces to ‘be alone together.’ It was only on lunchtime weekends that coffeehouses were full of the sound of conversations. Further study should consider the effect of eliminating free Wi Fi and banning laptops on social engagement and whether other place attributes need to be incorporated to promote social engagement.     

Geographies of high frequency trading – Algorithmic capitalism and its contradictory elements

This paper investigates the geographies of high frequency trading. Today shares shift hands within microseconds, giving rise to a form of financial geographies termed algorithmic capitalism. This notion refers to the different spatio-temporalities produced by high frequency trading, under the valuation of time. As high frequency trading accelerates financial markets, the paper examines the spatio-temporalities of automated trading by the ways in which the speed of knowledge exploitation in financial markets is not only of interest, but also the expansion between different temporalities. The paper demonstrates how the intensification of time–space compression produces radical new dynamics in the financial market and develops information rent in HFT as convertible to a time rent and a spatio-temporal rent. The final section discusses whether high frequency trading only responds to crises in microseconds or constitutes them. It argues that automated trading will not only contribute to accelerate crises, but also deepen them by the ways in which it differentiates the dynamics between financial, fixed and productive capital.