Über die Erzeugung großräumiger homogener Magnetfelder zum Studium des Verhaltens von Magnetkompassen und Kompensiermitteln auf verschiedenen magnetischen Breiten

Zusammenfassung Zur Herstellung schwacher, homogener Magnetfelder wird die Verwendung quadratförmiger Rahmenspulen vorgeschlagen, die gegenüber gleich großen Kreisringspulen den Vorteil einer größeren Homogenität des Feldes bieten. Es wird die Feldverteilung auf der Achse und in der Ebene einer Quadratspule angegeben. Für die Kombination von zwei Quadratspulen wird die Ermittlung eines optimalen Spulenabstandes gezeigt für die Fälle, daß entweder größte lineare Ausdehnung des homogenen Bereichs auf der Feldachse oder größte flächenhafte Ausdehnung in der Mittelebene des Feldes gefordert wird. Im Helmholtzfall, für den der Abstand der Quadratspulen zum 0,5445fachen der Kantenlänge berechnet wird, erweist sich der räumliche Bereich mit 1 % Feldstärkeabweichung gegen den Feldmittelpunkt um rund 72 % größer als bei einer gleich großen Helmholtzspule der üblichen Form.

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On the production of uniform magnetic fields of great volume for studying the behaviour of magnetic compasses and adjusting devices in different magnetic latitudes

Summary For the production of weak uniform magnetic fields it is proposed to use square frame coils which in contrast to circular coils of the same size offer the advantage of a greater field homogeneity. The field distribution on the axis and in the plane of a square coil is indicated. For the use of two combined square coils the determination of the optimum coil separation is given for the cases that there should either be required greatest linear extension of the uniform range on the axis or greatest homogeneous area in the median plane of the field. In the Helmholtz case, for which the separation between the square coils is calculated to the 0,5445th part of the edge's lenght the volume with 1 %deviation from the field intensity in the centre is by 72 % larger than that of a usual Helmholtz coil of the same size.

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Sur la production des champs magnétiques homogènes de grand volume pour l'étude du comportement des compas magnétiques et des dispositifs de réglage par différentes latitudes magnétiques

Résumé Pour pouvoir produire des champs magnétiques faibles et homogènes on propose d'employer des bobines à cadres carrés qui en comparaison avec des bobines annulaires de la même grandeur offrent l'avantage d'une homogénéité supérieure du champ magnétique. La distribution du champ sur l'axe et dans le plan d'une bobine à cadre carré est indiquée. Pour l'emploi de deux bobines combinées à cadres carrés on montre de quelle manière on peut déterminer la distance optimale pour les cas où il est nécessaire de demander de la plus grande extension linéaire de la région homogène sur l'axe du champ ou de la plus grande aire homogène dans le plan central. En cas des bobines Helmholtz dont la distance pour des bobines à cadres carrés a été déterminé à la 0,5445^e partie de la longueur de l'arête, le volume homogène avec une déviation de 1 % de l'intensité en comparaison avec l'intensité du centre du champ dépasse de 72 % le volume d'une bobine Helmholtz de la même grandeur et du type usuel.

     

Accuracy assessments of recent earth gravity models using crossover altimetry

Summary The calibrated variance-covariance matrices of the harmonic geopotential coefficients of the recent combined model JGM 2 has been tested and verified by independent crossover altimetry from TOPEX/Poseidon and ERS 1 using the Latitude Lumped Coefficients in the southern oceans area. Although orbits are not yet available for these missions with other recent models for which error matrices have been released, by comparison with JGM 2 results and field differences we also confirm that the error matrices for the satellite model GRIM 4S4p and the combined data model JGM 3 are also generally valid. Projections of these matrices for a variety of inclinations show that many unused orbits of even moderate altitude (≈ 800 km) will still yield trajectory crossover errors at a level of many tens of centimeters even with the latest orbitgeopotential models.     

Ship-board measurements and estimations of air-sea fluxes in the western tropical pacific during TOGA COARE

Direct air-sea flux measurements were made on R/V Kexue #1 at 4 ° S, 156 ° E during the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean-Atmospheric Response Experiment (COARE) Intensive Observation Period (IOP). An array of six accelerometers was used to measure the motion of the anchored ship, and a sonic anemometer and Lyman-α hygrometer were used to measure the turbulent wind vector and specific humidity. The contamination of the turbulent wind components by ship motion was largely removed by an improvement of a procedure due to Shao based on the acceleration signals. The scheme of the wind correction for ship motion is briefly outlined. Results are presented from data for the best wind direction relative to the ship to minimize flow distortion effects. Both the time series and the power spectra of the sonic-measured wind components show swell-induced ship motion contamination, which is largely removed by the accelerometer correction scheme. There was less contamination in the longitudinal wind component than in the vertical and transverse components. The spectral characteristics of the surface-layer turbulence properties are compared with those from previous land and ocean results. Momentum and latent heat fluxes were calculated by eddy correlation and compared to those estimated by the inertial dissipation method and the TOGA COARE bulk formula. The estimations of wind stress determined by eddy correlation are smaller than those from the TOGA COARE bulk formula, especially for higher wind speeds, while those from the bulk formula and inertial dissipation technique are generally in agreement. The estimations of latent heat flux from the three different methods are in reasonable agreement. The effect of the correction for ship motion on latent heat fluxes is not as large as on momentum fluxes.     

Pulsed Air Sparging in Aquifers Contaminated with Dense Nonaqueous Phase Liquids

Air sparging was evaluated for remediation of tetrachloroethylene (PCE) present as dense nonaqueous phase liquid (DNAPL) in aquifers. A two-dimensional laboratory tank with a transparent front wall allowed for visual observation of DNAPL mobilization. A DNAPL zone 50 cm high was created, with a PCE pool accumulating on an aquitard. Detailed process control and analysis yielded accurate mass balances and insight into the mass-transfer limitations during air sparging. Initial PCE recovery rates were high, corresponding to fast removal of residual DNAPL within the zone influenced directly by air channels. The vadose zone DNAPL was removed within a few days, and the recovery in the extracted soil vapors decreased to low values. Increasing the sparge rate and pulsing the air injection led to improved mass recovery, as the pulsing induced water circulation and increased the DNAPL dissolution rate. Dissolved PCE concentrations both within and outside the zone of air channels were affected by the pulsing. Inside the sparge zone, aqueous concentrations decreased rapidly, matching the declining effluent PCE flux. Outside the sparge zone, PCE concentrations increased because highly contaminated water was pushed away from the air injection point. This overall circulation of water may lead to limited spreading of the contaminant, but accelerated the time-weighted average mass removal by 40% to 600%, depending on the aggressiveness of the pulsing. For field applications, pulsing with a daily or diurnal cycling time may increase the average mass removal rate, thus reducing the treatment time and saving in the order of 40% to 80% of the energy cost used to run the blowers. However, air sparging will always fail to remove DNAPL pools located below the sparge point because the air will rise upward from the top of a screen, unless very localized geological layers force the air to migrate horizontally. Unrecognized presence of DNAPL at chlorinated solvent sites residual and pools could potentially hamper success of air sparging cleanups, since the presence of small DNAPL pools, ganglia or droplets can greatly extend the treatment time.     

Daily variability in abundance and population characteristics of tidal salt-marsh fauna

Nekton and macrocrustacean population levels and characteristics were studied in two similar tidal marsh creeks. Absolute and area-adjusted data were analyzed to determine variability resulting from daily population fluctuations. Two sampling schemes—intensive 3-d seasonal and periodic 1-d—were examined by constructing probability matrices to compare the accuracy of data comparisons resulting from each scheme. The probability of inaccuracies in comparisons of abundance using nonreplicated sampling schemes ranged from 0% to 100%. Significant differences between consecutive day data were observed for population characteristics such as blue crab (Callinectes sapidus) sex ratios, spot (Leiostomus xanthurus) length-weights, killifish: sciaenid ratios, and killifish: blue crab ratios. These data support the need to account for short-term variability when assessing mobile aquatic fauna abundance in estuarine wetlands.     

Impacts of sea-level rise on deltas in the Gulf of Mexico and the Mediterranean: The importance of pulsing events to sustainability

In deltas, subsidence leads to a relative sea-level rise (RSLR) that is often much greater than eustatic rise alone. Because of high RSLR, deltaic wetlands will be affected early by an acceleration of eustatic sea-level rise. If there is sufficient vertical accretion, wetlands can continue to exist with RSLR; however, lack of sediment input eventually leads to excessive water logging and plant death. Areas with low tidal range, such as the Mediterranean and Gulf of Mexico, are especially vulnerable to rising water levels because the elevational growth range of coastal vegetation is related to tide range. Reduction of suspended sediments in rivers and prevention of wetland flooding by river dikes and impoundments have reduced sediment input to Mediterranean and Gulf of Mexico deltaic wetlands. This sediment deficit will become more important with an acceleration in sea-level rise from global warming. Most sediment input occurs during strong pulsing events such as river floods and storms, and management policies and decisions are especially designed to protect against such events. Management approaches must be reoriented to take advantage of pulsing events to nourish marsh surfaces with sediments. We hypothesize that deltas can be managed to withstand significant rates of sea-level rise by taking advantage of pulsing events leading to high sediment input, and that this type of management approach will enhance ecosystem functioning.     

Sediment Melts at Sub-arc Depths: an Experimental Study

The phase and melting relations in subducted pelites have beeninvestigated experimentally at conditions relevant for slabsat sub-arc depths (T = 600–1050°C, P = 2·5–4·5GPa). The fluid-present experiments produced a dominant paragenesisconsisting of garnet–phengite–clinopyroxene–coesite–kyanitethat coexists with a fluid phase at run conditions. Garnet containsdetectable amounts of Na₂O (up to 0·5 wt%), P₂O₅ (upto 0·56 wt%), and TiO₂ (up to 0·9 wt%) in allexperiments. Phengite is stable up to 1000°C at 4·5GPa and is characterized by high TiO₂ contents of up to 2 wt%.The solidus has been determined at 700°C, 2·5 GPaand is situated between 700 and 750°C at 3·5 GPa.At 800°C, 4·5 GPa glass was present in the experiments,indicating that at such conditions a hydrous melt is stable.In contrast, at 700°C, 3·5 and 4·5 GPa, asolute-rich, non-quenchable aqueous fluid was present. Thisindicates that the solidus is steeply sloping in P–T space.Fluid-present (vapour undersaturated) partial melting of thepelites occurs according to a generalized reaction phengite+ omphacite + coesite + fluid = melt + garnet. The H₂O contentof the produced melt decreases with increasing temperature.The K₂O content of the melt is buffered by phengite and increaseswith increasing temperature from 2·5 to 10 wt%, whereasNa₂O decreases from 7 to 2·3 wt%. Hence, the melt compositionschange from trondhjemitic to granitic with increasing temperature.The K₂O/H₂O increases strongly as a function of temperatureand nature of the fluid phase. It is 0·0004–0·002in the aqueous fluid, and then increases gradually from about0·1 at 750–800°C to about 1 at 1000°C inthe hydrous melt. This provides evidence that hydrous meltsare needed for efficient extraction of K and other large ionlithophile elements from subducted sediments. Primitive subduction-relatedmagmas typically have K₂O/H₂O of 0·1–0·4,indicating that hydrous melts rather than aqueous fluids areresponsible for large ion lithophile element transfer in subductionzones and that top-slab temperatures at sub-arc depths are likelyto be 700–900°C. KEY WORDS: experimental petrology; pelite; subduction; UHP metamorphism; fluid; LILE     

Evaluation of numerical simulations of capillary barrier field tests

This paper presents a comparison of numerical simulations to the measured response of capillary barrier field tests. The simulations were of pilot-scale tests of four 7 m long, 1.2 m thick capillary barriers, two with 5% slopes and two with 10% slopes, with and without an included unsaturated drainage layers. The unsaturated drainage layer was included to encourage lateral drainage. The 5% sloped barriers were subjected to a period of constant infiltration for a period of 74 days, while the 10% system with the drainage layer had water added for 26 days and the conventional capillary barrier with a 10% slope was subjected to 43 days of infiltration. The numerical modelling was conducted using both drying and wetting soil moisture characteristic curves to determine their influence on the results. Differences between the field test and the model data were found, but in general the simulations appeared to adequately reproduce the response of the test systems. It was found that the use of wetting curve data provided a better fit to the field data, more accurately predicting the amount and timing of the percolate produced. © Rapid Science Ltd. 1998