Forced oscillation measurements of seismic wave attenuation and stiffness moduli dispersion in glycerine-saturated Berea sandstone

Fluid pressure diffusion occurring on the microscopic scale is believed to be a significant source of intrinsic attenuation of mechanical waves propagating through fully saturated porous rocks. The so-called squirt flow arises from compressibility heterogeneities in the microstructure of the rocks. To study squirt flow experimentally at seismic frequencies the forced oscillation method is the most adequate, but such studies are still scarce. Here we present the results of forced hydrostatic and axial oscillation experiments on dry and glycerine-saturated Berea sandstone, from which we determine the dynamic stiffness moduli and attenuation at micro-seismic and seismic frequencies (0.004–30 Hz). We observe frequency-dependent attenuation and the associated moduli dispersion in response to the drained–undrained transition (∼0.1 Hz) and squirt flow (>3 Hz), which are in fairly good agreement with the results of the corresponding analytical solutions. The comparison with very similar experiments performed also on Berea sandstone in addition shows that squirt flow can potentially be a source of wave attenuation across a wide range of frequencies because of its sensitivity to small variations in the rock microstructure, especially in the aspect ratio of micro-cracks or grain contacts.     

Determination of the drainage structure of a watershed using a digital elevation model and a digital river and lake network

Distributed hydrological models require a detailed definition of a watershed's internal drainage structure. The conventional approach to obtain this drainage structure is to use an eight flow direction matrix (D8) which is derived from a raster digital elevation model (DEM). However, this approach leads to a rather coarse drainage structure when monitoring or gauging stations need to be accurately located within a watershed. This is largely due to limitations of the D8 approach and the lack of information over flat areas and pits. The D8 approach alone is also unable to differentiate lakes from plain areas.

To avoid these problems a new approach, using a digital river and lake network (DRLN) as input in addition to the DEM, has been developed. This new approach allows for an accurate fit between the DRLN and the modelled drainage structure, which is represented by a flow direction matrix and a modelled watercourse network. More importantly, the identification of lakes within the modelled network is now possible. The proposed approach, which is largely rooted in the D8 approach, uses the DRLN to correct modelled flow directions and network calculations. For DEM cells overlapped by the DRLN, flow directions are determined using DRLN connections only. The flow directions of the other DEM cells are evaluated with the D8 approach which uses a DEM that has been modified as a function of distance to the DRLN.

The proposed approach has been tested on the Chaudière River watershed in southern Québec, Canada. The modelled watershed drainage structure showed a high level of coherence with the DRLN. A comparison between the results obtained with the D8 approach and those obtained by the proposed approach clearly demonstrated an improvement over the conventionally modelled drainage structure. The proposed approach will benefit hydrological models which require data such as a flow direction matrix, a river and lake network and sub-watersheds for drainage structure information.     

Utilisation des données d''un radar météorologique pour la simulation des écoulements en rivière à l''aide d''un modèle hydrologique matriciel

A good knowledge of the spatial and temporal distribution of precipitation is required for streamflow forecasting with a mathematical model. Thus, an operational method for correcting and integrating radar and station data with a minimum of operator intervention is under development. A total of 297 CAPPI maps at a height of 2 km have been used, each map representing a 2 h accumulation for 43 rainy days in the summer of 1978.Preliminary results have shown that it is necessary to detect and correct ground clutter as well as echoes coming from anomalous propagation. Thus, an algorithm has been developed to do so. In order to obtain a better idea on the necessity of correcting those echoes and on the efficiency of the algorithm, estimations of the mean areal daily rainfalls on the basin calculated from station data, are compared to estimations computed from (a) raw radar data, (b) radar data corrected with a ground clutter mask, and (c) radar data corrected with the developed algorithm, before and after calibration with station data. Similar comparisons between measured streamflows and streamflows computed by the CEQUEAU model, are presented, except for raw radar data. Actual results show that the identification and correction algorithm behaves very satisfactorily and could permit the operational use of radar data with hydrological models. Moreover, the sensitivity of the calibration factor on the number of meteorological stations is studied. It is shown that a relatively small number of stations may be used to calibrate the radar data. However, this number should be greater for convective than for continuous rain.

Résumé

La prévision des écoulements sur un bassin versant à l'aide d'un modèle mathématique requiert la connaissance de la distribution spatiale et temporelle des précipitations. Pour ce faire, une méthode opérationnelle de correction et d'intégration des images de radar et des données des pluviomètres demandant un minimum d'intervention de l'opérateur est actuellement en cours de développement. Les recherches effectuées jusqu'à maintenant ont porté sur l'étude de 297 images de radar de type CAPPI cumulées sur 2 h à une altitude de 2 km et réparties sur 43 jours pluvieux de l'été 1978.Les premiers résultats obtenus ont vite démontré la nécessité de détecter et de corriger les échos de montagnes et les échos provenant de propagation anormale. Un algorithme a donc été développéà cet effet. Afin de mieux juger de la nécessité de corriger ces échos et de l'efficacité de l'algorithme, des estimations de la pluie moyenne sur le bassin effectuées à l'aide des données aux stations, ont été comparées aux estimations obtenues à l'aide (a) des données de radar brutes, (b) des données de radar corrigées à l'aide d'un masque d'échos de montagnes, et (c) des données de radar corrigées à l'aide de l'algorithme développé, avant et après étalonnage par les données des pluviomètres. Des comparaisons similaires ont été effectuées entre les débits observés et les débits simulés à l'aide du modèle CEQUEAU sauf pour les données de radar brutes. Les résultats obtenus démontrent que l'algorithme d'identification et de correction développé se comporte de façon très satisfaisante et pourrait permettre d'utiliser de façon opérationnelle des données de radar avec un modèle hydrologique. De plus, une étude de la sensibilité du coefficient d'étalonnage en fonction du nombre de stations météorologiques utilisées indique qu'un nombre relativement restreint de stations peut être utilisé pour étalonner les données de radar. Toutefois, ce nombre doit être plus élevé pour un épisode de pluie convective que pour un épisode de pluie continue.     

浙江舟山海岸带古木埋藏区铁的微生物成矿作用

以浙江舟山海岸带铁矿石为研究对象, 通过对铁矿石及其周围环境背景样品的形态学显微观察、矿物学及地球化学分析, 结果显示渗漏水沉淀铁泥中存在大量形貌与Leptothrix ochracea和Gallionella ferruginea中性铁氧化菌极为相似的微生物鞘, 该微生物可促进Fe2+的氧化和Fe3+的快速沉淀, 并且细胞最终被完全矿化后将永久保存起来.与此相对应的是: 在铁矿石内部存在大量的似球形和丝杆状的针铁矿, 并且还保留了死亡的铁细菌外鞘, 这些特征揭示该铁矿石与微生物历史活动密切相关.将现代渗漏水铁泥中铁细菌的矿化作用和铁矿石中保留的微生物活动记录相对比, 为该环境下的铁矿石生物成矿作用及其成因机制提供了良好的佐证.铁矿石的形成与古木堆积密切相关, 古木埋藏腐烂过程产生的腐植酸加剧了基岩及其周围土壤中的铁淋滤进入到潮间带, 从而为铁矿石形成提供充足的铁来源.该研究有助于更好理解和认识地史时期腐植质及微生物在铁矿床形成中的作用.      

An improved discrete element method based on a variational formulation of the frictional contact law

An improved algorithm based on the contact dynamics approach is proposed. Like previous developed algorithms it involves two stages. In the first one (local stage) for each particle, forces are computed from the relative displacement using an interaction law, which models frictional contact and shock. In the second stage (global stage) Newton's second law is used to determine, for each particle, the resulting acceleration which is then time-integrated to find the new particle positions. This process is repeated for each time step until convergence is achieved. The two distinguishing features of the present algorithm are the local integration of the frictional contact law and the convergence criterion. By adopting a variational statement of the frictional contact law based on the bi-potential concept, the integration procedure is reduced to a single predictor-corrector step and a new convergence criterion is introduced. Both aspects significantly reduce the computing time and enhance the convergence. Numerical applications show the robustness of the algorithm.     

Total Microbial Activity and Sulfur Cycling Microbe Changes in Response to the Development of Hypoxia in a Shallow Estuary

—We examined the effects of changing from oxic to anoxic conditions on microbial communities using both biogeochemical and molecular approaches in a     

Predicting Snow Depth in a Forest–Tundra Landscape using a Conceptual Model Allowing for Snow Redistribution and Constrained by Observations from a Digital Camera

Estimation of snow depth in the forest–tundra landscape remains a challenge because of a lack of reliable and frequent observations on precipitation and snow depth. Snow models forced by gridded meteorological datasets are often the only option available for assessing snow depth at the local scale. Unfortunately, these models generally do not take into account the snow redistribution process between open and forested areas which frequently occurs in the forest–tundra landscape. A simple modification to an existing snow accumulation and melt model is proposed in order to allow for snow redistribution. Along with a technique for taking advantage of snow depth observations obtained from a digital camera, the model is shown to provide accurate predictions of snow depth at the local scale when forced with precipitation data from Environment Canada's Canadian Precipitation Analysis. Results from this study suggest that instrumenting automated weather stations with a digital camera, together with small modifications to an existing model used operationally for snow depth prediction, could result in significant improvements to snow depth prediction and analysis in this environment. Further testing at sites where snow water equivalent of the snowpack is available should, however, be performed to fully validate the method.     

Correcting wind‐induced bias in solid precipitation measurements in case of limited and uncertain data

Automatic precipitation gauges tend to underestimate solid precipitation in the presence of wind. Loss as a function of wind speed is typically evaluated by comparing the gauge with a more accurate measurement made using a double‐fence intercomparison reference gauge (DFIR). For small precipitation events, small errors in the observations can induce large errors in the ‘catch’ ratio, i.e. the ratio of the automatic gauge measurement to the DFIR observation. For this reason, precipitation events of less than 3 mm are typically discarded before performing the regression analysis. This can mean discarding more than 90% of the observations. This paper shows how the method of weighted least squares can be used to perform a regression analysis that can take into account the whole sample to provide a more accurate estimation of the relationship between the catch ratio and the wind speed. This methodology is then used to obtain an adjustment curve for a shielded Geonor T‐200B precipitation gauge in Northern Québec. Copyright © 2008 John Wiley & Sons, Ltd and Her Majesty the Queen in right of Canada.     

Spatial and temporal variability of soil biological activity in the Province of Québec, Canada (45–58 °N, 1960–2009)—calculations based on climate records

Climate records of air temperature (AT) and total precipitation (TP) are standard inputs for soil carbon dynamic models, i.e., for calculating temperature and moisture effects on soil biological activity. In this study our objective was to determine both spatial and temporal differences in soil biological activity in the Province of Québec, Canada. Soil biological activity was here calculated on a daily basis with the ICBM re_clim parameter using data from weather stations. When keeping soil and crop properties constant, re_clim (unitless) allows us to assess relative differences in soil biological activity. The magnitude of the temporal changes in re_clim, AT and TP were analyzed using Sen’s slope, which is a nonparametric method used to determine the presence of a trend component. The re_clim varied across Québec from 0.50 (58 °N) to a high of 1.66 (45 °N). Considering only the area with significant agricultural production, re_clim varied from 0.99 at Gaspé (48 °N) to 1.66 at Philipsburg (45 °N), i.e., soil organic carbon (SOC) decomposition rate is 68 % higher at the latter site (1.66/0.99) and correspondingly more C input is needed to maintain SOC. Soil biological activity increased from 1960 to 2009, with a mean slope variation in re_clim of about +10 %. The temporal variation in AT had more influence than that of TP. For 1980–2009 the mean annual slope of re_clim was significantly different from zero for 29 out of 49 climate records (mean?=?+14 %; N?=?29). We also emphasize that analysis of seasonal changes in AT is an issue that needs further attention, as well as modeling climate-induced changes in SOC dynamics based on future climate scenarios.