Apparent flow-dimension approach to the study of heterogeneous fracture network systems

Hydrogeology Journal - The generalized radial flow (GRF) model in well-test analysis employs noninteger flow dimensions to represent the variation in flow area with respect to radial distance from...     

Montagne Pelée volcano (Martinique,in the French Lesser Antilles) hydrogeological system revealed by high-resolution helicopter-borne electromagnetic imagery

Montagne Pelée, on the French island of Martinique, eastern Caribbean Sea, has been one of the deadliest volcanoes in the world, with 30,000 victims following the 1902 eruption. Thousands of people still live nearby, and this volcano is a strategic “water tank” for Martinique Island, providing 40% of the island’s water supply. This research aimed to better understand its hydrogeological functioning and the relationship with its complex volcanological evolution, taking advantage of a high-resolution helicopter-borne geophysical survey correlated with hydrogeological data from the boreholes and springs databases. Electromagnetic data, correlated with hydrogeological data, allowed for the identification of unsaturated zones, aquifers, and seawater intrusions, as well as the main geological units. In addition, data synthesised from pumping tests revealed that the older the unconsolidated pyroclastic deposits, the lower their hydraulic conductivity. The structural asymmetry between the northeastern and southwestern volcano flanks impacts its hydrogeological functioning. Consequently, the Montagne Pelée hydrogeological conceptual model is marked by several distinguishable aquifers. The upper perched aquifer within recent lava domes is directly involved in, and impacted by, phreatic eruptions, and it supports low flowrate springs. The remaining effective rainfall infiltrates to depth and recharges the hydrothermal system through vertical fractures. The other aquifers are categorized into three groups: northeastern, southeastern and southwestern flank aquifers. This research is a new step toward a better understanding of the Lesser Antilles volcanoes and more broadly of the central and proximal parts of the andesitic active volcanoes.

     

Mapping surface water erosion potential in the Soummam watershed in Northeast Algeria with RUSLE model

The present study aims to estimate the annual soil loss in the Soummam watershed in the northeast of Algeria, using the Revised Universal Soil Loss Equation(RUSLE), geographic information system(GIS), and remote sensing(RS). RUSLE model has been used for modelling the main factors involved in erosive phenomena. The Soummam watershed covers a surface area of 9108.45 km2 of irregular shape, northeast –southwest towards southeast. It is characterized by an altitude varying between 2 m in the northeast and 2308 m in the northwest. Results showed that the average erosivity factor(R) is 70.64(MJ·mm)/(ha·h·year) and the maximum value reaches 140(MJ·mm)/(ha·h·year), the average soil erodibility factor(K) is 0.016(t·h·ha)/(MJ·ha·mm) and maximum values reach 0.0204(t·h·ha)/(MJ·ha·mm) in the southeast regions of the watershed, the average slope length and steepness factor(LS) is 9.79 and the mean C factor is estimated to be 0.62. Thematic maps integration of different factors of RUSLE in GIS with their database, allowed with a rapid and efficient manner to highlight complexity and factors interdependence in the erosion risk analyses. The resulting map for soils losses, with an average erosion rate of 6.81 t/(ha·year) shows a low erosion(7.41 t/(ha·year)) which covers 73.46% of the total area of the basin, and a medium erosion(7.42 to 19.77 t/(ha·year)), which represents 17.66% of the area. Areas with extreme erosion risk exceeding 32.18 t/(ha·year) cover more than 3.54% of the basin area. The results can certainly aid in implementation of soil management and conservation practices to reduce the soil erosion in the Soummam watershed.     

Experimental study of the ground motion on a large scale topographic hill at Kitherion (Greece)

We present the results of an experiment about the effects on the seismic ground motion of a hill of large dimensions (6km long, 3km large and 700m high) near Corinth (Greece). We installed 7 seismometers across the hill, and analyzed the ground motion with different methods: the classical spectral ratios (CSR) and the horizontal to vertical spectral ratios calculated both on noise (HVNR) and earthquake data (RF). The amplification at the resonant frequency (around 0.7Hz) is low, as expected, but one of the two stations installed at the top exhibits large amplification (up to a factor of 10) at 3Hz. The comparison between the H/V results and the CSR ones suggests that the former method is able to point out the fundamental frequencies of a hill. Amplifications and spatial localization predicted by numerical 3D modeling are consistent with the experimental data only at frequencies lower than 1Hz.     

Nature and biological composition of the New Caledonian outer barrier reef slopes

The Grande Terre of New Caledonia is enclosed by one of the longest barrier reefs in the world. For the first time, the fore-reef slopes of this barrier reef have been sampled by dredging, from 40 to 320 m deep, in order to analyze their sedimentological and biological characteristics. The rocks and sediments can be divided into seven sedimentary facies: bindstones dominated by coralline algal crusts, bindstones dominated by foraminiferal crusts, bindstones dominated by bryozoan crusts, coral framestones, bindstones and interstratified packstones rich in skeletal debris, packstones/wackestones and grainstones rich in rock gravels. Radiocarbon dating performed on encrusting organisms (coralline algae and acervulinids) and corals provide ages relatively young. These ages confirm that the encrusting organisms are modern and the corals mainly dated of Holocene are reworked due to the instability of the fore-reef slopes, especially during storms. Because the biological assemblages are distributed according to a bathymetric range depending on light intensity, a model of distribution of modern encrusting calcareous organisms can be proposed from our observations and analyses. From the upper reef slopes to approximately 90 m, thick coralline algal crusts are dominant and distributed in three groups. Group C, the shallowest parts of the fore slopes, is mainly characterized by mastophorids (Hydrolithon reinboldii, H. cf. munitum, Lithoporella melobesoides, Aethesolithon cf. problematicum, Neogoniolithon sp. and undetermined species) and lithophylloids (Lithophyllum sp., L. pustulatum). Group B, composed of lithophylloids (Lithophyllum sp., L. cf. kotschyanum, L. cf. moluccence, L. pustulatum), Mesophyllum sp. and Peyssonnelia sp. occurs from 15 to 40 m. Group A, rich in Mesophyllum sp., M. cf. mesomorphum, Peyssonnelia sp. and Sporolithon sp. is characteristic of deep reef slopes up to 90 m. Below approximately 90 m, when the light intensity decreases, the encrusting foraminifera acervulinids progressively replace the coralline algal crusts. Such a model is particularly useful to interpret and reconstruct the past Quaternary reef environments rich in crusts of coralline algae and/or foraminifera.     

Sensitivity experiments for polar low forecasting with the CMC mesoscale finite‐element model

Abstract

High‐resolution versions of the Canadian operational regional finite‐element model (RFE) have been developed to assess their potential in simulating mesoscale, difficult‐to‐forecast and potentially dangerous weather systems commonly referred to as polar lows. The operational (1989) 100‐km version and a 50‐km version of the model have been run for two different polar low cases: one over Hudson Bay and one over Davis Strait. More integrations have also been performed on the Hudson Bay event both at 50 and 25 km to assess the model sensitivity to ice cover. As expected, the reduction in spatial truncation errors provided by the increase in resolution results in a better simulation of the systems. Moreover, when run at higher resolutions the model shows a significant sensitivity to ice cover. The results of the ice‐cover experiments also put into perspective the interaction between the heat and moisture fluxes at the surface, the low‐level wind structure, and the relation of these to the development of the polar low. This study suggests that the improved forecast accuracy obtained from increased resolution is limited by the correctness of the analysis of the ice cover, which acts as a stationary forcing for the entire forecast period.     

Estimating aquifer thickness using multiple pumping tests

A method to estimate aquifer thickness and hydraulic conductivity has been developed, consisting of multiple pumping tests. The method requires short-duration pumping cycles on an unconfined aquifer with significant seasonal water-table fluctuations. The interpretation of several pumping tests at a site in India under various initial conditions provides information on the change in hydrodynamic parameters in relation to the initial water-table level. The transmissivity linearly decreases compared with the initial water level, suggesting a homogeneous distribution of hydraulic conductivity with depth. The hydraulic conductivity is estimated from the slope of this linear relationship. The extrapolation of the relationship between transmissivity and water level provides an estimate of the aquifer thickness that is in good agreement with geophysical investigations. The hydraulically active part of the aquifer is located in both the shallow weathered and the underlying densely fractured zones of the crystalline basement. However, no significant relationship is found between the aquifer storage coefficient and initial water level. This new method contributes to filling the methodological gap between single pumping tests and hydraulic tomography, in providing information on the variation of the global transmissivity according to depth. It can be applied to any unconfined aquifer experiencing large seasonal water-table fluctuations and short pumping cycles.     

Geochemical modelling of fluid–rock interactions in the context of the Soultz-sous-Forêts geothermal system

The development of the Enhanced Geothermal System (EGS) at Soultz-sous-Forêts (France) has given to scientists an interesting opportunity for the application of geochemical modelling of water–rock interactions, combining theoretical studies with field and experimental data. The main results of four successive and complementary studies are summarized: geochemical modelling of fluid–rock interactions with prediction of dissolution/precipitation of minerals, feed-back effects on the mineralogy and petrography of the rock (major role of silicates in the geological past and of carbonates in the near future of the exploitation), experimental control of the dynamics of silicates under thermal gradient and relation between the evolution of the petrophysics of the rocks and the heat and mass transfers. The thermal cycle of the fluid, between 200 °C and 65 °C in the geothermal loop, may be responsible for dissolution/precipitation of minerals which modify the porosity and permeability of the granite, as it happened in the geological past, in relation with hydrothermal circulations in the Rhine Graben.