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.     

Multi-Beam Echo Sounders–INS Automatic Latency Calibration

Abstract

This paper presents an automatic, rigorous, and robust method to determine a Multi-Beam Echo Sounder (MBES) and Inertial Measurement Unit/Inertial Navigation System (IMU/INS) latency calibration. The latency may be due to the IMU/INS itself, but also to the time-tagging configuration, which is generally left to the survey systems user. One survey strip over a flat seafloor is the required configuration of line for using this approach, called Multibeam IMU/INS Latency Automatic Calibration (MILAC), standing for MBES to IMU/INS Latency Automatic Calibration. The algorithm considers only data with maximum latency effects and integrates an automatic data selection for this purpose. The latency estimation procedure is based on an iterative Least Square adjustment method followed by a statistical analysis. Our process can deal with beam acoustic refraction caused by the speed of sound in sea water. The accuracy of MILAC is about 2?ms (millisecond) and its average precision of 0.15?ms. MILAC is able to determine the latency with an average resolution of 5?ms. However, the morphology of the survey line is restricted to smooth and regular seafloor and the survey platform should have a relatively high attitude rate.     

High-temperature infrared properties of forsterite

Polarized emittance measurements were acquired for synthetic forsterite, the pure magnesium end member of the olivines group, on the whole infrared spectral range and up to the melting point by using CO₂ laser heating. The experimental data, fitted with a semi-quantum dielectric function model, allowed the retrieval of the temperature dependence of the absorption coefficient of forsterite both in the opaque and semi-transparent regions. The analysis of the phonon parameters indicates that the lattice dynamics evolve drastically with increasing temperature. The normal modes involving motions of the magnesium cations located in site 1 are the more impacted, and some of them vanish around 1,200 K. The results confirm that the enhancement of the lattice anharmonicity and the increasing mobility of the magnesium cations are closely linked and are at the origin of the anomalies observed in the evolution of the thermophysical properties. This complete set of spectroscopic data may be a step toward a more precise evaluation of the impact of thermal radiation heat transfer inside systems involving forsterite and quantification of their heat budget.     

Penetrative convection in the upper ocean due to surface cooling

Abstract

A new non-linear model of mixing and convection based on a modelling of two buoyant interacting fluids is applied to penetrative convection in the upper ocean due to surface cooling. In view of simple algebra, the model is one-dimensional. Dissipation is included, but no mean shear is present. A non-similar analytical solution is found in the case of a well-mixed layer bounded below by a sharp thermocline treated as a boundary layer. This solution is valid if the Richardson number, R _i , defined as the ratio of the total mixed-layer buoyancy to a characteristic rms vertical velocity, is much greater than unity. The model predicts a deepening rate proportional to R _i ^?3/4. The thermocline remains of constant thickness, and the ratio thermocline thickness to mixed-layer depth decreases as R _i ^?3/4 as the mixed layer deepens. If the surface flux is constant, the mixed-layer depth increases with time as t ^½. The vertical structure throughout the mixed layer and thermocline is given by the analytical solution, and vertical profiles of mean temperature and vertical fluxes are plotted. Computed profiles and available laboratory data agree remarkably well. Moreover, the accuracy of the simple analytical results presented here is comparable to that of sophisticated turbulence numerical models.     

Long term high frequency sediment observatory in an alpine catchment: The Arc-Isère rivers,France

We present a dataset on to the Arc-Isère long-term environmental research observatory, which is part of the Rhône Basin Long Term Environmental Research Observatory. This alpine catchment located in the French Alps is characterized by high Suspended Particulate Matter (SPM) in anthropogenized valleys. Suspended Sediment Concentrations (SSC) naturally observed in the river are very high, ranging from a few tens of milligrams per litre at low flow to tens of grams per litre during major natural hydrological events (floods, debris flows) or river dam hydraulic flushes. One research objective related to this site is to better understanding the SSC dynamics along the river using a system of nested catchments (Arvan, Arc, and Isère) in order to assess both temporal and spatial dynamics. The data allow the quantification of fine sediment yields and also the evaluation of possible morphological changes due to fine sediment deposition or resuspension. Additionally, the observatory database support studies on contaminants (either dissolved or particulate contaminants). Our monitoring includes six stations with high frequency (2–30 min) streamflow, SSC measurement using turbidity sensors, and associated automatic sampling. Discharge is measured via water level measurements and a rating curve. The oldest station (Grenoble-campus) started recording discharge and concentration data from April 2006 while others stations were built between 2009 and 2011. Data are available in an online data website called ‘Base de Données des Observatoires en Hydrologie’ (Hydrological observatory database, https://bdoh.irstea.fr/ARC-ISERE/ ) with a DOI reference for the dataset. The hydrological and sediment transport time series are stored, managed and made available to a wide community with unfettered access in order to be used at their full extent. This database is used as a data exchange tool for both scientists and operational end-users and there is an associated online tool to compute integrated fluxes.