Estimate of hydrodynamic parameters with a coupled hydrogeophysical inversion using GPR surveys

<p>We develop a methodology to estimate soil hydrodynamic parameters from a water infiltration experiment monitored with a GPR (Ground Penetrating Radar). Such an experiment, carried out on both controlled and natural site, consists in applying a water charge in a tank on the soil surface. During the water infiltration, the water layer thickness above the soil surface in the tank and the GPR response on the infiltration water front are monitored. The infiltration experiment is then modelled numerically using hydrogeological parameters which describe the constitutive relationships between water content, pressure and hydraulic conductivity. In that goal, we use the WAMOS-1D code which combines the 1D Richards equation and the Mualem – van Genuchten model. From the hydrogeological models outputs and petrophysical relationships, corresponding GPR velocity models are created to generate the resulting GPR signals. Then, an inversion algorithm couples both the hydrogeological and the geophysical models to seek the optimal hydrodynamic parameters. The inverse problem objective function is calculated from the estimated arrival time of the GPR waves reflected by the water infiltration front and compared to the measured ones. Preliminary inversion tests explore the hydrodynamic parameters space using synthetic data. First results show that the saturated hydraulic conductivity parameter can be estimated. Further tests are performed to improve both our experimental set-up and methodology and allow an estimation of the other hydrodynamic parameters. An emerging idea is to complete the objective function by analyzing the arrival time corresponding to additional reflectors to the water infiltration front.</p>

Reducing Threshold Pressure for Infiltration of Al-12Si Alloys into Carbon Particle Compacts by Placing a Thin Layer of Sn at the Infiltration Front

The oxide layer that usually covers the surface of liquid aluminum and its alloys, is one of the main factors that hinders infiltration of these alloys into graphite particle compacts. The oxide film increases the threshold pressure for infiltration and the porosity of the resulting composites is large because the wetting at the metal/carbon interface is reduced. Infiltrating graphite compacts with tin requires, however, a much lower pressure, less than half of that required to infiltrate the eutectic Al-12Si alloy. As the surface tension of tin is half that of the Al-12Si alloy, this result indicates that wetting at the Sn/C interface is slightly better. As a result, porosity in the infiltrated samples is reduced. In order to reduce the threshold pressure and improve the properties of Al-Si/graphite composites, a novel method has been used in this work that consists in placing a thin film of tin at the compact end through which infiltration takes place. During the infiltration process the graphite particles are firstly infiltrated by tin, which is pushed by the aluminum alloy, thus avoiding the oxidation of the latter. The method proved to be very effective in reducing the threshold pressure, while keeping almost constant the infiltration rate.

Macro Modeling of Reactive Infiltration Using Level Set Finite Element Formulations

The process of reactive melt infiltration can be used to fabricate ceramics and ceramic matrix composites. This process involves a liquid metal being allowed to infiltrate a medium with which the liquid reacts to form a resultant ‘matrix’ along with the already present reinforcing fibers. The authors’ previous work on this area revealed that the transient porosity and permeability of a porous medium can be determined for certain geometries from the reaction kinetics and coupled heat and mass transfer problem occurring at the pore level. But the formulation at the macro level, which is essential to optimize the process, has been limited. Towards this end, this paper solves the macro reactive flow problem in a porous medium analytically as well as numerically. The focus of this article will be on the solutions for the advance (displacement) of the ‘infiltration front’ with progressive chemical reaction occurring between the medium and the infiltrant. A finite element formulation is used to solve the problem computationally; a level set formulation is used to track the infiltration front during the process. Excellent agreement is obtained between the analytical and computational solutions thereby validating the level set finite element formulations.

Timing of Nitrate Leaching from Turfgrass after Multiple Fertilizer Applications

The leaching of nitrogen from surface-applied fertilizer to groundwater is an environmental concern. Nitrogen fertilizer is routinely applied to turfgrass from spring to late autumn in Canada. The main objective of this study was to determine the contribution of N applied in May, July and September to leaching. The leaching of applied chloride (May and September only) was also monitored and the transport of nitrate and chloride were simulated using the model LEACHM (within EXPRES) to assist in fulfilling the main objective. The accuracy of the model simulation for transport, not nitrogen losses, was also addressed. Field lysimeters (Guelph, Ontario) were packed with a three-horizon profile of a sandy loam soil, topped with Kentucky bluegrass (Poa pratensis) sod and monitored for 1 year. Based on soil water samples taken from suction samplers placed at depths of 10, 17, 29, 43, 54, 64 and 85 cm, part of the solute from spring/summer applications remained in the soil during the unusually dry summer. This residual solute was later transported downward with the ensuing infiltration front in late autumn, building upon the autumn application, resulting in excessive concentrations. Predictions by LEACHM of solute concentration profiles generally were similar to field measurements.