Yves Benderitter

Determination of recharge in unsaturated soils using temperature monitoring

Can shallow temperature measurements on vertical profiles be used to determine the recharge? As a first approach to answer this question, we consider here the soil temperature data recorded by meteorological stations. A group of annual cycles allows us to determine the average value of vertical water seepage: in the presence of infiltration the apparent diffusivity deduced from the damping of the amplitude of the temperature between two different depths differs from the apparent diffusivity deduced from the temperature phase shift between the same pair of depths, and the average water flow can be easily deduced from this difference. For the determination of recharge over shorter periods of time it would be better to reconsider the sampling step of the data in both time and space domains. These limitations can be overcome by installing specialized vertical temperature profiles.

Flow characterization through heat transfer evidence in a carbonate fractured medium: First approach

Water temperature records at the outlet of a fractured carbonate system during an annual cycle show two types of variation, slow variations over a small range, resulting from the thermal equilibrium between water and rock in the aquifer, and more rapid variations over a wider range, transmitted more quickly due to the inflow of water through fractures or karstic pipes. Starting at the outlet we present first a model of thermal exchange for flow in a conduit, then a model of thermal exchange in a porous medium between the surface and the reservoir. The comparison with the data recorded at Guichy (Nievre, France) allows us to establish the fact that on this site the vertical flow between the surface and the reservoir is rapid through a fracture network as is the case for the flow at the outlet.

Geophysical study of the industrial waste land in Mortagne- du-Nord (France) using electrical resistivity

The industrial waste land in Mortagne-du-Nord (France) has been the site of a zinc smelter unit and a lead smelter unit linked to sulphuric acid production. It has been systematically explored by drillings and pits. Geophysical data have led to the location of mineralized soil and water, and to the identification of water seepage towards the river drain. Various geophysical devices are used for subsurface exploration and for studies with higher spatial resolution. The prospect area covers a surface of 4.5 ha. Data obtained from trenches has been used to calibrate and to validate the geophysical interpretation. It has been found that variations in the depth of the interface between resistive and conductive layers are due to two types of waste: ‘non-active’ (carbonates and oxides) and ‘active’ (sulphides and sulphates) materials. Lower resistivities than expected are due to the presence of mineralized water in all geological formations.

Subsurface DC resistivity mapping: approximate 1-D interpretation

Abstract Resistivity prospecting is the main tool used to investigate the shallow structure of the ground. A series of new techniques for determining the 2-D and 3-D geometry of the ground is now finding increasing use, but the light and simple Wenner prospecting technique remains a practical and efficient tool for rapidly mapping lateral variations in resistivity. When the resistivity changes are smooth, 1-D modelling can be used to interpret the data, and the criteria governing this approximation can be defined from synthetic data generated by a 3-D slab-model. For a Wenner array, two quadripole configurations can be used, Normal and Dipole-Dipole. For these two configurations the width of the transition zone, the apparent anisotropy effect and the precision of the resistivity values recovered from 1-D inversion differ. However the simultaneous inversion of both sets of data gives better results than for either configuration by itself. Two examples illustrate that in geological contexts where the thickness of the weathered zone causes the changes in the apparent resistivity value, this parameter can be recovered from 1-D inversion.

Application of the electrostatic quadripole to sounding in the hectometric depth range

Abstract A current is injected with and electrostatic quadripole by electrostatic influence from two poles located above the ground surface. In the air, the voltage is also measured, between two other poles. This technique can be used whatever the ground surface and it overcomes the difficulties which arise in the DC resistivity method when the contact between the electrodes and the ground is not good enough. The satisfactory results obtained for low investigation depths, one to a few metres, led us to make an attempt to use this method for greater depths. Among other avantages, the method is easy to use on very resistant surfaces (dry, frozen, uncompacted, etc.) and extends those offered by EM methods. Here, we present comparisons between DC electrical soundings and electrostatic ones in order to establish the practicability of this new technique. These comparisons include the presentation of solutions allowing to overcome technical measurement problems and to interpret the results by taking into account the effect of induced currents.

Mise en évidence par résistivité électrique des écoulements préférentiels et de l'assèchement par le maı̈s d'un CALCISOL de Beauce irrigué

Abstract Soil electrical resistivity decreases with water saturation. During irrigation of a soil of Beauce with a corn crop cover, a non-destructive multi-electrode 2D electrical method, spatially integrative, was tested jointly with TDR measurements (Time Domain of Reflectometry) of water content to realise a study of soil water flow with time. The results presented show that the method employed is well suited to the question investigated. The soil volumes dried up by corn roots, the wetting front movement and the preferential flow directions were identified and located.

The industrial waste land in Mortagne-du-Nord (59) – III – Methodological approach for a non-destructive geophysical study of sites contaminated by mineralized water

The site of zinc and lead metallurgy, and sulphuric acid making of Mortagne-du-Nord (France) is a waste landfill where contamination is the main preoccupation. The physical property sensitive to mineralization is electrical resistivity, because of the water content and quality dependance. Geophysical techniques taking into account this property, such as slingram electromagnetic mapping and electrical 2D imaging have been carried out. Surveys at different depths and over a wide area allow to locate mineralized water areas, to identify the water leakage toward river drain, and to locate the clayey alluvial deposits.

Invariants for correcting field polarisation effect in MT-VLF resistivity mapping

Abstract MT-VLF resistivity mapping is well suited to perform hydrology and environment studies. However, the apparent anistropy generated by the polarisation of the primary field requires the use of two transmitters at a right angle to each other in order to prevent errors in interpretation. We propose a processing technique that uses approximate invariants derived from classical developments in tensor magnetotellurics. They consist of the calculation at each station of . Both synthetic and field cases show that they give identical results and correct perfectly for the apparent anisotropy generated by the polarisation of the transmitted field. They should be preferred to verticalization of the electric field which remains of interest when only transmitter data are available.