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Details
  • Dr. Damien Jougnot, CNRS, UMR METIS, Université Pierre et Marie Curie, França
  • Date: June, 7, 2016 12:00 am
  • Place: Sala de Juntes de la Facultat de Geologia (UB)
  • Location: Carrer de Martí i Franquès, s/n,  Barcelona
  • Further information: Pilar Queralt (UB)
Abstract

Geophysics offers a variety of non-intrusive techniques to study near-surface processes of relevance for environmental sciences. One such technique is the self-potential (SP) method that is, among other contributors, sensitive to water fluxes. This contribution, the so-called streaming potential, results from the presence of an electrical double layer at the mineral-pore water interface. When water flows through the pore space, it gives rise to a streaming current and a resulting measurable electrical voltage. Streaming current generation is well understood in water-saturated porous media, but its modeling under partial saturation is still an area of active research. To evaluate how SP data and state-of-the-art models can be used to characterize flow and transport phenomena in the vadose zone, we conducted field-based monitoring of vertical variations of the SP signal during 36 months. The investigations were carried out at the Voulund agricultural test site of the Danish hydrological observatory (HOBE). The site is instrumented to monitor suction, water content and temperature down to a depth of 3 m, together with meteorological variables and repeated geophysical crosshole surveys. We installed 15 non-polarizable electrodes at 10 different depths within the vadose zone and placed the reference electrode below the water table. Data were acquired every 5 minutes and the record included various hydrologic events, such as natural infiltration, water table rises, and a high salinity tracer infiltration. We developed a fully coupled numerical scheme to simulate water fluxes and ionic transport in order to predict bulk electrical conductivity and the SP signal. The simulation results of the tracer test agreed rather well with the measured SP data. This model will now be used together with the 36 months of data to carry out a detailed assessment of the predictive value of the SP method in vadose zone hydrology, with specific focus on in situ monitoring of water flux