Use of geophysics for non-destructive characterisation of soil properties

Scientific leader: Claude Doussan <claude.doussan@inrae.fr> (INRAE, UMR EMMAH, Avignon).

A model coupling soil water transfers and soil electrical resistivity has been developed in the VSoil platform. This model is used in inversion mode to estimate hydraulic and petrophysic parameters from soil electrical resistivity measurements obtained by electrical resistivity tomography (ERT).

Details of the work carried out under this theme are presented below.

• Julien Ardouin (master's internship from February to August 2022)

Julien Ardouin (Savoie Mont Blanc University) used the VSoil platform during his master internship (February - August 2022) for a topic entitled "Sensitivity analysis and inversion of a coupled hydro-geophysical model". This internship is the continuation of the work started by Ismaïl Djellouli (see below).

Julien improved the module implementing the "petrophysics" process by adding an option allowing to correct the true resistivity by the temperature provided by a module solving the heat transport equation. Julien then used the Morris sensitivity analysis method available in the VSoil platform to estimate the relative importance of hydraulic and geophysics parameters on electrical apparent resisitivity and water volumetric content values. Julien also used and improved the SCE-UA optimisation algorithm available in the VSoil platform to infer hydraulic and geophysics parameters from reference or measured values of electrical apparent resistivitiy and water volumetric content. Julien launched his simulations on the Linux VSoil calculation server available in EMMAH (INRAE, Avignon). At least, Julien developed a Python script allowing to create easily the files (electrical mesh and configuration of electrodes) to be read by the module implementing the "electrical transport" process.

• Ismaïl Djellouli (master's internship from March to August 2021)

Ismaïl Djellouli (Sorbonne University, Paris) and Claude Doussan (INRAE, EMMAH, Avignon) used the VSoil platform during Ismaïl's master internship (March - August 2021) for a topic entitled "Inversion of soil hydraulic properties with a coupled modelling of soil water transfers and electrical resistivity tomography survey".

Julien and Claude added in VSoil two processes named "petrophysics" and "electrical transport" and two C++ modules implementing these processes. The module implementing the "electrical transport" process uses Gimli, a library for geophysical modelling and inversion. A fully coupled model was built in VSoil taking into account these processes in addition to water and heat transfer in soil, crop development, climate, ... In order to estimate hydraulic and petrophysical parameters, the SCE-UA (Shuffled Complex Evolution - University of Arizona) optimisation algorithm from rtop package was added in the list of R parameter estimation methods available in VSoil. The Morris sensitivity analysis method (from sensitivity R package) available in VSoil was previously used to determine the least sensitive parameters which are thus fixed in the parameter estimation.