iSOIL - Interactions between soil related sciences -
Linking geophysics, soil science and digital soil mapping

WP 3: Pedophysics

Lead Partner: UPD

Objectives

Geophysical data can be extremely useful in mapping soil properties but there is a need to establish sound physical relationships between geophysical parameters and soil properties.

The aim of this work-package was to develop novel, physically-based geophysical pedo-transfer functions (GPTFs) that could be used (i) to quantitatively analyze the geophysical signature of a soil and (ii) link the soil response to soil state, in particular to static properties, such as texture (e.g. porosity, grain-size distribution, compaction) and mineralogy.

This was achieved by

  • developing a theoretical constitutive model (or GPTF) that incorporates explicitly the textural parameters; and
  • developing empirical correlations based on laboratory experiments and field-scale validation.

The theoretical model links the electrical response of a porous medium to its structural characteristics and its water content, both expressed in terms of the well-known Archie’s law parameters, but reaching the capability of describing with the same common parameterization dielectric constant, electrical conductivity and spectral induced polarization response of the same porous medium. Empirical analysis complemented the theoretical investigation by analyzing: (a) the dependence of electrical response upon temperature, (b) the dependence of the complex dielectric constant on clay content, (c) the dependence of gamma ray emission on soil parameters, e.g. soil texture (grain size distribution).

Methods of investigation

Laboratory measurements

Laboratory measurements on samples that have different structures, under different ambient forcings, and under different conditions

Pore scale modeling

Pore scale modeling reproducing typical soil structures, the fluids therein and their geophysical response

Analysis of most significant geophysical data combinations

Analysis of most significant geophysical data combinations based on physical considerations as well as statistical evidence, towards the development of applicable constitutive relationships

Results of the work

Substantial progress has been made i.e. in the development of Geophysical Pedo-Transfer Functions (GPTFs), capable of linking (combinations of) geophysical data with specific soil properties and characteristics. We conducted a well calibrated mixture of theoretical developments of novel GPTFs and empirical verification and testing. In particular the novel GPTFs are focussed on the description of the electrical response of soils in terms of their structural and state parameters. This description involves direct current (and EMI) electrical conductivity, dielectric properties and spectral induced polarization using an overarching ensemble of common, well-identified physical parameters describing the micro-structure of porous media. Thus one step was to develop a model that combines measurements of DC electrical conductivity and high-frequency permittivity to map soil properties. The novel GPTF is based on the formal equivalence of the governing macroscale equations describing electrical permittivity (under quasi-static conditions) and the low frequency limit of electrical conductivity. Within iSOIL, we began developing a description for the spectral behaviour of SIP phase shift. We focused on the quantitative description of SIP spectra, either measured in the lab or inverted from SIP field measurements, in terms of relaxation time distributions, as relaxation time is directly linked to an effective length scale, given the diffuse nature of the SIP phenomenon in soils and sediments. An inversion algorithm proved capable of correctly recovering the input relaxation time distributions from the SIP data. Based on this procedure, the next step will be to convert the relaxation time distribution into a pore size distribution according to the space-time inter-dependence for diffusive processes and then estimate hydraulic conductivity from the obtained pore size distribution. Such an overall approach will establish a direct link between measured SIP data and soil hydraulic conductivity and thus represents an important contribution to the target GPTFs. Other physical characteristics of soils, such as magnetic and gamma emission properties, could not be described within the given theoretical framework as the controlling parameters are obviously not correlated with the ones controlling the (wide sense) electrical response. The GPTFs have been tested and verified against data from the iSOIL test sites and on other datasets made available by external collaborators. The overall result is satisfying, opening also new pathways of research aiming at a fully-comprehensive construction of GPTFs that could also consider non-electrical properties of soils.

Further Information

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Important Information

Project ended in November 2011

CWA 16373 "Best Practice Approach for electromagnetic induction measurements of the near surface" is published and available at CEN or DIN Berlin

iSOIL data are available at http://eusoils.jrc.ec.europa.eu/projects/isoil/data.html

iSOIL is a member of the SOIL TECHNOLOGY CLUSTER of Research Projects funded by the EC

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FP7