A number of existing geophysical tools for soil characterization have been tested and new tools developed within the project to study pollution in groundwater and soil and to collect relevant hydrogeological data from specific sites. In particular, the project team is striving to relate soil pollutants to certain geophysical parameters such as the electrical conductivity.
The Polares device is a field instrument using damage-reducing techniques for measuring the soil conductivity and polarization that has been developed within the project framework.
The new field instrument is based on the measurements of complex electrical conductivity to perform indirect measurements of the soil the conductivity and induced polarisation (IP) response in the frequency range between 0.1 Hz up to about 120 Hz. The induced polarisation effect at different frequency is observed by estimate the phase shift of the voltage signal with respect to a sinusoidal current signal. The ground acts as a capacitor, and the phase shift at different frequency is measured and used for identifying the nature of the subsurface. The IP response is different in clean soil than in contaminated, as the signal is usually stronger in the polluted soil. This instrument is able to control more than 100 hundred electrodes and is designed to work in borehole-configuration for in-hole or cross-hole measurements, on surface. Thus, the field instrument provides a robot link between geophysical parameters and pollutants, used to observe the presence of pollutants and enabling time-lapse monitoring of contaminants in the subsoil. The conventional way of conducting this kind of analysis is sampling the soil and performing laboratory analysis, a time-consuming and complex activity. Also, sampling can only provide point measurements. Cross-borehole applications of geophysical techniques such as ground penetrating radar and electrical conductivity have been employed over a number of years, but are still not used routinely, partly due to time constraints of measurements collection and also need for expert knowledge. The innovative aspect of the instrument developed here is the possibility to make observations at high frequencies; this provides double advantage of the instruments: data set involving thousands of observations can be collected in few minutes; spectral analysis of data (from lower to upper frequency) gives a better potential to detect soil pollution.
The project team has also developed new geophysical software for 2D and 3D data processing and new strategies for managing geophysical data.
The pollutants are related to presence of some organic compounds. The project team observed the concentration of some hydrocarbons present in soils at contaminated sites, collected electrical conductivity data from the sites and connected them with pollutant concentrations. Thus, the team has found empirical relations between the presence of hydrocarbons and the electrical conductivity. To validate these findings and to establish the correct relations, data analysing activities are now undertaken. As chemicals are not the single factor that can affect polarizability, this is a complex task. One of the solutions to this task is a combined analysis of the conductivity and IP effects observed at different frequencies of the electrical fields; the conductivity is mostly related to the soil porosity and fluid distribution, while IP is related to complex interaction between the soil matrix and the fluids in the pore volume. This means that the geophysical parameter (complex conductivity) in principle is sensitive to the degradation activity of the organic matter.
A prototype of the field instrument has been developed and has been tested in the field at several test sites, for instance in Italy and Norway. Thus, the technology readiness level of the instrument reaches a level 9 on the TRL scale.
Another development in the project is a new generation remotely controlled instrument for fast acquisition of subsoil electrical properties. The instrument was developed by IRIS instruments, a partner of the project, on the basis of their geo-resistivimeter Syscal-pro. This instrument is aimed towards sites where time-lapse measurements of electrical conductivity/chargeability and self-potential can be used for monitoring of changes in contaminant situation.