## 4 Discussion
The findings confirm the applicability of the tool to complex systems, as already proposed by other authors [16; 17; 18; 19; 20; 21] in similar studies and with similar conditions. The second challenging issue was to check the reliability of fast and accurate technology to monitor the process itself. In a previous study, tests using a time domain reflectometer (TDR) were adopted to measure the dielectric properties of the samples. In those cases, even if the approach seemed promising, some inaccuracies made the results ambiguous in terms of the relationship between the biomass activities and the observed geophysical parameters. The open-ended coaxial cable could, in theory, overcome some of the previous problems, as discussed by [PERSON] et al. [32], who carried out a detailed comparison of TDR and open-ended coaxial probes applied to non-contaminated agricultural soils. The two kinds of probes gave highly correlated results in determining the real part of permittivity at frequencies around 1 GHz. However, the open-ended coaxial cable investigated the frequency-dependent imaginary part with much more detail. It provided a wide frequency spectrum of analysis, in which different polarization phenomena influenced the complex dielectric permittivity. We did not find in the literature the use of open-ended coaxial cables in monitoring of bioremediation of contaminated soils, while these probes are commonly used in other application fields to evaluate biological substances or tissues [33]. In fact, the use of an open-ended coaxial cable to monitor the effects of biodegradation on dielectric properties of soil gave interesting results, because of the capability to monitor both the real and the imaginary part in a wide frequency range. Variations in diesel oil content can significantly affect the real part of the dielectric permittivity of a soil matrix, as also observed by [PERSON] with TDR [34], and by [PERSON] with an open-ended coaxial cable [25] in non-bioremediated contaminated soils. Instead, the variations in the imaginary component are related to progressive changes in the pore water chemistry due to biological phenomena. The latter are observed especially at low frequencies (<1 GHz): after 130 days of bioremediation, the imaginary component at 200 MHz decreased to near-zero values, indicating that the conductivity of pore water decreased during the process. Fluctuations in pore water conductivity of a soil under bioremediation were also observed in [PERSON] et al. [35]. The use of an open-ended coaxial cable is a fast method to estimate both electrical permittivity and conductivity. The application of this technology to soil bioremediation is a new research field, and these initial experiments allowed us to focus on different phenomena of the degradation process, such as hydrocarbon depletion or salt depletion. However, the correlation between biological findings and geophysical ones remains a challenge for researchers. Conceptualization, F.C. and A.G.; methodology, F.C., A.G.; software, C.M.R. ; validation, C.M.R., F.C. and A.V. ; formal analysis, F.B. ; investigation, C.M.R. and A.V. ; resources, B.R. ; writing--original draft preparation, C.M.R., F.C. and A.V. ; writing--review and editing, C.M.R., F.C. and A.G.; supervision, F.C. and A.G.; project administration, F.C. ; funding acquisition, F.C. All authors have read and agreed to the published version of the manuscript. This research was funded by the project \"GEOPHYSICAL METHODS TO MONITOR SOIL BIOREMEDIATION\" funded by the Italian Ministry of Foreign Affairs and International Cooperation in the frame of the Executive Programme of Scientific and Technological Cooperation between the Republic of India and the Italian Republic for the years 2017-2019--SIGNIFICANT RESEARCH. The authors are personally grateful to [PERSON] for the useful discussions and kind support during the gas-chromatographic activity. They also thank [PERSON] and [PERSON] of Department of Electronics and Telecommunications for their contribution to open-ended coaxial cable measurements.