Comparison of GPR and Capacitance Probe laboratory experiments in sandy soils

The integration of different techniques for the estimation of the volumetric water content θ in low-loss sandy soils may allow to obtain more reliable measure, after a proper evaluation of the techniques limits and their pros and cons. In particular, the integration of direct laboratory measurements performed on samples $\theta$ values measured) with geophysical data collected on a soil column using a Ground Penetrating Radar (GPR) as well as a Capacitance Probe (CP), allowed us to compare the results and evaluate their accuracy. Our experimental measures, performed on two typical sandy soil outcropping in Central Italy, show that the GPR reflected pulses provide similar permittivity $(\varepsilon_{\mathrm{r}})$ values for both soils at very low θ. The measured $\varepsilon_{r}$ values seem to progressively differ by increasing the soil moisture of the two sands. The CP shows a clear difference of measured permittivity already at lower soil moisture. As θ values in the media increase approaching the soil saturation, the CP $\varepsilon_{r}$ values measured on both the two soils show a larger difference. In conclusion, the comparison between GPR and CP measures in two selected sands under controlled condition $\pmb{(0.05\ < \theta < 0.3)}$, shows that the latter tends to overestimate $\varepsilon_{\mathrm{r}}$ on the entire range investigated. Nevertheless, if a specific laboratory calibration is carried out, as in the present work, reliable $\theta$ values estimations can be obtained by both methods. Other measurement techniques will be tested and compared in further experiments; moreover, the calibration and integration of GPR and CP is advised not only in laboratory studies, but also to better constrain possible field applications.