GNGTS 2017 - 36° Convegno Nazionale

724 GNGTS 2017 S essione 3.3 The spectra were filtered by removing the high frequency components with a fitting procedure of a triple Cole-Cole model (Kemna, 2000). Fig. 1 shows an example of the prediction of the HAS-IP model, where both membrane polarizations and EDL polarization are integrated in a total complex resistivity response. This example is modelled assuming parameters that are common for a fine sand. The Stern polarization occurs at lower frequencies, while the membrane polarization has a stronger effect at higher frequencies. In Fig. 2 the spectra of measurements on 3 different sand types are plotted together with the modelled spectra. In two cases (sand types “Standard” and “Blaster coarse”), there are experimental spectra at 2 different water conductivities (50 and 100 mS/m). For the latter data, the HAS-IP model have been inverted using a multi-object function, taking into account both water conductivities, since the petrophysical parameters are the same for both measurements, but water conductivity. The HAS-IP model describes the SIP response with a high accuracy (Fig. 2), reproducing the same shape of the experimental curves, i.e. accurate forecast of magnitude and phase pick. According to these preliminary results, it seems that the membrane polarization is less relevant than the Stern layer polarization for this specific type of fine sands. Conclusions. The petrophysical parameters used are coherent with literature values, even if the fitting of the model is complicated and the parameters could be correlated. Further studies about parameter correlations should be done. The use of the same model parameters for fitting different water conductivities is a key point to better constrain the model and do not fit noisy data. The filtering of the high frequencies spectra through a Cole-Cole model is a delicate procedure that can easily remove part of the signal of the membrane polarization. Archie’s law controls mainly the resistivity, while sand petrophysical parameters controls the phase response. Fig. 2 - Application of the model on fully saturated conditions. In the plot are presented 3 different types of sand (Tab. 1). The top panel shows spectra only with a water conductivity of 108 mS/m; while the other two panels display SIP data with 2 different water conductivities (50 and 100 mS/m).