GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 3.3 723 ions and the formation of a membrane potential. There are several mechanistic models that consider this polarization as the main out-of-phase response (e.g. Titov et al. , 2002); 3. ���������� ������ ����� ����� ������������� ���� ������������ ��������� ��� ���� electrical double layer (EDL) polarization. This polarization mechanism has been considered only in the recent years, and its modelling seems to be able to identify textural and hydraulic properties of porous materials (De Lima and Sharma, 1992; Revil and Florsch, 2010). These models are based on the theories of Schurr (1964) and Schwarz (1962). This type of polarization is mainly responsible for the low frequency complex signals (below 10 Hz). This paper aims to analyze a mechanistic model, derived from Brovelli and Cassiani (2011), for induced polarization in frequency domain, that incorporates both grain and membrane polarization (Fig. 1) and uses a non-linear mixing rule to combine the (complex) contribution of the three phases (water, air and solid grains). The model, here referred as HAS-IP is tested on three different lab experimental datasets. Methodology and discussion. The mechanistic model HAS-IP has been applied on different experimental data acquired on saturated sand samples (Tab. 1). A brine was prepared from deionized water and NaCl salt at different salinities: 50, 100 and 108 mS/m. The samples were packed at water saturation, obtaining a mean porosity of 35%. The acquisitions were performed at a stable temperature of 20-21°. Tab. 1 - Sand properties. Sand type Porosity D50 (mm) Geometric Std Dev Sw Quartz (%) Natural A 0.35 0.340 3 1 70.3 Standard 0.35 0.065 3 1 99.9 Blaster coarse 0.35 0.185 4.5 1 75.0 The sample holders used in the experiment follow the general geometric recommendations for SIP measurements, i.e. the current electrodes are stainless steel plates as large as the section of the sample; the potential electrodes are placed externally from the current path (at least 1 cm apart). The instrument for SIP acquisition is a ZEL-SIP04 impedance meter from the Forschungszentrum Juelich and we used a frequency range of 0.01-10000 Hz (Fig. 2). Fig. 1 - Example of predictions obtained using the HSA-IPmodel. The solid black line shows the convoluted response, in terms of bulk resistivity (left panel) and phase shift (right panel). The red dashed line shows the model results when only the Stern layer polarization is considered, while the blue dashed line is relevant to the case with membrane polarization only. For this example, porosity was set to 0.39, the cementation factor to 1.85 and the D50 of the grain size distribution to 0.16 mm.