GNGTS 2014 - Atti del 33° Convegno Nazionale

Membrane polarization by constrictivity of pores: its effects on DC and TEM geo-electromagnetic measurements V. Hallbauer-Zadorozhnaya 1 , G. Santarato 2 , N. Abu Zeid 2 , S. Bignardi 2 1 Council for Geoscience, South Africa 2 University of Ferrara, Italy Introduction. It is well known that the induced polarization (IP) phenomenon is due to at least four known mechanisms, i.e. the so-called electrode polarization, electro-osmosis polarization, Maxwell-Wagner effect, although very weak, and membrane polarization due to presence of clay minerals in rock pores and/or by constrictivity, i.e. the polarization effect is due to change in pore size (Schön, 1996). Of course freshwater saturated pores is essential for the occurrence of the IP phenomena, on the contrary, increased concentration of dissolved salts affects negatively membrane polarization until its complete nullification at elevated concentrations. Ingeneral, traditional practice for IPdatacollection isdonebycommercial geo-resistivitymeters while acquiring subsurface resistance data, for example by means of the Electrical Resistivity Tomography (ERT: Barker, 1989) technique. In this case, the IP information is measured in Time- domain by observing the IP decay curve over one or several time windows after current switch- off and the measured quantity determines the value of the chargeability (M). The chargeability M provides additional independent information, which in many circumstances helps to better understand the subsurface. For instance, it is used, since its discovery, for metallic minerals explorations, nowadays, the method is used for solving environmental issues related to the presence of pollutants in the subsurface, both of inorganic (e.g.Abu-Zeid and Santarato, 2004) and of organic origin (see e.g. the review paper byAtekwana and Atekwana, 2010). Throughout the seventies of the past century, together with the increasing use of the Transient (or Time-Domain) Electro-Magnetic (TEM or TDEM) technique, also, for mineral exploration, geophysicists become aware of the fact that IPmay affect the observed decay curve observed after current switch-off of the inducing magnetic field (Sidorov and Yakhin, 1978; Walker and Kawasaki, 1988). Their presence was dealt with by developing specific algorithms being implemented in computer codes to both model this behavior (Ingeman-Nielsen and Baumgartner, 2006) and consequently to remove it (Antonov and Shein, 2008). Nevertheless, geo-electromagnetic community agrees that the IP phenomenon is “linear”, i.e. not dependent neither on intensity nor on charging time length. Recently,whileconducting intensive resistivityandchargeabilitymeasurementsonfreshwater saturated samples of different rock types, Zadorozhnaya, discovered that both chargeability and resistivity depend on time and intensity of the polarizing direct current (Zadorozhnaya, 2008; Zadorozhnaya and Maré, 2011), although at values that are generally above charging times and current densities which are commonly used in the field. In particular, she observed that chargeability always decreases with increasing current intensity, while resistivity can assume both behaviors. The effect of increasing time of current feeding increased both resistivity and chargeability values. The observed non-linearity of chargeability increase with time is known to be a common occurrence and can be easily understood; however, the remaining non-linear phenomena were quite surprising. That IP could be, under specific conditions, a non-linear phenomenon due to current increase, was a circumstance of which, in early times of the method, scientists were aware of (Bleil, 1953), although the reasons were not investigated further. Few decades later, one of the authors of this paper was involved in laboratory IP measurements on samples of loose sediments (Iliceto et al. , 1982), where it was claimed that all the measurements were performed in the range of supplied currents that ensured the linear behavior. To our knowledge, nobody has published field observations related to the observed remaining non-linear phenomena, in particular, on the dependence of resistivity on charging time and 158 GNGTS 2014 S essione 3.2