Observed Linear and Non-linear IP Effects - A Summary of Joint Italy-South Africa Bilateral Projects 2007-2014

Abstract

Among the several types of induced polarization effects, electro-osmosis and membrane polarization by
constrictivity of pores my result in non-linear behaviour, i.e. chargeability is not constant with supplied dc
current. A new model, based on the second Fick’s equation succeeded in predicting, on laboratory samples,
this behaviour. In fact, The non-linear behaviour was shown to correlate well with Pore Size Distribution
(PSD) of laboratory tested samples whose PSD distribution was measured experimentally using a mercury
injection capillary pressure test method. The model predicts also the non-linearity of the electrical
resistivity for sufficiently high current densities. In this paper, we show that non-linear behaviour of
resistivity and chargeability vs. supplied current density can be met in field measurements, provided they
are carefully planned. In a site located in Northern Apennines, where a saturated sandstone outcrops,
different values of resistivity and chargeability were obtained at different supplied electric current whose
achieved results agree with the forward model. Moreover, the non-linearity can also perturb TEM data as
being affected by this IP effect, as the model predicts a dependence of non-linearity also on the current
supply length. An example of anomalous TEM data collected in Northern Apennines is also presented and
discussed.