Field evidence for non-linear IP effects in a volcanic hydrothermal system using reciprocal data.

Lore Vanhooren , Wouter Deleersnyder, Warre Dekoninck, Adrian Flores Orozco, Frédéric Nguyen, Corentin Caudron, Thomas Hermans 

In electrical geophysical methods it is often assumed that the resistivity or chargeability, behaves linearly with the applied electrical current. However, non-linearity has been reported in Induced Polarization (IP) and can be caused by oxidation-reduction reactions or reactions at the surface of clay minerals. Methods to determine the presence of non-linear effects exist and involve analysis of the harmonic distortion of spectral IP data. Measurement of reciprocal data is standard practice in many ERT surveys in the context of data filtering and error quantification. When assuming a system behaves linearly, the normal and reciprocal measurements should yield the same distribution of values. Consequently, a systematic misfit between the two can indicate non-linearity. Here, we present the case of time domain IP (TDIP) data acquired in a volcanic hydrothermal system (VHS), with a strong discrepancy in both observed data and inversion of normal and reciprocal data, presented as a positive shift of the reciprocal decay curves (>100 mV/V). The inversion shows a strong discrepancy in the imaginary part, of 20 mS/m, localized on the southern part of the profile. In the complex environment of a VHS we interpret the observed non-linear IP effect as the result of oxidation-reduction reactions at the interface of iron oxides or sulfides. Our study suggests that analysis of normal-reciprocal misfit
can be further used as an interpretation tool for the presence of non-linear effects.