Induced polarization images alteration in stratovolcanoes

We investigate the complex conductivity of 32 volcanic rock samples from two stratovolcanoes, La Soufrière Volcano (Guadeloupe Island, Caribbean) and Papandayan volcano (Java Island, Indonesia). These stratovolcanoes are characterized by high degrees of kaolinite-related alteration associated with the upwelling of acidic ground waters as well as the formation of smectite-rich clay caps. Our goal is to assess the dependence of two geoelectrical properties, the electrical conductivity and normalized chargeability, on the conductivity of the pore water and the Cation Exchange Capacity (CEC) for volcanic rocks at near-neutral pH conditions. An alteration index based on CEC is built for both smectite- and kaolinite-rich zones. The data are discussed in the context of a previously acquired experimental dataset based on samples collected from shield volcanoes in Hawaii. We show that all the core samples display the same trends in their petrophysical geoelectrical properties whatever the type of volcanoes. Surface conductivity and normalized chargeability are strongly controlled by the CEC of the material and the bulk tortuosity of the pore space (product of the formation factor by the porosity). This implies in turn that the conductivity and the normalized chargeability are controlled by rock alteration in the same way as long as surface conductivity dominates the bulk conductivity contribution, which is salinity and pH dependent (both being interrelated in acidic pore waters). These petrophysical results are then applied to the interpretation of a 3D induced polarization survey performed at Papandayan stratovolcano in Indonesia. The tomograms are used to image the alteration of the volcanic edifice and we conclude that surface conduction is non-negligible in these volcanoes. Only above a pore water conductivity higher than 10 S m−1 at 25 °C) the bulk conductivity dominates the surface conductivity associated with alteration.