| Summary: | The electrical signature of volcanoes is affected by several characteristics of rocks: volume and salinity of pore fluid, abundance of conductive minerals, rock temperature and presence of molten crust (magma). Electromagnetic soundings are widely used to image the underground structure of volcanoes and look for high-temperature geothermal resources. However, the relative contribution of the above-mentioned elements to the measured resistivity is often an unsolved question when interpreting resistivity inversions. This thesis aims at improving the interpretation of electrical resistivity structures at active volcanoes, in order to develop innovative tools for the assessment of high-enthalpy geothermal resources. Focus is on conductive minerals, which can either be solid ionic conductors (clay minerals, in particular smectite) or electronic semi-conductors (pyrite and iron-oxides). Also investigated are the effects of porosity, salinity, temperature and melting. Iron-oxides are mostly formed during the primary crystallization of magma, while smectite and pyrite are formed upon hydrothermal alteration of volcanic rocks, thereby witnessing hydrothermal convections. Krafla volcano, Iceland, is used as a laboratory area, where extensive literature, borehole data, core samples, surface soundings and infrastructures are available. The contribution of smectite to the electrical conductivity of volcanic samples saturated with pore water at different salinity is first investigated in the laboratory (room temperature) by electrical impedance spectroscopy, also called complex resistivity. Non-linear variations of the conductivity at 1 kHz with salinity are observed and interfoliar conduction is suggested as an important mechanism by which smectite conducts electrical current. The influence of pyrite and iron-oxides on the charge-storage (capacitive) properties of the rock is then analyzed, using the frequency-dependent phase-angle of the impedance. Maximum phase-angle (MPA) higher than 20 mrad can be attributed to pyrite if ...
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