Electrical properties of hydrothermally altered rocks : observations and interpretations based on laboratory, field and borehole studies at Krafla volcano, Iceland.

• Main Author: Lévy, Léa
• Other Authors: Laboratoire de géologie de l'ENS (LGENS), Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Département des Géosciences - ENS Paris, École normale supérieure - Paris (ENS-PSL), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-École normale supérieure - Paris (ENS-PSL), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL), Université Paris sciences et lettres, Háskóli Islands, Pierre Briole, Freysteinn Sigmundsson
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2019
• Subjects: Geothermal energy; Electrical conductivity; Smectite; Pyrite; Induced polarization; Geophysical inversion; Géothermie; Conductivité électrique; Polarisation provoquée; Inversion géophysique; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; Krafla; Iceland; Islande
• Online Access: https://tel.archives-ouvertes.fr/tel-02179415; https://tel.archives-ouvertes.fr/tel-02179415/document; https://tel.archives-ouvertes.fr/tel-02179415/file/Levy-2019-These.pdf

Electromagnetic soundings are widely used to image the underground structure of volcanoes and look for hightemperature geothermal resources. The electrical resistivity of volcanoes is affected by several characteristics of rocks: volume and salinity of pore fluid, abundance of conductive minerals, rock temperature and presence of magma. This thesis aims at improving the interpretation of electrical resistivity structures around active volcanoes, in order to develop innovative tools for the assessment of geothermal resources. I focus on conductive minerals, which can either be solid ionic conductors (clay minerals, in particular smectite) or electronic semi-conductors (pyrite and iron-oxides), but I also investigate the effects of porosity, salinity, temperature and presence of magma. I use Krafla volcano as a laboratory area, where extensive literature, borehole data, core samples, surface soundings and infrastructures are available. Smectite and pyrite are formed upon hydrothermal alteration of volcanic rocks and thus witness hydrothermal convection. On the other hand, iron-oxides are mostly formed during the primary crystallization of magma and dissolved by hydrothermal fluids. The contribution of smectite to the electrical conductivity of volcanic rocks saturated with pore water at different salinity is first investigated in the laboratory (room temperature) by electrical impedance spectroscopy “complex resistivity”. Non-linear variations of the conductivity at 1 kHz with salinity are observed and discussed. Interfoliar conduction is suggested as an important mechanism by which smectite conducts electrical current. The influence of pyrite and iron-oxides on induced polarization effects is then analyzed, using the frequency-dependent phase-angle of the impedance. A maximum phase-angle higher than 20 mrad is attributed to pyrite if the rock is conductive and to ironoxides if the rock is resistive. The maximum phase-angle increases by about 22 mrad for each additional per cent of pyrite or iron-oxide. These ...