ABSTRACT
Geoelectrical methods have been used extensively to study the structure of volcanoes. They provide an image of the internal form as a distribution of resistivities which must be interpreted in terms of volcanic layers and bodies. Because the methods used for volcano monitoring differ little from those used for more classical geoelectrical surveys in geophysics, only the general principles and procedures will be described, unless an experiment requires a unusual methodology. Volcanic rocks exhibit a wide range of porosities, from low-porosity dense lava flows or intrusions to high-porosity vesiculated lava flows or poorly compacted pyroclastic deposits. Direct-current resistivity techniques have been employed in geoelectrical surveys for decades, and exhaustive descriptions of these methods may be found, therefore, in many geophysics textbooks. Apparent resistivity monitoring has been carried out at Izu-Oshima, an active basaltic volcano with a caldera and a nested central cone crowned by a summit crater, since 1975 using an axial dipole-dipole arrangement.
