Electrical conductivity and induced polarization investigations at Krafla volcano, Iceland

Abstract

Electrical conductivity and polarization properties of 6 samples from Krafla volcano (Iceland) were measured in the frequency range 1 mHz–45 kHz and compared to the data obtained on various basaltic rock samples from Hawaii. The results indicate that for altered samples, the surface conductivity, normalized chargeability, and quadrature conductivity of the core samples scales linearly with the cation exchange capacity, taken as a proxy of the alteration facies. The surface conductivity of fresh samples is also controlled by the cation exchange capacity but their normalized chargeability is influenced by the presence of magnetite, especially for unaltered samples. The temperature dependence of quadrature conductivity and normalized chargeability can be modeled with an Arrhenius equation with an activation energy of 16–19 kJ mol−1. The experimental results agree with a model in which the polarization of the metallic and non-metallic grains are both considered in a unified framework. These results are used to interpret two 3D induced polarization surveys performed in the South and East parts of Krafla volcano using two 1.3 km-long cables with 32 electrodes each. The electrical conductivity is in the range 0.3 (clay cap) to 5 × 10−5 S m−1 (unaltered rock) while the normalized chargeability is typically comprised between 10−2 (clay cap) and 10−5 S m−1 (unaltered rock). Induced polarization is used to image porosity and the cation exchange capacity. A long 5.6 km electrical conductivity profile was also performed connecting the two 3D sites and crossing a rhyolitic obsidian ridge called Hrafntinnuhryggur. Hrafntinnuhryggur is characterized by very low conductivity values on the order of 10−4 S m−1. The long conductivity profile shows the position of the inner and outer caldera rims and the feeder dike of Hrafntinnuhryggur. A self-potential survey performed along this long profile shows no shallow active geothermal features in this area, as expected from the low permeability of the clay cap.