Résumé
Electrical Resistivity Tomography (ERT) and Induced Polarization (IP) are two complementary methods of subsurface investigation, as they map two types of electrical properties: the electrical conduction and the electrical charge storage capabilities of rocks (polarization). These two electrical properties are particularly sensitive to the presence of three types of minerals: smectite, sulfides and iron-oxides. Smectite and sulfides being two subsurface witnesses of active hydrothermal circulations, detecting the presence of these minerals is of interest for geothermal exploration. The complex electrical conductivity of 88 brine-saturated cores samples from four boreholes in the Krafla high-temperature geothermal field (Northeast Iceland) was measured in the frequency range 100 mHz-1 MHz. These basaltic samples have variable amount of smectite (0-50 wt.%), pyrite (0-15 wt.%) and iron oxides (0-5 wt.%). Field measurements of ERT and Time-Domain IP (TDIP) were carried out along two sets of profiles, located around two of the boreholes. The measured DC resistivity and voltage decay were inverted with the software AarhusInv, taking into account the full transmitter waveform in order to resolve the spectral IP parameters from TDIP data. Inversions, logs and laboratory DC resistivity are consistent around the borehole with low and constant temperature (10 degrees C down to 300 m) and indicate a smectite-rich zone sandwiched between two crystalline layers. Around the borehole with a strong temperature gradient (180 degrees C at 200 m), field and laboratory resistivity show large deviations, interpreted as the consequence of hot water convection and thus preventing any mineralogy interpretation. Based on IP laboratory measurements, the presence of iron-oxides, associated to crystalline rocks with little alteration, corresponds to peak frequencies higher than 100 Hz. Pyrite-rich samples, on the other hand, exhibit relaxation frequencies between 0.1 Hz and 1 kHz and maximum phase angle between 20 and 180 mrad. The presence of pyrite is often associated with altered, smectite-rich rocks. Patches of high-maximum phase angles and low peak frequencies, consistent with laboratory values, are observed on 2D inversions of field TDIP data and interpreted as pyrite-rich zones.