Induced polarization of volcanic rocks. 5. Imaging the temperature field of shield volcanoes

International audienceKilauea is an active shield volcano located in Hawai'i. An induced polarization survey was performed in 2015 at the scale of the caldera. The data were acquired with a 2.5 km cable with 64 electrodes and a spacing of 40 m between the electrodes. A total of 6210 measurements were performed. The apparent chargeability data were inverted using a least square technique to obtain a chargeability tomogram. The normalized chargeability tomogram is obtained by multiplying cell-by-cell the chargeability by the conductivity. Once the conductivity and normalized chargeability tomograms are obtained, they are jointly interpreted using a dynamic Stern layer conduction/polarization model, which explains the low-frequency polarization spectra of volcanic rocks. This conductivity/polarization model is tested here on new laboratory experiments performed on 24 samples from a drill-hole located on the Kilauea East Rift Zone (Hole SOH-2). We show that for Kilauea, the ratio between the normalized chargeability and the conductivity is equal to a dimensionless number R = 0.10 ± 0.02 proving that the conductivity and the normalized chargeability are both controlled by the alteration products of the volcanic rocks with a minor role of magnetite except close to the ground surface. In turn, the degree of alteration is controlled by temperature and therefore normalized chargeability and electrical conductivity can both be used as a non-intrusive temperature sensor. This approach is then applied to the field data. Meaningful temperature tomograms can be produced from both electrical conductivity and normalized chargeability tomograms