Induced polarization of volcanic rocks. 9. Anatomy of a rising thermal plume

SUMMARY The dynamic Stern layer petrophysical model can be used to interpret field induced polarization data and can be applied to both magmatic (volcanic and igneous) rocks and sediments. Thanks to it, field-scale tomograms of conductivity and normalized chargeability can be transformed into tomograms of porosity, Cation Exchange Capacity (CEC) and temperature. Furthermore, kilometer-scale galvanometric induced-polarization surveys are nowadays doable thanks to the recent development of independent stations measuring the primary and secondary electrical fields. This approach reduces capacitive and inductive coupling effects inherent to systems based on long cables and allow for deeper investigations. We apply here this combined methodology (novel equipment and revised petrophysical model) to a geothermal prospect located at Mashyuza, Republic of Rwanda, in Eastern Africa. At this site, the rifting activity led to the occurrence of an extensional regime favouring the occurrence of a rising thermal plume at the intersection between two faults. The existence of this plume is expressed at the ground surface by the presence of a hot spring at a temperature of ∼52 °C (a well nearby provides a temperature of ∼65 °C). A time-domain large-scale induced polarization survey is performed. The current source signal is produced by a VIP-5000 squared signal injector and injection current values range from 1 to 4 A, with stacking of 1 s on–off signals ranging from 100 to 300 stacks to improve the signal-to-noise ratio. The size of the 3-D array is ∼1.5 km and allows a tomography down to a depth of ∼300 m. The data are inverted with the deterministic least-square technique, penalizing the roughness of the resulting tomograms. The conductivity and normalized chargeability tomograms are combined to get the temperature, porosity and CEC distributions. The temperature distribution is consistent with the temperature of the hot spring and well. The results are interpreted in terms of ground water flow pattern and dilution of the mineralized thermal water with the fresher surface meteoric water. The survey images a rising plume of warm water from a depth of at least 300 m along intersecting fracture systems.