A case study of resistivity and self-potential signatures of hydrothermal instabilities, Inferno Crater Lake, Waimangu, New Zealand

Abstract : Inferno Crater Lake, Waimangu, one of the largest hot springs in New Zealand, displays vigorous cyclic behavior in lake level and temperature. It provides a natural small-scale laboratory for investigating the geo-electrical signature of fluid flows. We measured self-potential and electrical resistivity to see whether the huge variations of fluid volume, approximately 60,000 m3 during a mean cycle period of 40 days, could be detected. Electrical resistivity measurements revealed spectacular changes over time, with the medium becoming more conductive as the lake receded. This result is consistent with analog models, where the vapor phase is replaced by liquid at recession. The self-potential survey did not detect temporal changes related to fluid movements. This can be explained by the pH of the pore water (∼2.3), which is close to the point of zero charge of silica.