Self-potential and passive seismic monitoring of hydrothermal activity: A case study at Iodine Pool, Waimangu geothermal valley, New Zealand

Iodine geyser, located in the Waimangu Geothermal Valley (New Zealand), has been studied by both passive electrical and seismic methods. The activity of the geyser was monitored at various distances from the vent using self-potential method. The self-potential signals display cyclic negative variations with respect to a baseline drawn when the geyser is quiet. The minimum in the self-potential signals coincides with the maximum overflow. We provide a numerical model able to explain both the polarity and magnitude of the observed signal. This model is based on the fluctuations of the hydraulic head in the conduit of the geyser; the divergence of the streaming current density is created at the interface between the pipe and the surrounding rock. Passive seismic experiments were used to localize ambient noise sources. These signals have been processed with the so-called Matched-Field Processing technique (MFP); a dominant source emerged from this processing, that we characterized in range and depth with a good accuracy.