Review on morphological insights of self-potential anomalies on volcanoes

Energy and thermal transfers in active volcanoes can play an important role in controlling their dynamics depending on the hydrothermal state. Much geothermal energy is released through the groundwater circulation, hot gas emission and thermal conduction. Therefore, it is very important to know the hydrological and thermal environments associated with volcanoes from the volcano-energetic point of view. However, it is difficult to evaluate these because of the availability of only a few borehole data on the summit of volcanoes. Recent studies reveal that self-potential (SP) anomalies ( up to some hundreds of mV) are observed on volcanoes, active fissure zones and/or fumarolic areas, suggesting that the SP anomalies are closely related to heat-triggered phenomena such as thermoelectric and electrokinetic effects due to hydrothermal circulations. Therefore, SP studies can be appropriate for sensing the thermal and hydrothermal states of volcanoes. In addition, monitoring SP anomalies can be an efficient method for describing the change of thermal state and the evolution of the hydrothermal ( and volcanic) activities. In this paper, we have reviewed the origin of the SP anomalies associated with volcanic phenomena theoretically as well as experimentally. Subsequently, we have presented the results of many case studies and have classified the types of anomalies in accordance with possible mechanisms. We have also described the results of time variations of SP anomalies associated with volcanic activities. Time varying SP fields exhibit the dynamic aspects of volcanic activities corresponding to the evolution of hydrothermal activity, changes in ground water circulation and magma displacement. These morphological insights should lead to a quantitative interpretation of SP anomalies in volcanic regions.