13:45 〜 15:15
[SCG52-P06] Modelling a pair of positive and negative self-potential anomalies observed at the Oomuro-dashi hydrothermal field, Izu-Ogasawara arc, southeast Japan
キーワード：海底自然電位、海底熱水系、火山活動、物理探査
We have observed a pair of positive and negative self-potential anomalies in a hydrothermal field associated with a quarterly rhyolitic volcano, Oomuro-dashi, Izu-Ogasawara arc, south of Japan [Kawada and Kasaya, JpGU2019, SCG56-P14]. The amplitude is weak, on the order of one millivolt 5 m above the seafloor. The observation of a positive self-potential is a new finding, although negative self-potential anomalies have been reported frequently in various hydrothermal fields. In addition, long-term monitoring of sub-seafloor temperatures suggest the downward flow of several meters per year in the hydrothermal field. These observations may enable us to image the hydrogeological structure below the self-potential anomaly.
We model the results of the self-potential observation in several ways. First, we solve an inverse problem to explain the self-potential anomaly as a combination of buried electrical current sources. Positive and negative electric current sources are obtained ~30 m below the observed positive and negative self-potential anomalies, respectively. Secondly, we model electrochemical (geo-battery) processes around a buried conductive body that crosses a redox contrast. The reduction that occurs on an oxidative part results in a negative self-potential, while the oxidation on a reductive part results in a positive self-potential. Thus, the observed positive/negative pair may arise when a conductive body is lying down and crosses a non-horizontal (or sub-vertical) redox boundary. In contrast, when a conductor crosses a horizontal redox boundary that becomes more reductive at deeper depths, only a negative self-potential emerges, which corresponds to the common observations. The laying down geo-battery model is the most plausible in the current state.
Additionally, we model the streaming and thermal potentials, which respectively arise when the fluid flow and temperature anomaly exist. The streaming potential is known to appear at geological boundaries including the seafloor. However, the coupling coefficient is small in seawater environments with a large electrical conductivity, and the resulting self-potential cannot exceed 1 millivolt at the seafloor level. The coupling coefficient of the thermal potential has not well understood, especially in marine environments. It might contribute to a significant part, and future studies are desired.
We model the results of the self-potential observation in several ways. First, we solve an inverse problem to explain the self-potential anomaly as a combination of buried electrical current sources. Positive and negative electric current sources are obtained ~30 m below the observed positive and negative self-potential anomalies, respectively. Secondly, we model electrochemical (geo-battery) processes around a buried conductive body that crosses a redox contrast. The reduction that occurs on an oxidative part results in a negative self-potential, while the oxidation on a reductive part results in a positive self-potential. Thus, the observed positive/negative pair may arise when a conductive body is lying down and crosses a non-horizontal (or sub-vertical) redox boundary. In contrast, when a conductor crosses a horizontal redox boundary that becomes more reductive at deeper depths, only a negative self-potential emerges, which corresponds to the common observations. The laying down geo-battery model is the most plausible in the current state.
Additionally, we model the streaming and thermal potentials, which respectively arise when the fluid flow and temperature anomaly exist. The streaming potential is known to appear at geological boundaries including the seafloor. However, the coupling coefficient is small in seawater environments with a large electrical conductivity, and the resulting self-potential cannot exceed 1 millivolt at the seafloor level. The coupling coefficient of the thermal potential has not well understood, especially in marine environments. It might contribute to a significant part, and future studies are desired.