9:30 AM - 9:45 AM
[SEM15-08] Characteristics of spatial and temporal changes in self-potential distribution at Goshogake geothermal area in Hachimantai area, northeast of Japan
Keywords:self potential, geothermal area, hot spring, mud pot
We have carried out geophysical explorations at the Goshogake geothermal area in Towada-Hachimantai National Park, northeast of Japan. Mainly along the pedestrian trails in the active geothermal area, the methods of ERT (Electrical Resistivity Tomography), GPR (Ground Penetrating Radar), and multi-frequency electromagnetic induction method using the equipment GEM-2 have been utilized as well as SP (self-potential) method. From 2019 to 2024, we have gathered SP readings along the trails at least once in a year. With the total potential method and with copper-copper sulfate electrodes made by porous pottery, the SP values were acquired about every 30 m along the trail. We made at least three times of readings at a site with small deviation less than 5 mV by using the handy multimeter with 10 mega-ohm internal impedance. The positions of the sites were recorded by a handy GPS, for example by Garmin eTrex series.
We made color contour map for SP distribution for each campaign of measurements using the software of GMT (Generic Mapping Tool) and we got time-lapse changes of distribution of SP at the Goshogake geothermal area. It is difficult to recognize, at a glance, the trend and characteristics of time changes on the SP color contours, partly because they include the seasonal changes, properly SP readings have spatially scattered feature, the sites of SP readings were not exactly at the same points at different campaign observation each other, etc. But we can recognize the characteristics of the spatial distribution of SP. We found the positive anomalies of SP exist basically on the geological alteration zones. In addition, we can recognize the positive anomalies around Oname-Motome hot spring welling and Oyu-numa hot pond and negative anomaly around Odoro-Kazan mound of mud. We can not see the relationship between SP values and their elevations of observation points, which indicates the cause of SP is not due to the gravimetric flow of the ground water. Probably the causes of SP would be streaming potential of geothermal fluid convection or geochemical effect in associated with geological alteration.
We tried to make equivalent source models supposing positive and negative electric charges in the earth with an inversion with Marquardt method. The models can successfully predict the observed SP distribution. The values of pH for the upwelling hot water in this area are known very small and, in such cases, the streaming potential would be negative. We put equivalent source models of SP with negative electric charges existing in the area with active fumaroles and active mud pots.
In addition, we try to make models for understanding the spatial distribution and time changes of SP simultaneously. In such a numerical prediction model, the spatial SP distribution is modeled by quadratic equation in two directions within horizontal plane and the time changes of SP is modeled by sinusoidal equation with linear trend with time.
We made color contour map for SP distribution for each campaign of measurements using the software of GMT (Generic Mapping Tool) and we got time-lapse changes of distribution of SP at the Goshogake geothermal area. It is difficult to recognize, at a glance, the trend and characteristics of time changes on the SP color contours, partly because they include the seasonal changes, properly SP readings have spatially scattered feature, the sites of SP readings were not exactly at the same points at different campaign observation each other, etc. But we can recognize the characteristics of the spatial distribution of SP. We found the positive anomalies of SP exist basically on the geological alteration zones. In addition, we can recognize the positive anomalies around Oname-Motome hot spring welling and Oyu-numa hot pond and negative anomaly around Odoro-Kazan mound of mud. We can not see the relationship between SP values and their elevations of observation points, which indicates the cause of SP is not due to the gravimetric flow of the ground water. Probably the causes of SP would be streaming potential of geothermal fluid convection or geochemical effect in associated with geological alteration.
We tried to make equivalent source models supposing positive and negative electric charges in the earth with an inversion with Marquardt method. The models can successfully predict the observed SP distribution. The values of pH for the upwelling hot water in this area are known very small and, in such cases, the streaming potential would be negative. We put equivalent source models of SP with negative electric charges existing in the area with active fumaroles and active mud pots.
In addition, we try to make models for understanding the spatial distribution and time changes of SP simultaneously. In such a numerical prediction model, the spatial SP distribution is modeled by quadratic equation in two directions within horizontal plane and the time changes of SP is modeled by sinusoidal equation with linear trend with time.