2:30 PM - 2:45 PM
[HTT18-04] Repeated Measurements of Self-potential in Rokugo alluvial fan in Akita Prefecture, Japan
Keywords:Self potential, Topographic effect, Repeated measurements, Groundwater
When water flows underground, it is known that a streaming potential is generated depending on the pressure difference applied to the water, specific resistivity, zeta potential, etc. The self-potential (SP) method is a geophysical prospecting method that can obtain information on the direction of groundwater flow by measuring the electric potential distribution generated by the streaming potential. In this study, repeated SP observations on the same lateral line were conducted in the Rokugo alluvial fan in Akita Prefecture, Japan, and the obtained SP distributions were compared.
In the Rokugo alluvial fan, there are more than 70 springs, including large and small springs, and the groundwater status can be indirectly obtained through the discharge rate of these springs. In addition, groundwater level observation wells have been installed at several locations, and it is possible to obtain information on groundwater level fluctuations through observations using these wells. In this study, we conducted observations at different times of the year to obtain data that can be used to study the relationship between the groundwater level and the distribution of the groundwater potential. Observations were conducted from October 2015 to November 2019, and 16 measurements were taken on the same measuring line. The survey line was set along the central axis of the fan, along the main road extending in a straight line from the top of the fan to the edge of the fan. The measuring line length is about 4 km, and the measuring point interval is about 100 m.
The SP distributions obtained showed the influence of the "topographic effect," in which the potential value increases as the elevation decreases, although the degree of the effect was small. The slopes of the approximate straight lines were compared, and it was found that the slopes could be classified into three types: high (-2.4 mV/m), medium (-1.5 mV/m), and low (-0.2 mV/m). The magnitude of the topographic effect varied with the time of observation, suggesting that a natural potential distribution reflecting the hydrogeological conditions of the subsurface occurred.
In the Rokugo alluvial fan, there are more than 70 springs, including large and small springs, and the groundwater status can be indirectly obtained through the discharge rate of these springs. In addition, groundwater level observation wells have been installed at several locations, and it is possible to obtain information on groundwater level fluctuations through observations using these wells. In this study, we conducted observations at different times of the year to obtain data that can be used to study the relationship between the groundwater level and the distribution of the groundwater potential. Observations were conducted from October 2015 to November 2019, and 16 measurements were taken on the same measuring line. The survey line was set along the central axis of the fan, along the main road extending in a straight line from the top of the fan to the edge of the fan. The measuring line length is about 4 km, and the measuring point interval is about 100 m.
The SP distributions obtained showed the influence of the "topographic effect," in which the potential value increases as the elevation decreases, although the degree of the effect was small. The slopes of the approximate straight lines were compared, and it was found that the slopes could be classified into three types: high (-2.4 mV/m), medium (-1.5 mV/m), and low (-0.2 mV/m). The magnitude of the topographic effect varied with the time of observation, suggesting that a natural potential distribution reflecting the hydrogeological conditions of the subsurface occurred.