11:30 〜 11:50
[HTT17-10] 後生掛地熱地域における浅部電磁法探査
Since 2016 the high temperature watery mud has eroded the promenade called Goshogake Onsen Nature Trail at the Goshogake geothermal area, Kazuno city, Akita prefecture, northeast of Japan. For the purpose of delineate subsurface hydrothermal activity, geophysical explorations were carried out. Various methods were utilized like as GPR (Ground Penetrating Radar), ERT (Electrical Resistivity Tomography), SP (Self Potential) and electromagnetic induction method by the instrument called GEM-2. Especially GEM-2 can clearly delineate subsurface structure. The GEM-2 is an instrument that can observe changes of conductivity in the depth direction by using multiple frequencies simultaneously. In this study, five frequencies were used: 425 Hz, 1525 Hz, 5525 Hz, 20025 Hz, and 72225 Hz. The two types of exploration, VDM (vertical direction mode) on August 21, 2020 and HDM (horizontal direction mode) on October 21, 2020, were conducted over two days.
In advance of HDM measurement, free air calibration was performed at 4 m height above the ground using a pipe made by vinyl chloride but at the VDM measurement free air calibration couldn’t be done. Free air calibration is the process of a calibration setting the zero-level of the GEM-2 instrument free from the influence of the ground.
Within the active mud pots area, measurement range was set as 28 m in the direction of the survey line and 34m perpendicular to the survey line for GEM-2 measurement. Location of two mud pods in the measurement range were surveyed by GNSS. Electrical conductivity contour diagram was created based on the GEM-2 measurement data with median processing in the range of 3 m. Attached figure shows the conductivity maps arranged at equal intervals in the depth direction. The depths in the figure were derived based on the average values of the electrical conductivity at each frequency and calculated by the square root of skin depth by Huang (2005). In addition, topographical map was drawn by using GNSS data in the function of GEM-2 instrument and Geographical Survey Institute’s DEM data.
As a result, high electrical conductivity at lower elevations can be seen. The cause of the high electrical conductivity can be clay. The areas of high electrical conductivity in the deeper layers are the possibly the pathways of the clay and hot liquid.
In advance of HDM measurement, free air calibration was performed at 4 m height above the ground using a pipe made by vinyl chloride but at the VDM measurement free air calibration couldn’t be done. Free air calibration is the process of a calibration setting the zero-level of the GEM-2 instrument free from the influence of the ground.
Within the active mud pots area, measurement range was set as 28 m in the direction of the survey line and 34m perpendicular to the survey line for GEM-2 measurement. Location of two mud pods in the measurement range were surveyed by GNSS. Electrical conductivity contour diagram was created based on the GEM-2 measurement data with median processing in the range of 3 m. Attached figure shows the conductivity maps arranged at equal intervals in the depth direction. The depths in the figure were derived based on the average values of the electrical conductivity at each frequency and calculated by the square root of skin depth by Huang (2005). In addition, topographical map was drawn by using GNSS data in the function of GEM-2 instrument and Geographical Survey Institute’s DEM data.
As a result, high electrical conductivity at lower elevations can be seen. The cause of the high electrical conductivity can be clay. The areas of high electrical conductivity in the deeper layers are the possibly the pathways of the clay and hot liquid.