16:00 〜 16:15
[SEM16-09] 後生掛地熱地帯の自然研究路上におけるGPRとSPの繰り返し探査
キーワード：地中レーダー、自然電位、比抵抗、誘電率、温泉、地熱地帯
Gshogake geothermal area in Towada-Hachimantai National Park is an active geothermal area located in Akita Prefecture close to the border of Iwate Prefecture, northeast of Japan. This geothermal area is behind the Goshogake hot spring inn and situated between the Akita-Yakeyama volcano to the west and the Hachimantai volcano to the east. Pedestrian paths called Goshogake Nature Research Path are constructed and maintained in the thermal area and we can see the uncommon landscape of mud pots, mud pools, fumaroles, hot springs, hot water ponds, etc.
We conducted geophysical exploration methods targeted subsurface zone related to the geothermal activity just beneath our feet. So far, we utilized GPR (Ground Penetrating Radar), ERT (Electric Resistivity tomography), SP (Self Potential) and the electromagnetic survey with multi-frequency portal instrument named GEM-2 (manufactured by Geophex Ltd., USA). Especially, GPR and SP were conducted repeatedly on the same survey line. GPR surveys were carried out 4 times on 2017/11/10, 2019/11/08, 2021/06/24 and 2021/10/29. SP measurements were carried out 6 times on 2019/11/04, 2020/07/09, 2020/10/08, 2020/11/12, 2021/05/27 and 2021/11/19.
The instrument used for GPR was SIR 3000 manufactured by GSSI (Geophysical Survey Systems, Inc.) with mainly 270 MHz and 400 MHz antennas. The antennas were encapsulated in exclusively used and hand-pushing vehicle with wheels. We set the repeated survey line along the south-east peripheral of the Oyunuma hot pond. After carefully adjusted operation of both ends of the survey line, we compared the reflections of B-scans of the GPR for respective surveys. We could not clearly recognize the time-change of the structure related to the geothermal activity. But clear reflections can be found at whitish altered silicified zone. At present the cause of GPR reflection is inferred due to the porous altered medium.
SP measurement points are around 30 m intervals along the Nature Research Path and the total length of survey line is up to 2000 m. We found the positive anomalies of SP is in accordance with the high temperature spots at 1 m depth. Positive SP anomaly is outstanding around the Oyunuma hot pond and Negative SP anomaly can be seen around the Odorokazan which is mounds of the hot mud. The time-change of the SP distribution is a little complicated and including seasonal change and change related to the geothermal activity. The figure of the time-lapse distributon of SP is shown.
We conducted geophysical exploration methods targeted subsurface zone related to the geothermal activity just beneath our feet. So far, we utilized GPR (Ground Penetrating Radar), ERT (Electric Resistivity tomography), SP (Self Potential) and the electromagnetic survey with multi-frequency portal instrument named GEM-2 (manufactured by Geophex Ltd., USA). Especially, GPR and SP were conducted repeatedly on the same survey line. GPR surveys were carried out 4 times on 2017/11/10, 2019/11/08, 2021/06/24 and 2021/10/29. SP measurements were carried out 6 times on 2019/11/04, 2020/07/09, 2020/10/08, 2020/11/12, 2021/05/27 and 2021/11/19.
The instrument used for GPR was SIR 3000 manufactured by GSSI (Geophysical Survey Systems, Inc.) with mainly 270 MHz and 400 MHz antennas. The antennas were encapsulated in exclusively used and hand-pushing vehicle with wheels. We set the repeated survey line along the south-east peripheral of the Oyunuma hot pond. After carefully adjusted operation of both ends of the survey line, we compared the reflections of B-scans of the GPR for respective surveys. We could not clearly recognize the time-change of the structure related to the geothermal activity. But clear reflections can be found at whitish altered silicified zone. At present the cause of GPR reflection is inferred due to the porous altered medium.
SP measurement points are around 30 m intervals along the Nature Research Path and the total length of survey line is up to 2000 m. We found the positive anomalies of SP is in accordance with the high temperature spots at 1 m depth. Positive SP anomaly is outstanding around the Oyunuma hot pond and Negative SP anomaly can be seen around the Odorokazan which is mounds of the hot mud. The time-change of the SP distribution is a little complicated and including seasonal change and change related to the geothermal activity. The figure of the time-lapse distributon of SP is shown.