[SVC44-P04] メディポリス地熱フィールドの下深さ3.6kmに見つかった高Vp/Vs 域が示唆する物理的意味
キーワード：地熱構造、分布型振動センサー（DAS)、高Vp/Vs、地震速度の温度依存性、水によるVp/Vs変化、坑内地震計測
We conducted the second seismic and temperature measurement at the Medipolis geothermal field in 2019 using optical fiber system in the borehole down to depth of 1,545 m. Using distributed temperature sensors (DTSs), the temperature in the borehole was measured. The temperature was 272°C and 155°C at 918 m and 1,530 m, respectively. Offset VSP data were obtained using distributed acoustic sensors (DASs). We operated the MiniVib seismic source at five locations.
The analysis of the P-to-S conversions observed by surface seismometers at the same geothermal field in 2018 suggested the presence of Vp/Vs=3 zone at depth of approximately 4 km (Kasahara et al., 2020b) (see Figs.1 & 2). By forward modeling and the migration processing of DAS data in 2019, we obtained Vp/Vs=3 zones at depth of approximately 700–900 m, 1,300–1,600 m and 3,600 m (Kasahara et al. 2020a). Because ordinary silicate rocks show approximately Vp/Vs=1.75 (e.g. Hoshino et al., 2001), Vp/Vs=3 is quite different from other rocks. Vp/V=3 does not mean exact number, but it means that Vp/Vs is much higher than 1.75 which is that for ordinal silicate rocks.
We considered the geophysical meaning of Vp/Vs ~3. Using the numerous Vp and Vs measurements under high pressure and temperature by Kern (1978), Kern and Richter (1981), and Kern et al. (1997), most quartz-free rocks showed very small temperature dependence of Vp and Vs. Rocks including quartz minerals demonstrated large temperature dependence on Vp due to that phase change from α to β quartz (Kern and Richter, 1981). For quartz-free rocks, Vp/Vs remained constant to be approximately 1.75 and Vp/Vs ~3 was not explained. The serpentine rock exhibited large decreases in Vs and Vp at temperatures higher than 500 °C (Kern and Richter, 1981). The amount of velocity decrease at high temperature was explained by the dehydration of antigorite, which is a high temperature-type serpentine mineral. The decrease in Vs was larger than that in Vp. Water saturation in rocks might be a cause of high Vp/Vs. In addition, the Vp/Vs is related to the Poisson’s ratio. Vp/Vs = 3 corresponds to 0.467 for the Poisson’s ratio. Because the Poisson’s ratio of water is 0.5, the zone at a depth of approximately 3.6–4 km could be filled by water. Winlker and Nur (1985) obtained Vp/Vs > 2 for the Massillon sandstone if the sandstone was 100% saturated by water. Considering the above discussion, high Vp/Vs suggests the presence of high-water saturation zones at depth of approximately 4 km. Because the temperature at the depth of 914 m is 264 °C in the IK-4 well, the zone around 4 km depth might be close to the supercritical state if the zone is filled by low salinity water.
ACKNOWLEDGHMENTS
This article is based on the results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).
References
Hoshino, K., Kato, T., Tanaka, S., Komata, A., Moriguchi, Y., Hattori, M., and Imamura, T., Handbook of mechanical properties of the Japanese rocks under high confining pressure, AIST Geological Survey of Japan Quick report No. 23, (2001).
Kasahara, J., Hasada, Y., Kuzume, H., Mikada, H., and Fujise, Y.: The second seismic study at the geothermal field in southern Kyushu, Japan using an optical fiber system and surface geophones, Proceedings, 45th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA (2020a).
Kasahara, J., Hasada, Y., and Kuzume, Y.: Possibility of high Vp/Vs zone in the geothermal filed suggested by the P-to-S conversion, Proceedings, 45th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA. (2020b).
Kern, H.: The effect of high temperature and high confining pressure on compressional wave velocities I quartz-bearing and quartz-free igneous and metamorphic rocks, Tectonophysics, 44, (1978), 185-203.
The analysis of the P-to-S conversions observed by surface seismometers at the same geothermal field in 2018 suggested the presence of Vp/Vs=3 zone at depth of approximately 4 km (Kasahara et al., 2020b) (see Figs.1 & 2). By forward modeling and the migration processing of DAS data in 2019, we obtained Vp/Vs=3 zones at depth of approximately 700–900 m, 1,300–1,600 m and 3,600 m (Kasahara et al. 2020a). Because ordinary silicate rocks show approximately Vp/Vs=1.75 (e.g. Hoshino et al., 2001), Vp/Vs=3 is quite different from other rocks. Vp/V=3 does not mean exact number, but it means that Vp/Vs is much higher than 1.75 which is that for ordinal silicate rocks.
We considered the geophysical meaning of Vp/Vs ~3. Using the numerous Vp and Vs measurements under high pressure and temperature by Kern (1978), Kern and Richter (1981), and Kern et al. (1997), most quartz-free rocks showed very small temperature dependence of Vp and Vs. Rocks including quartz minerals demonstrated large temperature dependence on Vp due to that phase change from α to β quartz (Kern and Richter, 1981). For quartz-free rocks, Vp/Vs remained constant to be approximately 1.75 and Vp/Vs ~3 was not explained. The serpentine rock exhibited large decreases in Vs and Vp at temperatures higher than 500 °C (Kern and Richter, 1981). The amount of velocity decrease at high temperature was explained by the dehydration of antigorite, which is a high temperature-type serpentine mineral. The decrease in Vs was larger than that in Vp. Water saturation in rocks might be a cause of high Vp/Vs. In addition, the Vp/Vs is related to the Poisson’s ratio. Vp/Vs = 3 corresponds to 0.467 for the Poisson’s ratio. Because the Poisson’s ratio of water is 0.5, the zone at a depth of approximately 3.6–4 km could be filled by water. Winlker and Nur (1985) obtained Vp/Vs > 2 for the Massillon sandstone if the sandstone was 100% saturated by water. Considering the above discussion, high Vp/Vs suggests the presence of high-water saturation zones at depth of approximately 4 km. Because the temperature at the depth of 914 m is 264 °C in the IK-4 well, the zone around 4 km depth might be close to the supercritical state if the zone is filled by low salinity water.
ACKNOWLEDGHMENTS
This article is based on the results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).
References
Hoshino, K., Kato, T., Tanaka, S., Komata, A., Moriguchi, Y., Hattori, M., and Imamura, T., Handbook of mechanical properties of the Japanese rocks under high confining pressure, AIST Geological Survey of Japan Quick report No. 23, (2001).
Kasahara, J., Hasada, Y., Kuzume, H., Mikada, H., and Fujise, Y.: The second seismic study at the geothermal field in southern Kyushu, Japan using an optical fiber system and surface geophones, Proceedings, 45th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA (2020a).
Kasahara, J., Hasada, Y., and Kuzume, Y.: Possibility of high Vp/Vs zone in the geothermal filed suggested by the P-to-S conversion, Proceedings, 45th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA. (2020b).
Kern, H.: The effect of high temperature and high confining pressure on compressional wave velocities I quartz-bearing and quartz-free igneous and metamorphic rocks, Tectonophysics, 44, (1978), 185-203.