5:15 PM - 6:45 PM
[SVC27-P08] The Structural Verification of Possible Source Fissure of Phreatic Eruption Using High-Density Electrical Prospecting
Keywords:Hakone volcano, crater topography, high-density electrical prospecting, resistivity structure, topographical interpretation
Recent development of two decades of airborne LiDAR surveys has enabled to detect small but clear crater topography that had been impossible from aerial photographs (e.g., Chiba et al., 2007). However, identifying source crater only based on topographic interpretation is arbitral and detecting its ejecta by trenching around the crater topography is the decisive geological proof even in present. On the other hand, it is not always possible to obtain volcanic ejecta by trenching. Because trenching does not always a guarantee to detect deposit, and as a practical matter, trenching is difficult in national parks and on private land.
Using airborne LiDAR data, several topographical research has been identified small fissure topographies, which are considered to be sources of hydrothermal eruption in Hakone volcano too (Kobayashi et al., 2022; Oikawa, 2023). However, boring or trenching surveys are difficult to conduct in this area because Hakone volcano is a national park. We thus conducted high-density electrical prospecting at fissure vents in the northwest of Owakudani in order to detect source structures such as volcanic conduit beneath the topographic crater and to establish a new approach to detect source craters. We set two test lines (Line1 and 2) both of which were 160 m long with 4 m of electrode spacing. The depth of exploration was approximately 30 m.
In Line 1 (Fig. 2), a high-resistivity zone considered to representing andesite lava is detected at 950 to 960 m in elevation. Low-resistivity zone beneath the high-resistivity zone was interpreted as a hydrothermal alteration deposit. It is noteworthy that just beneath the crater topography (about 100m from the western end of the survey line), the high-resistivity zone disappears, and a medium to low-resistivity zone appears to fill the gap. This medium to low-resistivity zone seems to be connected to the hydrothermal deposit. We interpret this low-resistivity zone as volcanic conduit based on geological and topographical context. The low-resistivity of the conduit was formed by volcanic eruption that removed original lava distributed in this area and filled with hydrothermal deposit provided from deeper instead. Line 2 (Fig. 3) has a similar structure to Line 1 in terms of high resistivity in upper and low resistivity in lower and interpreted as lava and hydrothermal deposit respectively. However, the high-resistivity zone is not continuous unlike in Line 1 and its distribution is interrupted in places by medium-resistivity zones. These medium-resistivity zones may represent conduits beneath the survey line; however, the influence of the high resistivity-zones on both sides of the crater topography could deteriorate the resolution.
Our investigation suggests that high-density electrical prospecting helps to detect volcanic conduits and is a potential alternative approach to identify the source crater without conventional geological investigations. High-density electrical prospecting is relatively low-cost and able to survey and analysis swiftly. High-density electrical prospecting is considered useful not only as evidence for crater identification, but also for preliminary investigations (e.g. selection of trenching survey points) for more detailed geological surveys. Based on our investigation, we recommend that multiple prospecting lines are better to set orthogonal to the fissure trend.
In selecting survey sites, topographical interpretation of the crater was conducted in the area around the vents in the northwestern direction of Owakudani. Red relief image map made from airborne LiDAR survey data (laser point cloud density 0.5m/point) acquired by Kanagawa Prefecture was used for topographic interpretation (Basic survey of water source forests in FY-H31, Approved by the Director of the Green Policy and Reforestation Division, Environment and Agriculture Bureau, Kanagawa Prefecture ,2023, Mori No. 1306) . Topographical interpretation revealed new crater topography not found by Kobayashi et al. (2022) or Oikawa (2023).
References
Chiba,T.,Tomita,Y., Suzuki,Y., Arai,K., Fuji,N., Miyaji.N.,Koizumi,S., and Nakashima,K. (2007) AnalysisofmicrotopographyoftheAokigaharaLavaflows,Fujivolcano,Yamanashi Institute of Environmental Sciences,
p. 349-363
Kobayashi,M,. Mannen,K. Yamaguchi,T. and Nagai,M.(2022) Topography and deposits associated with the latest eruption activities of Hakone volcano. The earth monthly, Vol.44, No.3
Oikawa,T.(2023) Volcanic craters data of Hakone Volcano (Hakoneyama) . Research Institute of Earthquake and Volcano Geology, Geological Survey of Japan, AIST
Using airborne LiDAR data, several topographical research has been identified small fissure topographies, which are considered to be sources of hydrothermal eruption in Hakone volcano too (Kobayashi et al., 2022; Oikawa, 2023). However, boring or trenching surveys are difficult to conduct in this area because Hakone volcano is a national park. We thus conducted high-density electrical prospecting at fissure vents in the northwest of Owakudani in order to detect source structures such as volcanic conduit beneath the topographic crater and to establish a new approach to detect source craters. We set two test lines (Line1 and 2) both of which were 160 m long with 4 m of electrode spacing. The depth of exploration was approximately 30 m.
In Line 1 (Fig. 2), a high-resistivity zone considered to representing andesite lava is detected at 950 to 960 m in elevation. Low-resistivity zone beneath the high-resistivity zone was interpreted as a hydrothermal alteration deposit. It is noteworthy that just beneath the crater topography (about 100m from the western end of the survey line), the high-resistivity zone disappears, and a medium to low-resistivity zone appears to fill the gap. This medium to low-resistivity zone seems to be connected to the hydrothermal deposit. We interpret this low-resistivity zone as volcanic conduit based on geological and topographical context. The low-resistivity of the conduit was formed by volcanic eruption that removed original lava distributed in this area and filled with hydrothermal deposit provided from deeper instead. Line 2 (Fig. 3) has a similar structure to Line 1 in terms of high resistivity in upper and low resistivity in lower and interpreted as lava and hydrothermal deposit respectively. However, the high-resistivity zone is not continuous unlike in Line 1 and its distribution is interrupted in places by medium-resistivity zones. These medium-resistivity zones may represent conduits beneath the survey line; however, the influence of the high resistivity-zones on both sides of the crater topography could deteriorate the resolution.
Our investigation suggests that high-density electrical prospecting helps to detect volcanic conduits and is a potential alternative approach to identify the source crater without conventional geological investigations. High-density electrical prospecting is relatively low-cost and able to survey and analysis swiftly. High-density electrical prospecting is considered useful not only as evidence for crater identification, but also for preliminary investigations (e.g. selection of trenching survey points) for more detailed geological surveys. Based on our investigation, we recommend that multiple prospecting lines are better to set orthogonal to the fissure trend.
In selecting survey sites, topographical interpretation of the crater was conducted in the area around the vents in the northwestern direction of Owakudani. Red relief image map made from airborne LiDAR survey data (laser point cloud density 0.5m/point) acquired by Kanagawa Prefecture was used for topographic interpretation (Basic survey of water source forests in FY-H31, Approved by the Director of the Green Policy and Reforestation Division, Environment and Agriculture Bureau, Kanagawa Prefecture ,2023, Mori No. 1306) . Topographical interpretation revealed new crater topography not found by Kobayashi et al. (2022) or Oikawa (2023).
References
Chiba,T.,Tomita,Y., Suzuki,Y., Arai,K., Fuji,N., Miyaji.N.,Koizumi,S., and Nakashima,K. (2007) AnalysisofmicrotopographyoftheAokigaharaLavaflows,Fujivolcano,Yamanashi Institute of Environmental Sciences,
p. 349-363
Kobayashi,M,. Mannen,K. Yamaguchi,T. and Nagai,M.(2022) Topography and deposits associated with the latest eruption activities of Hakone volcano. The earth monthly, Vol.44, No.3
Oikawa,T.(2023) Volcanic craters data of Hakone Volcano (Hakoneyama) . Research Institute of Earthquake and Volcano Geology, Geological Survey of Japan, AIST