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[ACC32-P03] Electrical Resistivity and GPR Surveys of Permafrost in Mt. Sashirui, Shiretoko Mountain Range
Keywords:Shiretoko Mountain Range, Permafrost, Electrical resistivity survey, GPR
1. Introduction
Recent surveys have confirmed the presence of permafrost in wind-exposed areas near the summit of Mt. Sashirui (1564 m a.s.l.) in the Shiretoko Mountain Range. The Shiretoko Mountain Range is located near the southern limit of permafrost distribution, making it an important indicator of long-term environmental changes. However, detailed data on the thickness and distribution of permafrost in this region remain insufficient, necessitating further investigation. This study aims to clarify the thickness and distribution of permafrost by conducting electrical resistivity and ground-penetrating radar (GPR) surveys in the wind-exposed areas near the summit of Mt. Sashirui.
2. Survey Methods
An electrical resistivity survey was conducted on 8 October 2021 using a Syscal R1 Plus manufactured by IRIS Instruments. A 48-meter survey line was established, extending from the wind-exposed area to the dwarf pine zone. A pole-pole method was applied with electrodes arranged at 1 m and 2 m intervals. The maximum survey depth was approximately 10 m.
A ground-penetrating radar survey was conducted on 18 September 2024 using a pulseEKKO system manufactured by Sensors & Software. An antenna with a frequency of 225 MHz was used. The survey depth was approximately 2–3 m and profiles for ten sections were obtained using the common offset survey (COS) method. The common midpoint (CMP) method was also used to calculate the electromagnetic wave propagation velocity structure.
3. Results and Discussion
The electrical resistivity cross-section showed a high-resistivity zone exceeding 50 kΩm at depths of 1.5–5.0 m in the wind-exposed area. Ground temperature observations confirmed the presence of permafrost within this depth range. Additionally, the high-resistivity zone disappeared upon entering the dwarf pine zone, suggesting that this zone corresponds to the permafrost, which is estimated to be several meters thick.
Data analysis from the CMP method in the GPR survey revealed a two-layer structure. The upper layer, with a velocity of 0.70 m/ns, was separated from the lower layer, with a velocity of 0.12 m/ns, at a depth of approximately 1.1 m. The lower layer exhibited velocity characteristics typical of permafrost. Additionally, the radar profiles at around 1.1 m depth observed strong reflections indicative of a frozen boundary. However, these reflections were only partially present, with strong reflections corresponding to permafrost detected in six of ten sections. In contrast, clear reflections of permafrost were not observed in the remaining four sections.
4. Conclusion
This study conducted electrical resistivity and ground-penetrating radar surveys in wind-exposed areas near the summit of Mt. Sashirui in the Shiretoko Mountain Range to investigate permafrost thickness and distribution. The results indicate that permafrost thickness is approximately several meters, and its distribution is limited. Future research should include further surveys in other wind-exposed areas and continuous monitoring to improve our understanding of permafrost variations.
Recent surveys have confirmed the presence of permafrost in wind-exposed areas near the summit of Mt. Sashirui (1564 m a.s.l.) in the Shiretoko Mountain Range. The Shiretoko Mountain Range is located near the southern limit of permafrost distribution, making it an important indicator of long-term environmental changes. However, detailed data on the thickness and distribution of permafrost in this region remain insufficient, necessitating further investigation. This study aims to clarify the thickness and distribution of permafrost by conducting electrical resistivity and ground-penetrating radar (GPR) surveys in the wind-exposed areas near the summit of Mt. Sashirui.
2. Survey Methods
An electrical resistivity survey was conducted on 8 October 2021 using a Syscal R1 Plus manufactured by IRIS Instruments. A 48-meter survey line was established, extending from the wind-exposed area to the dwarf pine zone. A pole-pole method was applied with electrodes arranged at 1 m and 2 m intervals. The maximum survey depth was approximately 10 m.
A ground-penetrating radar survey was conducted on 18 September 2024 using a pulseEKKO system manufactured by Sensors & Software. An antenna with a frequency of 225 MHz was used. The survey depth was approximately 2–3 m and profiles for ten sections were obtained using the common offset survey (COS) method. The common midpoint (CMP) method was also used to calculate the electromagnetic wave propagation velocity structure.
3. Results and Discussion
The electrical resistivity cross-section showed a high-resistivity zone exceeding 50 kΩm at depths of 1.5–5.0 m in the wind-exposed area. Ground temperature observations confirmed the presence of permafrost within this depth range. Additionally, the high-resistivity zone disappeared upon entering the dwarf pine zone, suggesting that this zone corresponds to the permafrost, which is estimated to be several meters thick.
Data analysis from the CMP method in the GPR survey revealed a two-layer structure. The upper layer, with a velocity of 0.70 m/ns, was separated from the lower layer, with a velocity of 0.12 m/ns, at a depth of approximately 1.1 m. The lower layer exhibited velocity characteristics typical of permafrost. Additionally, the radar profiles at around 1.1 m depth observed strong reflections indicative of a frozen boundary. However, these reflections were only partially present, with strong reflections corresponding to permafrost detected in six of ten sections. In contrast, clear reflections of permafrost were not observed in the remaining four sections.
4. Conclusion
This study conducted electrical resistivity and ground-penetrating radar surveys in wind-exposed areas near the summit of Mt. Sashirui in the Shiretoko Mountain Range to investigate permafrost thickness and distribution. The results indicate that permafrost thickness is approximately several meters, and its distribution is limited. Future research should include further surveys in other wind-exposed areas and continuous monitoring to improve our understanding of permafrost variations.