Study on Applicability of Electric Sounding for Interpretation of the Internal Structure of Glacial Moraines

Transcription

1 51 Study on Applicability of Electric Sounding for Interpretation of the Internal Structure of Glacial Moraines Kengo OHASHI 1*, Toru KOIKE 1 Shuhei TAKENAKA 1 and Jun UMEMURA 2 1 Earth System Science Co., Ltd., , Shinjuku, Shinjuku-ku, Tokyo 16-22, Japan 2 College of Engineering, Nihon University, Tamura-machi Tokusada, Koriyama , Japan * ohashi@ess-jpn.co.jp Abstract The internal structure of moraine dams, especially the distribution of core ice and bedrock, is one of the key factors in assessing GLOF risk. In this study, we discuss the interpretation of the results of 2D electric sounding at three moraines in northern central Bhutan from the standpoint of not only their specific resistivity values, but also distribution patterns, in conjunction with field observations and previous studies. We found that: 1) Zones of specific resistivity exceeding 1, ohm-m could be regarded as massive dead ice, while zones of 1, - 5, ohm-m were assumed to be a mixture of fragmented ice and rock debris, where the higher the value was, the higher the ice content. 2) Resistivity values of rock debris were 5, - 1, ohm-m depending on the water content of matrix materials and degree of freezing. 3) Resistivity values of bedrock overlapped those of rock debris in the moraine. However, in the case of the study area, some distinctive patterns of lower resistivity, such as steep angle zones that were concordant with lithological features of the bedrock of the area, were used to identify bedrock. Further studies employing electric sounding accompanied by other exploration methods are still necessary to improve the accuracy of the interpretation. Key words: dead ice, GLOFs, moraine dam, resistivity profile, two-dimensional electric sounding 1. Introduction Triggers and mechanisms of glacial lake outburst floods (GLOFs) have been a controversial issue. Many previous studies have revealed that GLOFs are induced by sudden increased overtopping flows over the moraine dam, which are related to calving and/or mass rock failure from the edge of the glacial body and lakeside deposits into the lake (e.g., Iwata et al., 22). It is reported that gradual subsidence of the moraine surface as a result of melting of dead ice in the moraine contributes to the stability of the moraine dam with a lower lake level (Takenaka et al., 21), whereas rapid melting of dead ice may promote topographical instability of the moraine dam. In any scenario, it is highly necessary to obtain significant information on the internal structure and physical and geotechnical properties of the moraine dam in order to assess the breach potential of the dammed lake. Owing to severe difficulties in accessibility of high altitude regions in the Himalayas, no systematical or comprehensive investigations including drilling surveys in moraine dams have been conducted until now. By contrast, geophysical exploration methods which are relatively portable have been commonly applied for clarifying the internal structures of moraine dams. In particular, the electric sounding method has been employed in many previous studies because of prompt operation for data acquisition even over a broad exploration area, and its adaptive flexibility to changes in terrain conditions (e.g., Hauck et al., 23; Gibas et al., 25). On the other hand, specific resistivity values of moraine substances obtained by electric sounding depend on not only their original electric character but also the condition of the water content, quality, temperature, etc. Therefore, a comprehensive interpretation of resistivity values is essential for identifying the materials. In this study, we carried out two-dimensional (2D) electric sounding at one debris-covered glacial moraine and two glacial lake moraine dams in northern central Bhutan for the purpose of verifying specific resistivity values indicating constituent materials such as dead ice, Global Environmental Research 16/212: printed in Japan 212 AIRIES

2 52 K. OHASHI et al. debris and bedrock. In addition, the interpretation of distribution patterns on resistivity profiles is discussed in consideration of the results of previous studies as well. 2. Study Area The target glacial moraines of the Tsho Jo Glacier, Lake Zanam C (tentatively named by Komori et al., 21) and Lake Metatshota are located in the High Himalayan region of northern central Bhutan as shown in Fig. 1. The Tsho Jo Glacier, a debris-covered glacier with a total length of 13 km and an average width of 1.7 km, is one of the largest glaciers in Bhutan. A sudden abnormal flood originating from the terminus of glacier was observed on 29 April 29. It was identified as a discharge from a supra-glacial lake through an internal channel in the glacier (Komori et al., 212, in this issue). Due to this sudden discharge, a large amount of soil and debris from the terminal moraine was flushed away, and thereby an ice body 5 m in height was exposed in the debris cliff near the outlet. 2D electric sounding was carried out in order to calibrate specific resistivity values between the ice and the surrounding debris materials. Lake Zanam C is located in a small tributary of Sacha Chu River, a branch of the Mangde Chu River. The lake size is rather small:.9 km long by.2 km wide. In our preliminary geomorphological study, the breach potential was presumed to be the highest in the Mangde Chu river basin because of the 2 m steep slope of the moraine dam. Lake Metatshota, the largest glacial lake in the Mangde Chu river basin with a surface area of 1.34 km 2, is ranked as one of 23 potentially dangerous glacial lakes in Bhutan (Mool et al., 21). Undulating moraine alignments encompassing the frontal lake have already been deeply eroded by the outlet flow. The present relative height of the moraine dam is only 25 m from the frontal plain. Thus it seems to be stable for breaching compared with Lake Zanam C. 3. Method of Exploration Expeditions were conducted on 5 September 21 and 11 October 211. A McOHM Profiler-4 (OYO Corporation, Japan) was utilized as the electric sounding machine. This model includes an Operation System with an SSD drive in its body; therefore, the user can monitor the real-time electric potential wave and decay curve during measurement. It provides a significant advantage in measuring by enabling the user to review abnormal values in case extremely high ground resistances are predicted. The specifications of the utilized equipment are shown in Table 1. Survey lines and electrode intervals of 2.5 m or 5. m were applied according to the depth to be sounded at each objective site. Several electrode alignments of pole-pole, pole-dipole and dipole-dipole were examined to clearly represent the resistivity profile in each moraine. In order to reduce the high ground electric resistance of debrisrich moraine surfaces, multiple iron poles were inserted into one electrode position. In addition, a bentonite solution with highly conductive fabric was buried into the ground. Fig. 1 Location of the target moraines in the study area.

3 Study on Applicability of Electric Sounding for Interpretation of the Internal Structure of Glacial Moraines Table 1 4. Results Electric sounding equipment utilized in this study. All items were produced by OYO Corporation. Multi-channel Digital Res istivity M eter: McOHM Profiler-4 XPE 214D Electrode Switching Sy stem : Scanner Boos ter: Power Booster 2142 Analy sis Software: 4.1 Tsho Jo Glacier moraine The debris cliff formed by the abnormal discharge in 29 at the terminus of the Tsho Jo Glacier, has receded 5 m since September 29. The outcrop of the ice body is visible only at the end of the retreating cliff, where the outlet flow is rapidly eroding the debris materials, as shown in Fig. 2. It is unclear whether the ice body is a part of the glacial ice lying under the moraine or just an ice fragment in the debris deposit. Electric sounding was carried out on the moraine surface near the outcrop of the ice body. The survey line was laid out for a length of 15 m parallel to the ice cliff, as shown in Fig. 3. Specifications Item Output Method: Alternative DC Output Voltage: Maximum 4V Output Current: Max 12mA Max Electrode: 64 electrodes Output Voltage: Maximum 2V Output Current: 25,5,75,1mA ElecImager / 2D Outlet Survey Line Fig. 2 Overview of survey area at the Tsho Jo Glacier moraine. The base view is portrayed by an ALOS-PRISM elevation model and panchromatic image. Survey Line 158 Ice Outcrop Outlet < 1, 2, - 5, Ice Outcrop Outlet Ohm-m Ohm-m 1, 2 5, > 1, 1, 4 Coexistnace of & Ice (fragment?) 53 Massive Dead Ice 5, Fig. 3 Survey line and result of electric sounding at the Tsho Jo Glacier moraine.

4 54 K. OHASHI et al. Two masses of high-resistivity zones showing over 1, ohm-m were observed 1 m below ground level. Relatively lower resistivity of 2, - 5, ohm-m at 5-1 m below ground level corresponds to debris and the ice body observed in the cliff. The current outlet was located between the two zones. The shallow area above 5 m below ground level basically showed a resistivity value of less than 1, ohm-m, as shown in Fig. 3. Consequently, the moraine materials of the Tsho Jo Glacier can be divided into three layers, comprising 1) a zone of more than 1, ohm-m, 2) a zone with a range of 2, - 5, ohm-m in debris with ice, and 3) a surface zone of one-order below. 4.2 Lake Zanam C moraine dam The moraine dam of Lake Zanam C takes the shape of a steep slope around 2 m high from the Sacha Chu riverbed. We set our survey line on the crest of the terminal moraine. The electric sounding was carried out using pole-dipole alignment at 5 m intervals. The location of the survey line and the results are shown in Fig. 4. Resistivity in the moraine dam is limited to values of less than 15, ohm-m. There is no high resistivity zone indicating dead ice until 8 m depth from the ground surface. The highest resistivity zone was observed in the down slope of the moraine dam, where a resistivity zone of 5, - 5, ohm-m is distributed at 4 m thickness. 27 Survey Line m , - 15, < 5, Ohm-m 1, 2 Seepage 4 5, 6 (saturated) 8 Bedrock with fracture zone 1, (less matrix) 1 5, Fig. 4 Survey location and result of electric sounding at the Lake Zanam C moraine dam. The base view is portrayed by an ALOS-PRISM elevation model and panchromatic image (blank arrows indicate the cross-section line in Fig. 5). Fig. 5 Assumed geological section of Lake Zanam C (horizontal scale : vertical scale = 2:1).

5 Study on Applicability of Electric Sounding for Interpretation of the Internal Structure of Glacial Moraines 55 Outlet 25m 155 Survey Line (less matrix) 5, - 1, (saturated) 2, - 5, Ohm-m 1, 5, 1, 5, 6 Fig. 6 Survey location and result of electric sounding at the Lake Metatshota moraine dam. The base view is portrayed by an ALOS-PRISM elevation model and pan-sharpened image This zone may be composed of less matrix debris deposits according to the field observation. On the other hand, a resistivity zone of less than 5, ohm-m near the lakeside indicates saturated debris materials. Though the resistivity value gradually increases toward the deeper portion, several lower resistivity zones with steep angles are observed below 25 m from the ground level, as shown in Fig. 4. As discussed later, this steep-angle structure can be interpreted as fracture zone bedrock. A cross section shown in Fig. 5 indicates that the bedrock elevation at the moraine dam is concordant with the deepest level of the lake bathymetry reported by Komori et al. (21). The bedrock elevation is 15 m higher than the riverbed of the Sacha Chu River. 4.3 Lake Metatshota moraine dam Electric sounding was carried out on the narrow flat plan just behind the highest moraine hill, as shown in Fig. 6. The surface around the survey line is composed of fine sand and silt with high water content that may have less ground resistance for better sounding accuracy. As the results in Fig. 6 show, no high resistivity zone was observed in any part of the moraine until 5 m below the ground level. High resistivity values ranging from 5, - 1, ohm-m were obtained at the end point of the survey line, where less matrix debris deposits are distributed. The saturated zone below the lake water level shows less than 5, ohm-m and decreases toward the deep direction. Since the resistivity distribution pattern shows a horizontal structure, no steep angle structure is observed, unlike at Lake Zanam C. Such a horizontal pattern is commonly observed in sediments. In the Lake Metatsota moraine dam, therefore, no bedrock can be expected to exist until deeper than 5 m below the ground level. 5. Discussion Inevitably each material has a specific range of electric resistivity values and it must be comprehensively verified by field observations in consideration of previous studies. The resistivity value range of the moraine dam materials can be summarized as shown in Fig. 7, using data obtained from previous studies and this study. 5.1 Interpretation from resistivity values and distribution patterns Resistivity value of ice Most studies have identified high resistivity zones showing more than 1, ohm-m as underlying dead ice bodies in the moraine (e.g., Hauck et al., 23). In this study, in the Tsho Jo Glacier moraine, likewise, a large zone of 1, ohm-m below 1 m from the ground level was regarded as dead ice. On the other hand, it can be clarified that the layer of ice-containing debris material showed relatively lower values of 2, - 5, ohm-m. Therefore, it is assumed that the zone indicating resistivity values of more than 2, ohm-m may contain ice which is either massive or fragmented in debris material conditions. In the case that a resistivity zone of 1, - 2, ohm-m appears in the deep potion of a moraine, it is also expected to involve some amount of ice fragments.

6 56 K. OHASHI et al Resistivity values of debris Resistivity values of debris deposits vary depending highly on the water content of the matrix substances and the frozen condition. The results at Lake Zanam C and Lake Metatshota in this study suggest that the debris with less matrix or coarse sandy materials near the ground surface shows 1, ohm-m, whereas the saturated zone near the outlet or lakeside shows less than 5, ohm-m Resistivity values of bedrock As the resistivity range of bedrock overlaps with that of debris materials in the moraine, it is difficult to identify the boundary between them with any definitivity. In case of the Lake Zanam C moraine dam, a resistivity distribution pattern with several steep angle zones of lower resistivity was observed. In general, such vertical structures are often seen in fracture zones of bedrock such as shear zones or joint systems. Indeed, the lithology of the study area is composed of granitic gneiss with the predominant structure dipping toward the north, which is concordant with the observed resistivity pattern in the moraine. Such an approach to interpretation can help to identify materials in case there are no distinct electric differences. However, it may not be applicable to massive rock without fractures or predominant joint systems. 5.2 Applicability of interpretation Consequently, the proposed procedure to identify the internal structure of moraine dams is summarized in Table 2 in consideration of resistivity ranges and distribution patterns. In order to generalize this classification method to make it applicable to other regions, the following issues must be considered. 1) Reducing the ground electric resistance of debrisrich moraine surfaces is a focal issue for securing accuracy of electric sounding, especially for scanning distribution patterns of resistivity. Using bentonite or electrolyte fluid and multiple electrodes is recommended as one practical solution for electric sounding. 2) Interpretation using the inter-structure of bedrock cannot be applied in the case that the survey line is lying parallel to the bedrock structure. If the existence of bedrock is suspected, the survey line must be extended and/or multiple survey lines added. Fig. 7 Ranges of resistivity values of moraine dam materials. Table 2 Flow chart for interpreting internal structure using resistivity ranges and distribution patterns. Resistivity Value Distribution Pattern Material < 1, ohm-m 1, - 2, ohm-m 2, - 1, ohm-m > 1, ohm-m with Vertical Structure Horizontal or Irregular Structure Limited at shallow zone Not limited at sallow zone Irregular Distribution Extensive and Mass block Bedrock with Ice Fragments Saturated Dead Ice? Dead Ice

7 Study on Applicability of Electric Sounding for Interpretation of the Internal Structure of Glacial Moraines 57 3) The depth limitation of 2D electric sounding is approximately 1 m in terms of securing sound data accuracy. When bedrock is expected in a deeper portion, TEM (Transient Electro-Magnetic method) or combination with appropriate physical exploration methods should be employed as supplemental methods. In this study, the micro-tremor array method was also carried out at each site. However, no dependable results could be obtained due to insufficient micro-tremor at the sites. 6. Conclusion In terms of GLOF risk assessment, an important issue is to understand moraine dam internal structures, especially the distribution of huge dead-ice bodies and bedrock, which may have an effect on the breach potential and total flood volume. In this study, we carried out 2D electric sounding at three moraines in northern central Bhutan. We also interpreted the appearance and distribution of dead ice, debris and bedrock, considering specific resistivity values and their distribution patterns. Electric sounding is one of the most popular and applicable methods of analyzing the internal structures of glacial moraines. Securing accuracy in sounding data could make a large contribution to the understanding of not only internal structures but also the breach potentials of moraine dams. Further empirical studies in conjunction with other physical exploration methods are highly anticipated in order to reveal the electric features of moraine materials based on both resistivity values and their distribution patterns. Acknowledgements We thank all the Bhutanese staff of Department of Geology & Mines for their great assistance and hospitality during the expedition into the high mountain area. Special thanks to our medical advisor, Mr. Kinley Dorji, for carefully nursing us through trouble. We are grateful to the members of the science workshop at Hokkaido University for their valuable counsel and discussion. This study was carried out as a part of the Study on Glacial Lake Outburst Foods in Bhutan Himalaya project under SATREPS supported by JST and JICA. References Gibas, J., G. Rachlewicz and W. Szczucinski (25) Application of DC resistivity soundings and geomorphological surveys in studies of modern Arctic glacier marginal zones, Petuniabukta, Spitsbergen. Polish Polar Research, 26(4): Hausler, H., D. Leber, M. Schreilechner, R. Morawetz, H. Lentz, St. Skuk, M. Meyer and C. Janda, E. Burgschwaiger (2) Final Report of Raphstreng Tsho Outburst Flood Mitigatory Project (Lunana; Northwestern Bhutan): Phase II. Institute of Geology, University of Viena, Austria. Hauck, C., M. Vonder and D. MÜHLL (23) Evaluation of geophysical techniques for application in mountain permafrost studies. In: L. Schrott, A. Hordt and R. Dikau, eds., Geophysical Applications in Geomorphology, Ikeda, A. (26) Combination of conventional geophysical methods for sounding the composition of rock glaciers in the Swiss Alps. Permafrost and Periglacial Processes, 17: Isaksen, K., R.S. Odegard, T. Eiken and J.L. Sollid (2) Composition, flow and development of two tongue-shaped rock glaciers in the permafrost of Svalbard. Permafrost and Periglacial Processes, 11: Iwata, S., Y. Ageta, N. Naito, A. Sakai, C. Narama and Karma (22) Glacial lakes and their outburst flood assessment in the Bhutan Himalaya. Global Environmental Research, 6: Komori, J., T. Phuntsho and S. Takenaka (21) Bathymetric Topgraphy of Glacial Lake in Bhutan Himalaya. Abstracts of Japan Geosciences Union Meeting 21. Komori, J., T. Koike, T. Yamanokuchi and P. Tshering (212) Glacial lake outburst events in the Bhutan Himalayas, Global Environmental Research, 16: (in this issue) Mool, P. K., D. Wanda, S. R. Bajracharya, K. Kunzang and S. P. Joshi (21) Inventory of Glaciers, Glacial Lakes and Glacial Lake Outburst Floods. Monitoring and Early Warning Systems in the Hindu Kush-Himalayan Region Bhutan. ICIMOD and UNDP-ERA-AP, 244. Takenaka, S., T. Yabuta and H. Fukui (21) Distribution of dead ice inside moraine dam body of Imja Glacial Lake, Khumbu region, Nepal internal structure of moraine dam of Imja Glacial Lake. Journal of the Japanese Society of Snow and Ice, 72(1): Kengo OHASHI Kengo OHASHI is a consultant specializing in geophysical engineering, at Department of Environmental & Disaster Management at Earth System Science Co., Ltd. His fields of study include geophysical exploration for water resource development, landslide stability and construction of infrastructure. Groundwater development in arid regions of African counties by use of remote sensing and 2D / 3D electrical sounding is one of his main focuses these days. Toru KOIKE Toru KOIKE is a Deputy General Manager of the Environmental & Disaster Management Department at Earth System Science Co., Ltd. He has been working in the field of disaster management, including geological and river engineering under Official Development Assistance funding from the Japanese government. During 2-22, he lived in Bhutan and worked for the Department of Geology & Mines. Currently he is working in disaster related projects in Thailand, Vietnam, Bhutan, Bolivia and Caribbean countries. Shuhei TAKENAKA Shuhei TAKENAKA is dedicated staff of Earth System Science Co., Ltd., for the JICA/JST Bhutan GLOFs project. Although he got his M.Sc. in Glaciology from Nagoya University 3 years ago, prior to joining the project, he worked as a manager at an electronic device company. Some five years ago, he decided to return from cyber space to the real world looking for a chance to perform field work and more directly contribute to the human community. On Himalayan trails, he feels happy with his decision despite the aches in his back and knees. In addition to glaciers, he is interested in how local communities in remote areas are being changed by modern technologies like TVs and cell phones.

8 58 K. OHASHI et al. Jun UMEMURA Jun UMEMURA is a lecturer at the College of Engineering, Nihon University. He also is a member of the technical committee for waste disposal and conservation of the environment of Fukushima Prefecture. His speciality is geotechnical engineering and engineering geology, especially the prevention of landslide disasters. In 21, he was studying at Tribhuvan University, Nepal; and in 22, at the Swiss Federal Institute of Technology, Zurich. Recently his endeavors have focused on many of the major disasters in Japan, including those caused by the 28 Iwate-Miyagi Nairiku Earthquake and the 211 Tohoku Earthquake off the Pacific coast. (Received 26 December 211, Accepted 13 February 212)