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A experimental Feasibility of Magnetic Resonance Based Monitoring Method for Underground Environment

지하 환경 감시를 위한 자기공명 기반 모니터링 방법의 타당성 연구

  • Ryu, Dong-Woo (KIGAM (Korea Institute of Geoscience and Mineral Resources)) ;
  • Lee, Ki-Song (Chungbuk National University) ;
  • Kim, Eun-Hee (ETRI (Electronics and Telecommunications Research Institute)) ;
  • Yum, Byung-Woo (KIGAM (Korea Institute of Geoscience and Mineral Resources))
  • Received : 2018.10.15
  • Accepted : 2018.12.10
  • Published : 2018.12.31

Abstract

As urban infrastructure is aging, the possibility of accidents due to the failures or breakdowns of infrastructure increases. Especially, aging underground infrastructures like sewer pipes, waterworks, and subway have a potential to cause an urban ground sink. Urban ground sink is defined just as a local and erratic collapse occurred by underground cavity due to soil erosion or soil loss, which is separated from a sinkhole in soluble bedrock such as limestone. The conventional measurements such as differential settlement gauge, inclinometer or earth pressure gauge have a shortcoming just to provide point measurements with short coverage. Therefore, these methods are not adequate for monitoring of an erratic subsidence caused by underground cavity due to soil erosion or soil loss which occurring at unspecified time and location. Therefore, an alternative technology is required to detect a change of underground physical condition in real time. In this study, the feasibility of a novel magnetic resonance based monitoring method is investigated through laboratory tests, where the changes of path loss (S21) were measured under various testing conditions: media including air, water, and soil, resonant frequency, impedance, and distances between transmitter (TX) and receiver (RX). Theoretically, the transfer characteristic of magnetic field is known to be independent of the density of the medium. However, the results of the test showed the meaningful differences in the path loss (S21) under the different conditions of medium. And it is found that the reflection coefficient showed the more distinct differences over the testing conditions than the path loss. In particular, input reflection coefficient (S11) is more distinguishable than output reflection coefficient (S22).

도시 기반시설이 노후화됨에 따라 도시 재난 발생 가능성이 증가하고 있다. 특히, 하수관로, 상수도관망, 지하철 등 노후화된 지하 시설물은 도심지 지반함몰을 유발하는 잠재적 원인이 된다. 도심지 지반함몰은 토양 침식 혹은 유실로 인해 생성된 지하 공동이 확장하여 지역적이고 갑작스런 지반 붕괴까지 이르는 현상으로 정의할 수 있다. 이는 석회암과 같은 용해성 암반에서 발생하는 싱크홀과는 구분된다. 지반 거동과 관련된 전통적인 계측 방식은 좁은 측정 범위와 각 센싱 지점에서의 계측값을 제공하기 때문에 불특정 다수 지역에서 발생할 수 있는 지반함몰 감시체계로서 한계가 있다. 따라서, 도시에 발생하는 지하 공동에 의한 지반함몰을 예방하기 위한 감시체계로서는 적절하지 않으며 지반 내 물리적 환경변화를 감시할 수 있는 새로운 상시 영역 감시 기술이 필요하다. 본 연구에서는 비방사 유도 자기장(자기공명) 기반 감시 체계의 기술적 타당성을 실험적으로 검토하였다. 공기, 물, 흙 등 매질과 공진 주파수, 임피던스 그리고 송 수신기 거리 등과 같은 환경변수에 따른 경로 손실 변화를 측정하는 방식으로 이루어졌다. 이론적으로 자기장의 전달 특성은 매질의 밀도와 독립된 것으로 알려졌으나, 실험 결과 매질의 조건에 따라 경로 손실에 의미있는 차이를 보이는 것으로 나타났다. 또한, 매질의 물리적 환경변화에 따라 경로손실보다는 반사계수가 명확한 차이를 보였으며, 입력 반사계수가 출력 반사계수에 비해 보다 판별이 용이한 것으로 나타났다.

Keywords

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Fig. 1. System model for a magnetic resonant WPT system

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Fig. 2. Equivalent circuit of a magnetic resonant WPT system

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Fig. 3. MI coil deployment in the underground environment (Tan et al., 2015)

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Fig. 4. Received signal strength in underground environment with different VWC (Tan et al., 2015)

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Fig. 5. Helical type coils and soil tank for the test

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Fig. 6. S21 parameter against distance between antenna coils, frequency and medium

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Fig. 7. Distribution of magnetic fields for the magnetic resonant wireless power transfer (WPT)

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Fig. 8. Relative positions of soil tank to antenna coils

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Fig. 9. Variations of S11 and S22 parameters over distance at f=4 MHz

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Fig. 10. Variations of S11 and S22 parameters over distance at f=6.78 MHz

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Fig. 11. Variations of S11 and S22 parameters over distance at f=12 MHz

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Fig. 12. Variations of S11 and S22 parameters over distance at f=4 MHz

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Fig. 13. Variations of S11 and S22 parameters over distance at f=8 MHz

Table 1. Classification of subsidence

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Table 2. Magnetic permeability and conductivity of various materials

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Table 3. Comparison of power transfer efficiency at air and salt water over frequency range (Askari et al., 2015)

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Table 4. S21 values according to relation position of soil tank and frequency

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Table 5. Requirements for the magnetic resonance-based ground sink monitoring system

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