Development and feasibility assessment of the subsidence monitoring using Time Domain Reflectometry technology
|關鍵字:||地層下陷;時域反射方法;subsidence;time domain reflectometry method|
Monitoring methods for land subsidence are typically classified into underground monitoring and surface monitoring. Current monitoring practice in Taiwan is mainly via magnetic ring monitoring well (underground) and continuous Global Positioning System (GPS) measurement on fixed station (surface). The main purpose of this study is to improve the shortcomings of existing land subsidence monitoring techniques, such as single depth measurement limitation for a monitoring well, lack of automated monitoring potential, measurement accuracy inconsistencies due to operator errors. Therefore, this study developed a pilot land subsidence monitoring approach based on Time Domain Reflectometry (TDR), which aimed at automated subsidence monitoring, multiple depth measurement for a monitoring well, monitoring components with high accuracy and reproducibility. TDR subsidence meter (TDRSM) are comprised of metal casing, waveguide cable, and sensing ring. Initial prototype of TDRSM is to adopt the concept of an enlarged coaxial cable. By using coaxial and metal casing as transmission medium, when electromagnetic (EM) waves travel from transmission segment to sensing segment, reflection is induced at the sensing ring due to characteristic impedance drop caused by the cross-sectional area reduction of the outer conductor’s inner diameter (ID). Observing captured waveform from TDR instrument, strong reflection signals are found at the sensing segment and settlement amount can be deduced from the waveform variation. In the event of land subsidence, subsidence amount can be deduced quantitatively from the waveform offset differential analysis between the initial and current characteristic sensing waveforms. Selection of TDRSM component would influence the reflection signal intensity of the sensing ring and its corresponding analysis approach. This study adopted metal casing (fixed ID of 78 mm) as the outer conductor, whereas the optimal configuration of TDRSM is explored from the combination of inner conductor and sensing ring. Configuration of metal rod as inner conductor generated clear reflection signal in air but attenuated rapidly in water. Extra metal threads are required on the rods to facilitate extended connection on site. However, no connection issue would be encountered by using cable as inner conductor instead, with added benefits of rusting protection and signal transmission length increment due to the cable coating layer. This study hence selected aluminum casing in P3-500 coaxial cable as the inner conductor, reflection signals obtained in both air and water are sufficiently strong for waveform analysis. Sensing ring material and dimension combination are determined from the best configuration which produced good reflection signal and could move vertically with the subsided strata. This study examined the configuration using metal ring (ID of 25mm, 23mm, 20mm and 16mm) and plastic ring (ID of 21mm and 18mm). After series of experiment, best sensing ring configuration is found as metal ring with ID of 23mm. With additional sensing rings, total travel time would increase as a result of slight variation in EM wave velocity after passing the sensing rings. From waveform offset differential analysis between initial and current characteristic sensing waveforms, the deviated amount of the characteristic waveform are computed. In case of actual sensing ring offset by 3 cm, TDRSM measured data resulted in average of 2.92 cm with standard deviation of 0.04223 cm.