磁環式FBG地層下陷感測系統之改良

Abstract

現有磁環式光纖光柵(Fiber Bragg Grating, FBG)地層下陷感測器利用光纖感測技術，具有體積小、重量輕、防水、防電磁波干擾、長距離傳輸低失真與分佈式量測等優點。但現有技術於深度量測上使用捲尺並以目視辨識刻度，且感測器之收放與讀數紀錄皆仰賴人力，不僅費時費力，亦易產生人為誤差。本研究改良磁環式FBG地層下陷感測系統之量測方法，藉由改善訊號傳輸線材、結合編碼器(encoder)與提升解讀儀反應譜之擷取頻率，使讀數紀錄自動化，並於2014年在雲林土庫國中安裝一30m磁環式地層下陷監測井以及一30m之FBG分佈式水壓與沉陷監測系統，使二者量測結果得以相互驗證。結果顯示上述改良項目大幅減少試驗所需時間並提升系統截取頻率進而降低深度解析度。但結果亦顯示重複性低之缺失，並測試將傳輸線與編碼器吊掛線分離並於大型結構實驗室進行試驗，試驗深度為6m，其標準差約為7mm。分佈式水壓與沉陷監測系統之量測結果則可用於比較沉陷量與孔隙水壓間之關係。

The currently available optic fiber Bragg grating (FBG) sensored magnetic ring ground subsidence monitoring system has the advantages of optical fiber sensor technology, that include small sizes, light weight, water resistant and immune to electromagnetic interference. In current design however, the depth readings are taken manually using a graded tape which is time consuming and prone to mistakes. In this research, a strengthened fiber optic cable combined with an encoder was used to for depth detection of the magnetic rings. The improved system has a much higher sampling rate and the data are recorded automatically. The new FBG sensored magnetic rings along with FBG piezometers were installed in a 30 m deep borehole at a test site in Tuku Junior High School in Yunlin, in 2014, for comparison studies. The results showed that the depth resolution and efficiency in field operation can be improved significantly. However, the new system still lacks desirable repeatability. An additional test conducted inside of the structural laboratory within a 6 m deep casing showed that the readings fluctuated with a standard deviation of 7 mm. The piezometers showed pore pressure profiles that could be used to evaluate the mechanisms of ground subsidence.