光纖光柵震測水壓圓錐貫入儀之改良

Abstract

光纖光柵擁有不受水及電磁波干擾、可在單一條光纖上做多點分布感測且可遠距離傳輸後訊號衰減非常低等優點，在某些訊號須要遠距離傳輸、高電磁波干擾及高水壓的試驗環境，光纖光柵感測器具較高的穩定性。 本研究主要目的為改良既有一光纖光柵震測水壓圓錐貫入儀(SCPTU)系統，主要訴求可分為微型化光纖光柵水壓計之改良、光纖光柵加速度計之研發、貫入深度量測系統之建立以及解讀系統之整合。當中，改良後之微型化光纖光柵加速度計具有良好的重複性及靈敏度；力矩式光纖光柵加速度計具有較高的靈敏度；且利用光耦合器、光隔絕器以及光訊號放大器，使解讀系統在較高的擷取速度和多個光柵感測器串接下，光訊號強度仍足以辨識，最後進行現地試驗的測試，並與電子式圓錐貫入儀作比較。 光纖式圓錐貫入試驗因受解讀系統的限制，雖加入溫度量測以消除溫度造成的誤差，但因無法消除偏心效應以致量測結果不甚理想。震測試驗雖亦受設備限制，然而本研究研發之光纖光柵加速度計在訊號無須疊加的情況下，於深度17公尺仍可感測到震波，靈敏度大幅提高。

Optical fiber Bragg grating (FBG) has the advantage of being immune to moisture and electromagnetic interference (EMI), multiple sensors sharing a common fiber, and capable of long distance data transmission with limited signal degradation. FBG sensors are durable in applications where the sensors are subject to strong EMI and high water pressure and long distance data transmission. The goal of this research is to improve a previously developed FBG based seismic piezo cone (SCPTU). Major items include the improvement of the miniaturized piezometer, the FBG seismometers and integration of the penetration depth sensor with the data logging system. The modified miniature piezometer showed desirable repeatability. The moment type BG seismometer showed significantly improved sensitivity. With the help of optical coupler, isolator and laser ring amplifier, the strong and stable optical signals can be obtained at high frequencies and when multiple FBG’s are connected to a single optical fiber. The modified FBG SCPTU was tested in the field and its results compared with those from conventional electric SCPTU. Due to limitations of the FBG readout unit, the temperature compensation alone was not sufficient in offsetting errors caused by eccentric bending of the cone. Seismic tests using FBG seismometers were also hampered by the data logging system. The wave spectrum from the FBG seismometers could not be accumulated. However, the FBG seismometers, with their significantly improved sensitivity, were able to detect the shear wave arrival up till the depth of 17 m without signal accumulation.