Vibration detection method for optical fibre pre‐warning system

The measurement of optical fibre vibration is a key part of optic fibre pre‐warning system, which has gradually focused on phase‐sensitive optical time‐domain reflectometer. However, for this instrument, false alarm rate is very high and some unstable intrusion signals cannot be detected by using its fixed threshold method in the actual application. It needs to develop new vibration detection method to overcome the above defect. The vibration signals normally consist of three parts, that is, noise, interference and intrusion signals. After a large number of data analysis, the authors find that the system noise is time varying and follows the Rayleigh distribution. Hence, the authors innovatively use the constant false alarm rate (CFAR) method to detect this type of intrusion. Considering interference is also time varying and diverse, a good detection performance cannot be obtained only by using the conventional CFAR. For this reason, a background homogeneity adaptive CFAR (BHA‐CFAR) method is further proposed to detect the vibration signals in this study. The BHA‐CFAR consists of two detectors, cell averaging CFAR (CA‐CFAR) detector and greatest‐of/smallest‐of CFAR (GO/SO‐CFAR) detector. A parameter, homogeneity of background, is estimated first to classify the surrounding. Then CA‐CFAR and GO/SO‐CFAR are optionally used according to the surrounding is homogeneous or heterogeneous, respectively. This new detection method can adapt to any background surrounding and has a good detection performance. In order to check the feasibility and validity of the BHA‐CFAR method, several experiments were carried out in Da Gang oilfield. The detection results show that the proposed method can provide a good tradeoff between the detection performance and computation time.