Wavelength Characteristic Analysis of a FBG Array Embedded in Quaternary Unconsolidated Strata during a Deep Borehole Installation

Abstract

Real-time monitoring of settlement and deformation within a coal mines deep quaternary unconsolidated strata presents challenges with installation and signal analysis. This paper presents results from successfully installing a field-scale fiber Bragg grating (FBG) sensing system in a deep borehole for the purpose of achieving real-time monitoring of the settlement and deformation in a deep unconsolidated stratum. A 152m deep by 133mm diameter borehole was used to embed an array of 24 FBG sensors in 12 layers of gravel and clay from between 92.4m and 148.7m of unconsolidated quaternary strata. A wavelength bandwidth of 4.5nm was used with a wavelength division multiplexing and spatial division multiplexing technique to compose a 4-by-6 sensor array. During the four stages of installation, the real-time transmission characteristics and the changes in the FBG wavelength for this sensing system were evaluated. While the FBG sensing system was stable after installation, it was clearly shown that the engineering techniques associated with both positioning and grouting influenced the mechanical properties and transmission characteristics of the system. After installation, the sensor survival rate was found to be 78.26 with a maximum FBG wavelength shift of 1.447nm. This field-scale installation has provided information and experience that will improve future installations of buried fiber optic sensing technology throughout the underground coal mine industry.