Thermal strain sensing of optical cables using brillouin optical time domain reflectometry

Abstract

Recent advancement in distributed ?ber-optic sensing offers new possibilities for performance monitoring in the ?eld of geotechnical and civil engineering. Brillouin optical time-domain re?ectometry (BOTDR) is a commercially available technology that allows distributed strain measurements in the microstrain range along the full length of an optical ?ber. By integrating a single ?ber-optic cable into soil or a structure, an unprecedented amount of reasonably accurate (630 le), spatially resolved data could be obtained. Since the BOTDR data is in?uenced by both strain and temperature, it is important that methods to separate the two effects are fully understood. This paper describes the BOTDR temperature compensation method by implementing appropriate thermal expansion coef?cients of optical cables and structures to the raw data. In the laboratory study, validation of the instrumentation technique was conducted in a concrete beam by embedding two types of optical cables consisting of tight-buffered and loose-tubed coatings to measure thermal strains response during concrete curing. Temperature readings inferred from optical ?bers were found to be in accordance to the thermocouples. A ?eld study of axially loaded concrete pile subjected to cooling and heating cycle is presented. Measurements in the test pile and adjacent borehole indicate similar strain pro?les and temperature changes between BOTDR and conventional instrumentation such as vibrating wire strain gauges and thermistors. General steps to derive the temperature compensated strain pro?les observed in the thermal pile as a result of cooling and heating is presented. The data enables load-transfer pro?les to be interpreted and used as framework to understand pile response to temperature changes