Coherent optical time domain reflectometry: the theoretical basis for distributed acoustic sensing technologies

Matthew Eaid, Kristopher A. Innanen

The applications of distributed acoustic sensing (DAS) fibres for seismic acquisition, reservoir monitoring, and smart city applications continues to expand. As the interest in utilizing optical fibres for seismic acquisition continues to grow, it becomes increasingly important to understand the underlying technology that makes them effective. Understanding the theoretical means by which DAS fibres acquire seismic data allows for an improved understanding of their limitations and opportunities, and permits better decision making in how they are deployed. A grasp of the theory behind DAS technology also aids in the interpretation of the data, and trouble shooting when challenges arise. The way in which a given interrogator unit converts disturbances along the fibre to seismic strain varies and is typically proprietary. However, most implementations are based on a variant of coherent optical time domain reflectometry (COTDR), a technology used by telecommunications companies to detect flaws in installed optical fibres. In this short note we cover the topic of COTDR and discuss how it has been adapted for use in the acquisition of strain data resulting from seismic wave propagation.