Fiber optic shape-sensing, considerations for geophysical applications

The emergent technology of Fiber Optic Shape Sensing (FOSS) allows remote and instantaneous monitoring of the path taken by a structure or instrument. As this technology is an important component in medical instruments, robotics and civil engineering for structural health monitoring, we are introducing the technology to the geophysical community and sketching the outlook of the technology in our fields. In this talk, we introduce the basic theory of FOSS, from the derivation of curve variables from strain measurement in multi-core fibers to path-reconstruction numerical methods. These methods are established in the aforementioned fields thus only a general overview will be provided [1, 2, 3, 4, 5, 6, 7, 8]. The point of interest for this talk is to discuss the factors which must be considered to deploy this technology in geophysical and geotechnical applications. Specifically, we are interested in combining extended sensing ranges and performance in a highly dynamic environment using reliable and repeatable FOSS procedures and sensor designs. The guidelines we have established to this end include: The selection of appropriate interrogator technologies, including considerations for gauge pitch regarding sampling densities for shape reconstruction numerical methods as well as considering discrete vs. distributed strain-sensing methods. The selection of sensor geometries including the number of cores, the distance of the radial cores from the neutral axis and the cross-section of the sensor. The inclusion of special features such as a central core to mitigate environmental noise and hellically-wound fibers to model twist. The selection of an embedding method of the sensor into a structure or instrument depending on the environment of deployment and applications. The strategies available to locate and orient the reconstructed path in the global frame of reference and their advantages to quantify shape-sensing error. As we believe that FOSS may become an attractive solution to shape-sensing problems in geotechnical and geophysical applications, we will conclude by presenting some prospect applications of FOSS in these fields such as: real-time borehole orientation tracking during log-data acquisition; tracking topography for near-surface characterisation; waves, currents and flow tracking and subsidence and OBN depth monitoring. [1] Omar Al-Ahmad et al. “Improved FBG-Based Shape Sensing Methods for Vascular Catheterization Treatment”. In: IEEE Robotics and Automation Letters 5.3 (2020), pp. 4687–4694. doi: 10.1109/LRA.2020.3003291. [2] Moe Amanzadeh et al. “Recent Developments in Fibre Optic Shape Sensing”. In: Measurement 128 (June 2018). doi: 10.1016/j.measurement.2018.06.034. [3] Fouzia Khan et al. “Multi-Core Optical Fibers With Bragg Gratings as Shape Sensor for Flexible Medical Instruments”. In: IEEE Sensors Journal 19.14 (2019), pp. 5878–5884. doi: 10.1109/JSEN.2019.2905010. [4] Evan M. Lally et al. “Fiber optic shape sensing for monitoring of flexible structures”. In: Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2012. Ed. by Masayoshi Tomizuka, Chung-Bang Yun, and Jerome P. Lynch. Vol. 8345. International Society for Optics and Photonics. SPIE, 2012, 83452Y. doi: 10.1117/12.917490. url: https://doi.org/10.1117/12.917490. [5] Christoph M. Monsberger and Werner Lienhart. “Design, Testing, and Realization of a Distributed Fiber Optic Monitoring System to Assess Bending Characteristics Along Grouted Anchors”. In: Journal of Lightwave Technology 37.18 (2019), pp. 4603–4609. doi: 10.1109/JLT.2019.2913907. [6] Christoph M. Monsberger and Werner Lienhart. “Distributed Fiber Optic Shape Sensing of Concrete Structures”. In: Sensors 21.18 (2021). issn: 1424-8220. doi: 10.3390/s21186098. url: https://www.mdpi.com/1424-8220/21/18/6098. [7] Paul Westbrook et al. “Distributed sensing over meter lengths using twisted multicore optical fiber with continuous bragg gratings”. In: Furukawa Review (Mar. 2017), pp. 26–32. [8] Cheng Xiaomin, Wang Mingjun, and Yi Xinhua. “Deformation sensing of colonoscope on FBG sensor net”. In: TELKOMNIKA : Indonesian Journal of Electrical Engineering 10 (Dec. 2012). doi: 10.11591/telkomnika.v10i8.1693.