Mitigating Integration-Induced Displacement Drift in Distributed Fibre Optic Inclinometers Using an FBG Tiltmeter

Distributed fibre optic strain sensing (DOFSS) is increasingly adopted for inclinometer-based subsurface deformation monitoring due to its capability to provide continuous strain measurements along extended sensing lengths. In such systems, lateral displacement profiles are reconstructed through numerical integration of curvature derived from differential axial strain measurements. However, this strain-based reconstruction is inherently susceptible to integration-induced displacement drift arising from insufficiently constrained angular boundary conditions during double numerical integration. This study is the first to explicitly identify, explain, and experimentally mitigate integration-induced displacement drift in strain-based distributed fibre optic inclinometers using an independent angular reference. Combined field and laboratory evidence is presented to demonstrate systematic displacement drift caused by cumulative error propagation in strain-based reconstruction. Field monitoring at an active landslide site in Malaysia reveals progressive divergence between strain-reconstructed displacement and conventional inclinometer measurements, despite stable and internally consistent distributed strain responses, indicating that the observed drift originates from a fundamental limitation of the reconstruction methodology rather than sensor instability. To address this limitation, a hybrid measurement framework integrating a Fibre Bragg Grating (FBG) tiltmeter as an independent angular boundary reference is proposed and experimentally validated. Laboratory results demonstrate that incorporating an absolute, temperature-compensated angular constraint effectively suppresses integration-induced displacement drift while preserving the distributed deformation characteristics captured by the DOFSS system. These findings demonstrate that independent angular referencing is essential for reliable displacement reconstruction in strain-based distributed fibre optic inclinometers and provide a generalisable measurement strategy for long-term field deployment under complex deformation conditions.