Fiber Optic Depth Mapping Refinement Using Acoustic Velocity Profiling

Abstract Accurate depth mapping of downhole fiber-optic sensing channels is a key part of interpreting distributed sensing data. Because the deployed fiber rarely lies perfectly straight, the actual fiber position often drifts from the nominal measured depth. Buckling, slack redistribution, bending around curvature, and non-uniform coupling all introduce extra fiber length, causing the channel spacing along the fiber to differ from the true spatial spacing along the well. Here, we describe a straightforward way to refine the depth mapping profile by comparing two acoustic velocity estimates along the well. One is the thermodynamic speed of sound of the wellbore fluid, calculated from the local fluid pressure, temperature, and composition. The other is an apparent velocity obtained directly from DAS data. To obtain the latter, we apply noise interferometry, similar in principle to Green's function retrieval in seismology, by cross-correlating DAS measurements between different channels using a phase-weighted approach. The resulting travel-time estimates yield a channel-by-channel acoustic velocity profile. Where the DAS-derived velocity differs from the thermodynamic sound speed, we interpret the deviation as a geometric effect. In other words, the fiber is taking a slightly longer or more tortuous path than the wellbore centerline. The ratio of the two velocity profiles gives a local correction to effective channel spacing. We tested the workflow on synthetic injection-well data where compressive loading was expected to buckle the fiber deployment string over certain intervals. The DAS-derived wave velocity captured these variations reasonably well and allowed a continuous depth-correction trend to be recovered. The method is applicable to behind-casing installations, coiled-tubing-deployed fibers, and wireline interventions. It is most sensitive to deformation over meter-scale lengths; sub-meter variations remain below practical resolution. The practical benefit lies in improving the spatial accuracy of DAS interpretations, whether for flow profiling, seismic imaging, or thermal analysis, after the fiber has already been placed. Importantly, the correction can be applied locally wherever data quality is sufficient and leverages acoustic content already present in standard DAS datasets as an indicator of the fiber's mechanical condition.