The Reconstruction of Robust Shallow Velocity Profile using a Disposable Fibre

Summary Determining accurate near-surface velocity models in exploratory areas is quite challenging. It often requires on-demand high-resolution seismic refraction seismic experiment and/or drilling and logging numerous upholes, for near-surface velocity model building. We capitalised on the recent advancements in Distributed Acoustic Sensing (DAS) technology and used a pre-drilled water-supply well in an exploratory area to reconstruct a robust and cost-effective near-surface velocity model. A disposable fibre-optic cable was unspooled inside the water-supply well to a depth of approximately 479 meter in order to record the seismic wavefield along 471 densely spaced points along the fibre. This enabled the acquisition of a DAS Vertical-Seismic-Profiling (VSP) survey that delivers superior sampling compared to conventional uphole surveys. We extracted a large ensemble of first-break traveltimes from all gathers using all sweeps across 22 shot point positions centred around the water-supply well. We used a least-squares tomographic inversion, which yields a robust shallow near-surface model. The final model was constructed from inverting approximately 25,700 traveltime picks, and it revealed key lithological horizons that correspond to abrupt rate-of-penetration (ROP) sudden changes while drilling.