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4D seismic is an established approach in monitoring hydrocarbon reservoir production carbon storage. To reliably detect and track changes in the subsurface, time-lapse seismic data should achieve a high level of repeatability during acquisition.
The repeatability is affected by many factors. The mispositioning – the distance between a source or receiver location of the baseline data and monitor surveys – is one of the key factors during acquisition and significant effort is usually applied to maintain it as low as possible. Repeatability might be drastically reduced by the mispositioning of just a few meters.
The borehole seismic with distributed acoustic sensing (DAS) with the permanently installed optical fibre in a well eliminates the mispositioning for the receivers. However, source points on the surface are still prone to misposition errors.
To study the effect of mispositioning, we designed a dedicated experiment and acquired a DAS VSP survey at the Curtin GeoLab facility using a vibroseis source. Pairwise comparison of 76 shot points spread from 0.1 to 25 m apart showed a significant repeatability decrease due to small positioning errors as well as a strong effect of rapid spatial near-surface variations on repeatability.