Effects of Source Mispositioning on Repeatability for Onshore Das VSP Data

Summary 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.