Summary
For 4D acquisition with buried receivers we propose a simple and robust 4D binning scheme based on direct early arrivals. With buried receivers, the near-field downgoing energy can be recorded. Shots with poorly repeatable early arrivals are rejected to exclude gathers with the most unrepeatable reflections. The method has been applied to a field 4D dataset from Saudi Arabia with 11 repeat vintages. We confirm that both image quality and repeatability can be improved.
Introduction
For marine acquisition, seismic repeatability is often tied to reproducing geometry of the shots and/or receivers (Calvert, 2005). On land, there are other significant sources of non-repeatability (in addition to geometry) that are not present in marine environments (Jervis et al., 2012). In this study, we focus on buried receiver acquisition with surface vibroseis sources (Bakulin et al., 2012). While there are some geometry errors associated with repositioning surface vibrators, the tolerances are much smaller than in marine surveys (typically around 1-2 m). Attempts to see if geometry-based rejection may improve repeatability were not very successful. It turns out that the benefit of data rejection was quickly outweighed by reduction in fold, leading to deteriorating signal to noise ratio (SNR) and thus repeatability. Nevertheless, other factors related to variable source coupling and near-surface variations still remain significant sources of non-repeatability on land data despite well repeated shot geometry. Unlike acquisition geometry, these factors are hard to quantify based on simple metrics as generally they require assessment of the pre-stack traces, which have notoriously poor SNR in the Arabian Peninsula. For buried receiver data we have the luxury to record the downgoing arrivals that are used to illuminate the reservoir. The correlation between repeatability of these early arrivals and deep reflection data were reported in a previous study (Bakulin et al., 2014). Here we make use of this relationship and design a rejection scheme based purely on the pre-stack direct arrival NRMS and demonstrate that it can improve repeatability of the imaged reflection data.
