Usually, knowledge of where the sources and streamers were at a given shot instance is much more precise than the ability to control where they should have been. This implies that the operational ability to hold a predefined course for streamers and sources can be challenging even for systems with steerable devices (Brown and Paulsen, 2011). Normally, shot location is more easily repeatable than sensor location, even with steerable streamers. Four-dimensional processing therefore needs to include interpolation onto a common grid.
However, conventional data typically are sampled inadequately in the crossline direction, resulting in the leakage of aliased energy. It is now widely recognized that repeating source and receiver positions from the baseline survey is of utmost importance to minimize time-lapse background noise.
Landrø (1999) also finds that overburden heterogeneity has a significant impact on repeatability from analysis of a variogram of trace-to-trace differences against source-separation distance in a 3D VSP common-receiver gather. Landrø (1999) asserts that for a homogeneous earth, shot location should not matter, and recorded traces should be the same after allowing Kurt Eggenberger, Philip Christie, Dirk-Jan van Manen, and Massimiliano Vassallo, Schlumberger for different path lengths. The fact that there were differences between traces, that the differences were a function of shot separation, and that the variance spatially correlated with shallow overburden heterogeneity supported the inference that overburden heterogeneity limits the ability to repeat time-lapse seismic data and to deliver 4D signals with low background noise levels. Calvert (2005), Smit et al. (2005), Misaghi et al. (2007), and Naess et al. (2007) further discuss this issue in detail.