95 4. Near-surface reflection surveying a b Figure 4.4 3D land seismic acquisition (after J. Meunier, 2004, IFP School course). (a) Cross-spread build up: Cross-spread set of seismograms for which source positions belong to the same source line, and receiver positions belong to the same receiver line. (b) Stacking fold: number of overlapping crossspreads (G. Vermeer). The data are correctly sampled if the geophone interval i is sufficiently small (several meters) to avoid spatial aliasing. In 3D acquisition, it is necessary to use telemetric recording systems to simultaneously record several thousands of traces (an elementary shot being composed of several lines of receivers and several hundred receivers per line). The undersampling in distance can be done by applying a wave number filter in processing. Another solution is to use field arrays of sensors and a specific acquisition design called stacked array geometry introduced by Anstey (1986). For 2D surveys, a common midpoint can be viewed as a spatial filter, which is the convolution of a receiver array and a stack array. The geometry of acquisition must respect the following rules (Figure 4.5): • Shot points (SP) should be recorded with a symmetric split dip spread, • Source and receiver intervals should be the same,
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