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Figure 1 – The even-integer rule for onshore 3-D seismic design. This rule defines what the in-line and cross-line dimensions of a 3-D recording swath should be, so that 3-D stacking fold has a constant integer value across most of a 3-D acquisition grid. The rule specifies that the in-line dimension of a recording swath should span an even number of source-line spacings. This geometrical constraint causes the in-line stacking fold FIL to be a whole number, not a fractional number. A fractional value for FIL causes stacking fold oscillations in the in-line direction; a whole number value creates a constant in-line stacking fold. The even-integer rule further specifies that the cross-line dimension of the recording swath should span an even number of receiver lines. This constraint causes the cross-line stacking fold to be a whole number, not a fractional number, and results in a constant, non-oscillating integer-value stacking fold in the cross-line direction.