"Fold" in Seismic Reflection Surveying

The fold refers to the number of times a particular subsurface point has been sampled. It is equal to the number of traces in the CMP gather and is numerically evaluated by,

where n is the moveup rate in units of geophone spacing. "Moveup rate" is in fact (shot spacing)/(geophone spacing). For example, if geophones are 2m apart and shots are employed every 4 metres, then the moveup rate is n=4/2=2. This can be less than one if there are shots set more often than geophone spacing, a practice that is sometimes done in marine seismology, especially ocean bottom profiling.

The schematic below shows a single ended spread with 8 geophones and moveup rate of n=2. 

We see that each point in the subsurface is sampled only twice. Notice that the distance between the reflection points in the subsurface is half the geophone spacing.