FIG. C-9. Common-midpoint method. (a) In six-fold shooting with 24-geophone groups and the source point moved two group intervals between successive shots; the same subsurface is sampled six times (A⇒23, B⇒21, C⇒19, D⇒17, E⇒15, F⇒13). (b) A reflector that dips does not have a common reflecting point and common-midpoint stacking results in reflection-point smearing unless DMO (q.v.) processing or migration precedes stacking. (c) To achieve a common-reflection point in the case of dip requires unequal surface spacing. (d) A common-source gather is a collection of traces having the same source; (e) common-receiver gather; (f) common-offset gather. (g) If there are horizontal velocity variations, prestack migration is required to form a common-imaging-point gather. Compare Figure C-13. All diagrams assume constant velocity.