A summation of some of the traces in an upgoing vertical seismic profile (VSP) that has been processed to retain only primary reflection events and that has been time shifted to their two-way arrival times at the surface.
Summation is over a data window (a corridor) beginning at the first-break time and usually ending 200–300 ms later. This eliminates most multiples.
This is also called an outside corridor, and this is meant when no modifying adjective is used. The portion of the VSP remaining after the outside corridor has been removed is sometimes stacked to yield an inside cooridor stack that emphasizes multiples. See Figure C-16.