Much work is now underway to find a source that can be used in the borehole. This would allow a multitude of geophones to be located on the surface. Such a reverse VSP would markedly reduce survey time and cost. By using multiple-source locations on the surface, one can generate a 3-D VSP; however, such surveys today would require so much time in the borehole that they are run only rarely, but they should be feasible once more effective downhole sources are developed. A borehole energy source must be nondestructive to the well. One scheme uses the vibrations of the active drill bit as the energy source; this involves continuous recording for several minutes and cross-correlation of geophone outputs with the signal that travels up the drill stem, somewhat similar to Vibroseis processing.