Triggering for geophone triangulation
The more interesting plot would be what the signal from each geophone looks like if the whack is centered.  In other words, what is the variation the algorithm has to contend with?

If the nature of the whack and the sound transmission is reliable in that each waveform has about the shape you show, then it shouldn't be that hard.  You could pick off the time of the first negative peak below some value, or maybe the time of the positive zero crossing following it.

In any case, I really wouldn't do this with hard wired logic.  This looks like a job for a micro with a fast sampling A/D that can capture multiple channels at the same time.  Some of the dsPIC 33 or PIC 24H might be appropriate.  That processor can do the high speed analysis and report event detection and relative delays between channels to the higher level processor.