they could use seismic sounding techniques to find the tunnels

yes, they could do that, but it would take ages to do seismic surveys all over Gaza (it's a big fucking place!) even in the best of conditions and those surveyors could be captured/killed by Hamas in that time (justifiably, I think, in this situation). Also, finding the exact location of voids is hard even if you know where they are in advance, and when they're like within 10 feet of the surface. There's a reason why tunnels continue to be a problem for cross-border smuggling, as well as the odd prison escape, and those tunnels aren't usually 60 feet underground.