No doubt drug smugglers, as recently evidenced, would be willing to expend a lot of money and effort to tunnel under a fence since they will tunnel even when there is no fence so as to avoid detection. I don't know how effective sensors placed along a fence would be in detecting tunneling. And I don't know how deep a concrete foundation would have to be to deter all but the most resourceful tunnelers. But I am convinced that an Israeli-type fence would greatly diminish walk-across the border illegal immigrants. I'm open to suggestions for impeding the drug smugglers. Right now, I am most interested in ending the invasion of millions of illegal aliens.
Reinforced concrete footer that is wired for "sound" and "Shock therapy" should do the trick.