Why don't we just put ground sensors all around the border fence and save that 10 billion dollars for the american people, not a wall construction company
why would someone dig a tunnel if there's no wall
seismometers are exponentially more effective if there's an obstacle in the way i.e. in a tunneling situation where there is a wall above ground and multiple cubic meters of dirt to dig through.
The sensors would give much more advanced warning whereas depending on response times, if the sensors were installed without a wall, someone could probably just run across and get a considerable distance from the border itself before a response could be made.