"Are we installing electronic surveillance/monitors as well?" Yes. Buried seismic sensors, often paired with cameras, detect when a person (or animal, car or even a low-flying plane) is moving in the area, and the sensors ideally can differentiate among those things. The sensors ping border agents in a control center so they can take a look at camera footage of the area. Other aboveground sensors and ground-penetrating radar are used to find tunnels.