DEL RIO, Tex. — In this tiny town at the edge of South Texas brush country, a black steel fence runs along the Mexican border. About 15 feet high and topped by small metal spires that bend toward Mexico, it stretches west from a Border Patrol checkpoint for about a mile and a half. Then it stops.

Where the fence ends, a creek snakes off the Rio Grande, up through a private ranch where cattle wander between the mesquite trees. There, beside the creek gully, a small black cylinder sits atop a metal pole, looming over the small trees. It is a lidar sensor, the same laser-based technology that gives sight to self-driving cars. From its perch, it captures a three-dimensional image of anyone who walks into the area.

The sensor belongs to a Silicon Valley start-up called Quanergy, one of a number of companies trying to land business along the Mexican border. While President Trump still vows to build a physical border wall, these companies hope to help build a “virtual wall” that some policymakers believe would be more effective than miles of concrete and metal. A virtual wall would not offer a physical deterrence, but some people believe it would be cheaper to build and maintain.

“The only way to have operational control of the border is to look at all 2,000 miles of it at the same time,” said Representative Will Hurd, a Republican who represents Val Verde County, west of San Antonio. “And the only way to do that is through technology.”