What technique inspired the surveillance of the Ho Chi Minh Trail?

Operation Igloo White's first acoustic sensors were developed from the US Navy's anti-submarine sonobuoys of Project Jezebel, which recorded and processed sound using an audio spectrum analyzer. These battery-powered acoustic and seismic sensors, dropped from the air, were camouflaged and released in strings over predetermined geographic points along the logistics network of the Ho Chi Minh Trail.

The seismic sensor sensed vertical ground movement through an internal geophone. It could determine whether what was moving was a man or a vehicle at 33 and 109 yards (30 and 100 m) respectively. Dropped by gravity, the stake-shaped devices drove into the ground like a lawn dart and stayed buried except for their antennas, designed to look like undergrowth.

The continuous air effort over the trail was directed from Nakhon Phanom, Thailand, under the name Operation Igloo White. The system consisted of three parts: the strings of acoustic and seismic sensors dropped from the air, which gathered information about the trail; the computers of the Intelligence Collection Center in Thailand, which compiled that information and predicted convoy routes and speeds; and a relay and control aircraft that received the sensor signals and directed strike aircraft to targets as indicated by the center.