In representative embodiments, a system includes an acoustic emitter, a controller, an optical sensor, and an indicator for indicating a detection of a predetermined trace chemical vapor by the optical sensor. The acoustic emitter is positioned at a predetermined distance above the ground surface and configured to project a beam of acoustic energy toward the ground surface with a variable angle of incidence α. The controller is configured to control the acoustic emitter to vary the angle of incidence of the acoustic beam within the variable angle of incidence α, while the optical sensor is configured for standoff sensing of a trace chemical vapor proximate the ground surface. Excitation of the ground surface, particularly at a critical angle of incidence β, causes the release of trace chemical vapors from a buried source and the soil into the air above the buried source.