It is common in industrial and military scenarios for large tracts of real estate to be monitored for the illegal removal of equipment by intruders. Most commonly, closed circuit video cameras/recorders, pressure transducers, buried below ground radio wave cable based protection systems, microwave, laser, infra-red and ultrasonic surveillance systems are deployed to guard against this threat. They all work to some extent in providing protection, but cost escalates sharply as the area to be protected grows and the level of protection required is expanded. This invention focuses on the ability to provide a lower cost alternative to these options, for situations where a large tract of land has to be monitored for surface movement, i.e. either from human traversal or equipment movement into/out of the protected zone. The novel detection concept is based on the principle of electronically detecting the very sensitive perturbation of the intrinsic air mass within a closed length of small diameter flexible plastic tubing, arising as a result of physical vibrations being applied to the ground above the buried tubing. A mechanically modified electret microphone insert, coupled with electronic processing circuitry incorprating very low pass filtering and variable threshold discriminators, enables detection of the required signal only and rejection of unwanted audio level signals. This invention lays claim therefore to be able to detect at a relatively lower cost than current detection systems, utilizing relatively simple processing circuitry, very small changes in physical movement/vibration to the ground immediately above the buried plastic tubing. Unlike pressure transducers, this invention detection the very small shift in movement of the intrinsic air mass contained within a sealed length of flexible tubing. The shifting air mass occurs at a very low frequency. The claim is uniquely novel since no other like detection method is known to exist or be reported to exist.