Semi-Remote Acoustic, Electric, and Thermal Sensing of Small Buried Nonmetallic Objects

Three geophysical methods of sensing and rapidly locating small voids or non-metallic artifacts in the top 50 cm of the soil, without physical contact, have been studied. The acoustic method employed a search head consisting of one to three loudspeakers, used C. W. either as sources or sensors at around 100 Hz, which were held within 5 to 25 cm of the surface. It apparently detected local variations in acoustic compliance. The electric resistivity method employed a field of alternating current injected into the ground outside the area to be searched, and a symmetrical-coil magnetic-gradient search head. Both methods utilized induced proximity effects which diminish rapidly with distance, but experiments indicated that they are capable of detecting nonmetallic objects buried as deeply as 25 cm in bare or grass-covered soil, without disturbing or touching the ground being searched. The thermal method employed a differential radiation detector operating at infrared wavelengths greater than 5 microns. It has detected localized thermal and moisture barriers under 12 cm of a uniform, bare soil, using the diurnal insolation cycle as an energy source.