Passive detection of buried structures using elastic waves

Abstract

An investigation of the feasibility of detecting structures buried underground through passive listening techniques will be presented. Passive detection of structures will be analyzed using elastic wave sources originating inside the structure and from sources exterior to the structure and on the surface. The primary method of investigation will be numerical models using the finite-difference time-domain method (FDTD).A source inside the structure excites elastic waves in the structure, a portion of which travel upward along the walls of the structure and onward to the surface. An alternate form of excitation is a source such as a train, large vehicle, or an explosion located on the surface, away from the structure. Waves from this source interact with the structure and a portion of them travel up from the structure to the surface.An array of sensors is constructed to map the field at the surface and to determine the location and basic characteristics of the structure. Generally, structures examined will be on the order of the size of an underground tunnel complex or buried room and elastic wave sources will be in the low frequency range of large machinery or vehicles.

Feasibility study of the automated detection and localization of underground tunnel excavation using Brillouin optical time domain reflectometer

“…The analysis of the developed strains in the fiber is based on the following assumptions: [1] The fiber is always in contact with the soil, [2] the soil (in the vicinity of the fibers) is homogenous linear elastic continuum, [3] the tunnel is unaffected by the existence of the fiber (i.e. unaware of soil-fiber interaction).…”