The Design and Construction of an Acoustic Imaging System for Imaging Underground Facilities

As underground hidden facilities are used for various applications, imaging systems are requiredfor the detection and recognition of these facilities. In this paper, imaging by mechanical wavepropagation in the soil is investigated. Most underground facilities can be considered as airchambers located in the middle of the soil. Air and soil have a large difference in acousticimpedance, so underground facilities can produce a large reflective signal because the amplitudeof the reflected signal depends on the difference in impedance of the two materials. Seismic andnon-destructive concrete testing systems also use the process of propagation of mechanicalwaves in the material. Although seismic systems have been successful in the detection issue,they require several meters of space for equipment layout, and transportation complexity is alsoone of their problems. They are unsuitable for urban space because they are inherently designedto detect water sources at depths of several hundred meters. On the other hand, while thenon-destructive concrete testing system equipment have suitable dimensions, they have lowpenetration depth which is impractical for this purpose. Seismic and non-destructive testingsystems of concrete operate in the range of subsonic and ultrasonic waves, respectively. Thisresearch project proposes the idea that by choosing an operating frequency between the twomentioned ranges, it is possible to obtain equipment with the appropriate dimensions forimaging underground facilities. In this project we managed to image an undergroundconstructed cylindrical cavity with the diameter of 1 meter and the depth of 4 meters. Theaccuracy of this method depends on the sound propagation speed in the material. Since there areaccurate relationships for the propagation speed of sound in air, the diameter of the cylinder wasestimated with an accuracy of about 4%, but with the measurement of propagation speed beinginfeasible in this project, leading to the ambiguity in the value of propagation speed of sound insoil, the values cited in several reference sites were used, giving the accuracy of 4 to 20%.