Remote acoustical reconstruction of cave and pipe geometries

Abstract

Military and commercial interests desire to determine the geometry of caves, tunnels, and piping systems remotely. Advance knowledge of the geometry would allow military personnel to focus on the mission objectives rather than on cave exploration and mapping. Commercial applications of remote geometry mapping include determining locations of collapsed mines and tunnels for search and rescue missions, and determining locations of breaches or constrictions in pipelines. This research effort begins to extend acoustic pulse reflectometry to these applications and only requires simple measurements made at the waveguide entrance. This implementation first measures the acoustic reflection coefficient using a modified form of the two-microphone method and computes the waveguide impulse response. Then the system uses an inverse-scattering algorithm to solve for the cross-sectional area as a function of distance. Results are shown for a simple area expansion attached to a laboratory impedance tube with a diameter of 35 mm, and for a pedestrian tunnel with a nominal square cross-section of 5.9 m2. The impedance tube estimates match the known cross-sectional areas to within 10%. The results from the pedestrian tunnel show greater uncertainty due to frequency range limitations on the measurements, but can still capture important features of the geometry.