Time-Exposure Acoustics for Imaging Underground Structures

We propose to further develop a new technique for imaging underground facilities based on the passive monitoring of acoustic emissions from both stationary and moving equipment within such facilities. It is well known that all mechanical devices, such asmotors, gears, etc, emit acoustic signals. It is possible to identify an acoustic source based on its noise spectrum. During Phase I, we have demonstrated theoretically and experimentally the subject passive acoustic imaging method. Passive listening has been considered and employed in the past to detect underground structures based upon noise emitted from within. The fundamental difference in our approach is that we rigorously considered the inverse source problem subject only tothe assumption that the noise source is localized. Rather than using time delays across a sensor array to triangulate on the source, our method, known as time-exposure acoustics (TEA), coherently sums the data receiver over an array of sensors andback-propagates it into the host geologic formation. This procedure yields an image of the source that is similar to the image formation process used in reflection seismic exploration. The proposed technique can have military applications in locating and monitoring hidden and/or underground structures for targeting as well as commercial applications for locating people buried under collapsed structures that may be caused by caving,mudslide, and earthquakes.