Time-Exposure Acoustics for Imaging Underground Structures

"We propose to 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 as motors,gears, etc, emit acoustic signals. It is possible to identify an acoustic source based on its noise spectrum. In addition, by monitoring the acoustic emission from a particular source at several receptors, it is possible to quantify the source location.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 mostly commonly used techniques for the detection and imaging of underground facilities are active wave-based methods. For example, seismic reflection can to image an underground feature from the character of the recorded reflected wave that resultsfrom an applied wave at the ground surface. There are similar methods that exploit the transmission of waves.The proposed passive method has several advantages of active methods. First, the method does not require active sources that may be precluded in hostile areas. Second, the proposed TEA approach can easily accommodate a random distribution of sensors suchas those deployed by an air drop. Finally, TEA can operate unattended and in real time. The proposed method has many geophysical application in exploring geologic resources."