The Detection of Undersea Mines Using Single-Channel, Iterative, Time Reversal Acoustics
Numerical studies demonstrate enhanced signal-to-noise ratio and convergence to a narrowband signal using iterative time reversal of the monostatic scattered return from a resonating target [Pierson, David M., “Buried Object Detection Using Time-Reversed Acoustics,” PhD Dissertation, North Carolina State University, 2003]. The center frequency of the converged-upon signal is a resonance frequency of the target whose value can be used for classification and identification purposes. In this project, we are developing a technique for sensing and classifying objects buried in sediments using iterative, time-reversal acoustics. The procedure consists of exciting the target with a broadband pulse, digitizing the echo return, reversing the data stream in time, and using this signal as the source waveform for the next interrogation pulse. Free field results indicate that the spectrum of the echo rapidly converges to a frequency corresponding to a resonant mode of the target. The signal to noise ratio of targets buried in sand are enhanced as the iterations converge to the strongest available target resonance. This effort is funded by the US Navy Office of Naval Research. The investigative team includes Dr. Benjamin Dzikowicz at the Naval Surface Warfare Center, Panama City, Florida.
Acoustics Sensors and Array processing
In conjunction with the Woods Hole Oceanographic Institution, WHOI, we have developed a low noise state-of-the-art towed array, TA, towed from a small autonomous vehicle, AUV. Stable tows were achieved, vehicle radiated noise levels were found manageable and coherent signal processing in the narrow and broadband was found to be feasible. The near-term research objective was to utilize the autonomous-vehicle towed-array system to establish quantitative measures of array performance; to demonstrate the system capability to perform measurements; and to demonstrate its potential for sonar. The long-term objective was to demonstrate and quantify the performance characteristics of anautonomous-vehicle towed-array system collaborating with other systems to use this AUV-TA technology as a cost effective ocean measurement capability and to develop the necessary signal processing algorithms necessary for synthetic aperture Hankel transform processing.
The Nantucket experiments were conducted with the intent of using synthetic aperture processing techniques with an AUV-TA to characterize the waveguide. In proximity to the experimental site was the local shipping channel between the main land and the entrance to Nantucket Harbor. A ferry en-route to the island from the mainland passed in front of the vehicle from North to South and provided a means for demonstrating the bearing space problem (tracking) with improved resolution and improved estimation performance. Results from a ferry tracking are shown in figures 1 and 2. This type of tracking capability is useful for the cost effective use of this system for a variety of survey and surveillance applications. The use of acoustics to probe the coastal oceanographic conditions by the measurement of currents, fish , as well as internal waves and frontal conditions can provide valuable insight to the safe clean utilization of coastal waters for energy production.