Explosive charges have been widely used as wideband sources in underwater acoustic experiments. With a wide bandwidth and high source level, the explosive signals are often utilized to investigate the acoustic propagation characteristics of normal modes. Also, they can estimate the bottom properties according to the arrivals of different modes. However, the received shock waves are often polluted by the bubble pulses. That may increase the difficulty of identifying the arrival patterns of normal modes. Thus, canceling these bubble pulses will be helpful for many underwater acoustic applications.
NIU Haiqiang, CAS Member ZHANG Renhe and several other researchers from the Institute of Acoustics, Chinese Academy of Sciences propose a new method to cancel bubble pulses based on warping operators in the time-frequency domain.
For the sake of distinguishing the bubble pulses from the shock waves, the original received signal was transformed into a warped signal by warping operators. Taking advantage of the different features between the shock waves and the bubble pulses in the time-frequency domain for the warped signal, bubble pulses were filtered out successfully. In the meantime, the invertibility of the warping transformation ensures that the clean shock waves in the original time domain can be retrieved from the warped signal.
This method has been tested with the explosive data from an acoustic experiment in the Yellow Sea in November 2000. The time-frequency representation and the autocorrelation function show that the bubble pulses in the observation can be canceled effectively. Since the bubble pulses are suppressed effectively, the signals after processing can be used in subsequent applications based on the arrival times of the normal modes. However, it is stated that as there are still some overlaps between the shock waves and the bubble pulses in the warped time-frequency domain, the bubble pulse contribution cannot been eliminated entirely. Thus, the signals after processing may not be applied to the applications that require accurate knowledge of the modal amplitudes.
The research is supported by the National Natural Science Foundation of China under Grant Nos 11174312 and 11074269. It has been released online of http://iopscience.iop.org/0256-307X/30/8/084301/pdf/0256-307X_30_8_084301.pdf and published in Chinese Physics Letters (Vol. 30, No. 8, August 2013).
Contact:
NIU Haiqiang
State Key Laboratory of Acoustics, Institute of Acoustics, Chinese Academy of Sciences, Beijing 100190
Email: nhq@mail.ioa.ac.cn