Publications
Estimation and Interpretation of Broadband Mode Statistics
Abstract
The low-order acoustic modes constitute some of the most energetic arrivals at long ranges. Analysis of both simulated and experimental data indicates that, for ranges on the order of megameters, these arrivals have a complicated structure due to internal-wave-induced coupling. Despite the complexity, broadband receptions from the Acoustic Thermometry of Ocean Climate (ATOC) experiment demonstrate that the mode signals do retain travel-time information at ranges of 3515 km, e.g., there are detectable trends in the arrival time of the first 10 modes over a period of five months [1]. Recent work suggests that the relative arrival times of modes 1-10 may offer a measure of internal wave strength.
Understanding the mechanisms and effects of mode coupling is a prerequisite for using the mode signals in applications such as tomography or matched field processing. The North Pacific Acoustic Laboratory (NPAL) experiment has provided an opportunity for further research on the statistics of broadband mode signals. From July 1998 to June 1999, the NPAL billboard array located west of Sur Ridge, California recorded transmissions from the ATOC source north of Kauai. The billboard array consisted of four 20-element vertical line arrays (VLA's) and one 40-element vertical line array, which were located transverse to the 3900 km path to the source. Horizontal aperture of the billboard was 3600 meters and vertical apertures of the 20-element and 40-element arrays were 700 meters and 1400 meters, respectively. The 40-phone VLA was equipped with temperature sensors to facilitate evaluation of the environmental fluctuations near the receivers over the year-long deployment.
An initial analysis of the NPAL receptions shows frequency-selective fading in the arrivals that is typical of long-range propagation through random internal wave fields. Coherence times of the arrival peaks are on the order of 2-3 minutes, which is somewhat less than the 5.5 minutes seen in the ATOC analysis. Early arrivals appear to be less coherent than the late arrivals. One possible explanation for the reduced coherence is that interference from higher order modes is contaminating the estimates of modes 1 through 10. Failure of four hydrophones on the 40-element VLA reduces the ability to reject interference from modes above 10 with conventional mode filtering techniques. Scattering due to upslope propagation near the NPAL receivers also affects the mode statistics.
[1] K. E. Wage, A. B. Baggeroer, and J. C. Preisig, ``Modal Analysis of Broadband Acoustic Receptions at Megameter Ranges,'' in IEEE Sensor Array and Multichannel Signal Processing Workshop Proceedings, (Cambridge, MA), pp. 102--106, March 2000.
This talk was presented at the 2001 Underwater Acoustic Signal Processing Workshop (UASP) , sponsored by the IEEE Providence Section in cooperation with the IEEE Signal Processing Society. The abstract may be found on the UASP website.