Modal analysis of the range evolution of broadband wavefields in the North Pacific Ocean: Low mode numbers

Abstract

The results of mode-processing measurements of broadband acoustic wavefields made in the fall of 2004 as part of the Long-Range Ocean Acoustic Propagation Experiment (LOAPEX) in the eastern North Pacific Ocean are reported here. Transient wavefields in the 50-90 Hz band that were recorded on a 1400 -m long 40 element vertical array centered near the sound channel axis are analyzed. This array was designed to resolve low-order modes. The wavefields were excited by a ship-suspended source at seven ranges, between approximately 50 and 3200 km, from the receiving array. The range evolution of broadband modal arrival patterns corresponding to fixed mode numbers ("modal group arrivals") is analyzed with an emphasis on the second (variance) and third (skewness) moments. A theory of modal group time spreads is described, emphasizing complexities associated with energy scattering among low-order modes. The temporal structure of measured modal group arrivals is compared to theoretical predictions and numerical simulations. Theory, simulations, and observations generally agree. In cases where disagreement is observed, the reasons for the disagreement are discussed in terms of the underlying physical processes and data limitations.

Weakly dispersive modal pulse propagation in the North Pacific Ocean

“…These results are also consistent with prior data analysis [see Fig. 3(c) in Udovydchenkov et al (2012)], which suggests that three lowest number modes are expected to be weakly dispersive: Mode 1 due to a small value of the action I, and modes 2 and 3 due to a small absolute value of the waveguide invariant b. Note also that short time coherent averaging generally helps (presumably by increasing SNR), but the trends just noted hold for both individual receptions and coherently averaged receptions.…”

“…V for the definition of propagation-induced modal pulse broadening and also see Sec. IV (B) in Udovydchenkov et al (2012) for details on estimation of modal group time spreads]. In contrast, the temporal spread of the m ¼ 9 modal pulse increases significantly with increasing range.…”

“…Details are as described in Udovydchenkov et al (2012) except that the match-filtering step was omitted here because the transmitted m-sequence is treated as a communication sequence. Using this process, carrier-modulated amplitudes a m (t) of the first ten modal pulses were constructed.…”

“…Low-order modes have small I and hence small dispersive time spreads. In deep ocean environments low order modes have small scattering-induced time spreads because near-axial internal-wave-induced sound speed fluctuations (and hence also typical values of B) are small; see Virovlyansky et al (2007), Makarov et al (2010), and Udovydchenkov et al (2012) for a more detailed discussion of near-axial sound scattering. The second type of weakly dispersive modes consists of those modes for which the waveguide invariant parameter b is close to zero (Brown and Udovydchenkov, 2013).…”

“…Here Dt total is the data-based estimate of modal group time spread [see Sec. IV (B) in Udovydchenkov et al (2012) for details on estimation of Dt total ], and Dt bw is the reciprocal bandwidth contribution to the total modal group time spread; the assumed numerical value of Dt bw was 0.053 s (Udovydchenkov and Brown, 2008;Udovydchenkov et al, 2012). For communications applications a useful quantitative definition of a weakly dispersive modal pulse is one for which Dt Շ Dt bw .…”