Evolution of second‐order statistics of low‐order acoustic modes

Abstract

Low‐mode signals measured during long range tomography experiments, such as the Acoustic Thermometry of Ocean Climate (ATOC) and the North Pacific Acoustic Laboratory experiments, have a random arrival structure due to internal waves. At megameter ranges, the narrow‐band mode amplitude is predicted to be Gaussian and uncorrelated with other modes [Dozier and Tappert, J. Acoust. Soc. Am. 63, 353–365 (1978)]. Wage et al. measured the centroid, frequency coherence, time spread, and time coherence for the broadband ATOC mode signals received at ranges exceeding 3000 km [Wage et al., J. Acoust. Soc. Am. 117, 1565–1581 (2005)]. The 2004 Long Range Acoustic Propagation EXperiment (LOAPEX) provided an opportunity to observe how the mode statistics evolves with range. This talk investigates the mean, temporal covariance, and intermodal correlation of the low modes at ranges between 50 and 3200 km using LOAPEX data. Broadband parabolic equation simulations were performed to model internal wave effects on the low‐mode signals at the various LOAPEX stations. A kurtosis measure is used to study the amount of cross‐modal scattering with respect to range. Statistics of the measured and simulated mode signals are compared. [Work supported by ONR.]