Modal scintillation index: A physics-based statistic for acoustic source depth discrimination

Abstract

A new discriminant based on the scintillation of normal mode amplitudes is introduced for the problem of passive surface/submerged source classification in a shallow water waveguide. The scintillation of modal energies is often used to characterize and understand acoustic wave propagation in a randomly fluctuating ocean waveguide [D. B. Creamer, J. Acoust. Soc. Am. 99, 2825 (1996)]. This paper proposes a variant of the traditional modal scintillation index to treat the discrimination problem in a typical littoral oceanic waveguide. The approach is based on a modal decomposition of fluctuations in the received pressure field associated with the temporal modulation of the depth of an acoustic source about its mean value. Source depth fluctuations are the result of a platform’s response to surface or internal wave motion. The rms mode excitations due to source depth modulation are shown to exhibit a depth dependent signature that may be exploited to statistically separate surface and submerged source classes. In this work, the modal scintillation index (SI) is defined as the variance in the estimated magnitude of the modal excitation normalized by its expected value over some observation interval. The statistic is self-normalizing, so knowledge of source level and source range is not required to separate the two source classes. Estimation of the modal excitation statistics requires only knowledge of the water depth and the sound speed profile at the array. Classification performance predictions in terms of receiver operating characteristic (ROC) curves will be presented based on KRAKEN Monte Carlo simulations under conditions of known and unknown source depth and range in spatially white Gaussian noise. An ad hoc decision criterion, which compares the minimum scintillation index across all modes to a threshold, was used to illustrate the phenomenology. Vertical line array and horizontal line array endfire geometries were considered. The modal scintillation approach may provide a robust alternative to matched field processing for the problem of binary source depth classification.