Effects of water-depth mismatch on matched-field localization in shallow water

Abstract

This article discusses the impact of incorrect estimates of the water column depth on matched-field source localization in a shallow water environment. Computer calculations were performed for the case of a nominal 100-m depth water column subject to water-depth variations of up to ±3.5 m, which would be caused by long-period ocean swell or by tidal changes. The environment was assumed to be range independent (by proper choice of the geometry); thus the question of rough surface scattering was not an issue. The calculations incorporated source depths of 25, 50, and 75 m, a propagation distance of 4 km, an acoustic frequency of 150 Hz, and a linear vertical receiving array. The array consisted of 21 hydrophones with an interelement spacing of 2.5 m, and it spanned the center one-half of the water column (25- to 75-m depth). The matched-field algorithm utilized in this study is the high-resolution maximum-likelihood estimator. A primary result of the work is that, as the output of the matched-field processor degrades due to water-depth mismatch, the apparent source location varies in a systematic way; i.e., the source appears closer and deeper for increasing water depth and, conversely, the source appears farther and shallower for decreasing water depths. Another significant observation is that, as acoustic modes are stripped from the waveguide due to reduced channel depth, instabilities in the solution of the processor cause random variations in localization estimates.