Broadband matched-field processing of transient signals in shallow water

Range and depth source localization in shallow water amounts to the estimation of the normal‐mode structure of the acoustic field. As ‘‘seen’’ by a vertical array, and from a modeling point of view, the normal‐mode structure appears as a set of nonplane coherent waves closely spaced at a vertical angle. This paper presents a full‐wave‐field narrow‐band high‐resolution technique that uses the spectral decomposition of the sample covariance matrix to resolve the vertical arrival structure of the harmonic acoustic field. The broadband processor is obtained by weighted averaging of the narrow‐band range‐depth ambiguity estimates within the source signal frequency band. Results obtained on synthetic data show that its performance is always better than or equal to that of the generalized minimum variance processor, which itself largely outperforms the conventional matched‐field processor. It is shown, using both simulated and experimental data, that the effect of the broadband processor is to increase the stability of the source location estimate. Results obtained with this processor on short transient pulses collected during the North Elba’89 experiment with a 62‐m‐aperture vertical array, showed stable and accurate localizations over long time intervals. It is also shown that the sound field, received over a given frequency band, is relatively stable over time and is in agreement with the predictions given by a standard normal‐mode propagation model.