The effect of transient signal vertical directionality on hydrophone array performance

Abstract

Signals from explosive sources were measured by a near-bottom vertical array of hydrophones during an experiment conducted by the Naval Research Laboratory. Using a beamformer, estimates of the source energy spectrum were made with the array steered toward the direct-path arrival; estimates of the directional noise energy spectrum are made using the same look angle. Signal-to-noise (SNR) estimates were then computed for various ranges and steering angles. As expected, the SNR decreases rapidly at longer ranges and lower grazing angles. This decrease is due in part to lower signal levels, but is dominated by large increases in the ambient noise field for grazing angles less than the critical angle of the water–sediment reflection. The SNR for a vertical acoustic dipole formed by differencing two elements of the array also is computed. Despite the decreased response of the dipole beam at low grazing angles, the SNR remained unexpectedly high at longer range and exceeded that of the beam-steered array. As grazing angle decreased, bottom loss also decreased resulting in a large amount of signal energy in higher-angle, water-column multiples. These energetic signals fall into the broad dipole beam pattern, but are excluded from the beam-steered array. These results imply that to maximize SNR for the vertical array, the total vertical arrival structure of the signal (and of the noise) must be considered in choosing beamforming coefficients. [Work funded by ONT.]