Mirages in shallow water matched-field processing

Abstract

Broadband (50–200 Hz) matched field processing was performed on vertical line array data from a recent shallow water experiment. Although the actual water depth steadily decreased from 200 to 100 m over the source tow track, the replica vectors were calculated assuming a range-independent environment of 200 m depth. Rather than breaking up due to the increasingly severe environmental mismatch, the broadband matched field output peak in range and depth behaved in a consistent way; both the predicted range and depth of the source became increasingly greater than its true range and depth as the true water depth decreased. A simple analytical model was developed to predict the behavior of these MFP mirages. The good agreement between model predictions and actual results suggests that the model can be used to quantify the impact of unknown water depth changes (e.g., ocean swell on synthetic aperture studies) and to ‘‘calibrate’’ water depth in a region using the sinking mirages of surface ships of opportunity. By use of the concept of ‘‘effective depth’’ developed by D. E. Weston et al., the model also can be applied to studies of ocean bottom geoacoustic parameter mismatch. [Work supported by ONR, code 321.]