The effect of three-dimensional bathymetry on array processor performance

Array processing on underwater acoustic arrays traditionally ignores three-dimensional aspects of acoustic propagation. In shallow-water environments with sloping seafloors, paths of acoustic energy will ‘‘bend’’ out of the vertical plane containing the source and receiver, creating an acoustic field that may be significantly different from that predicted with a two-dimensional propagation model. Consequently, array processors such as conventional beamformers (CBF) or matched-field processors (MFP) may suffer serious localization errors and correlation degradations when arrays are placed in these environments. This study examines the effects of the two-dimensional propagation assumption on array processor (CBF on horizontal arrays, CBF and MFP on vertical arrays) performance when the field is actually three-dimensional. Computer simulations for the ASA benchmark penetrable wedge were used to systematically quantify degradations as a function of wedge angle, water depth, and distance from the array along a cross-slope track. Three-dimensional effects were also demonstrated experimentally for cross-slope source tow data (bottom slope =4–8 deg) recorded on a vertical line array during the fourth shallow-water evaluation cell experiment (SWellEX-4) off San Clemente Island near San Diego. In both cases, localization improvements obtained by incorporating known three-dimensional effects into the matched-field processor were determined. [Work supported by ONR/NRaD.]