Mirages in shallow water matched field processing

Broadband matched field processing (MFP)-derived estimates of 3D source location using data from hydrophone line arrays deployed in various geometries, i.e., vertical, horizontal, and tilted 45° from vertical, are presented in this paper. These data were collected in two shallow water (100–200 m) experiments off the coast of San Diego. Results show that estimates of source range and depth remain surprisingly coherent in the presence of large mismatch in bathymetry, but are offset from the true position by as much as 100%. The offsets are independent of array geometry. In contrast, for estimates of source azimuth, bathymetry mismatch typically causes a degradation in MFP correlation rather than an appreciable offset. However, errors in the assumed tilt from vertical of an array can lead to large offsets in the estimated source azimuth, particularly as the nominal angle of the array from vertical becomes smaller. Predictions from a simple analytical model based on adiabatic normal modes in ideal waveguides provide good fits to the broadband MFP results.