A SUBARRAY APPROACH TO MATCHED-FIELD PROCESSING

A simplified approach is presented for matched-field processing of large vertical arrays in typical ocean environments. The large array is divided into subarrays such that a plane-wave decomposition of the signal field is a good approximation over each subarray. A simple rule, based on the sound-speed profile, is presented for subarray sizing. Plane-wave beamforming is then performed to provide multiple beams from each subarray. For long-range situations, only those beams need to be formed that are steered to receive rays that are not strongly bottom interacting. When elements are spaced at one-half wavelength, the typical result of considering only these beams is a significant reduction in the size of the problem with far fewer beams required than elements. This first step is very robust and does not require a detailed knowledge of the signal field, or an assumption of signal coherence across the full array aperture. Beam outputs can then be combined to achieve full matched-field processing. The relative performance of suboptimal approaches such as incoherent combination of beam outputs or simply choosing the best beam is also examined. It is shown that full optimum coherent matched-field processing in spatially white noise yields an output signal-to-noise ratio that depends only on the total signal power intercepted by the aperture, and not on the details of the signal field. For an array that spans the full water column, this can be estimated simply on the basis of cylindrical power flow and falls off as the reciprocal of range. Detailed results are presented for a Pacific winter profile at three widely separated ranges. A small percentage of the beams is found to contain almost all of the signal power. The ray angle diagram (RAD) is used to relate rays, normal modes, and subarray beams, providing insight into the underlying principles. It is shown that the signal field has a fixed number of degrees of freedom and can be represented by that number of beams or normal modes. © 1990, Acoustical Society of America. All rights reserved.