ASA 125th Meeting Ottawa 1993 May

4aUW2. Holographic array processing in the presence of ambient noise.

Azmi A. Al-Kurd

Robert P. Porter

Appl. Phys. Lab., Univ. of Washington, 1013 NE 40th St., Seattle, WA 98105

A holographic array processing (HAP) algorithm is presented for source localization in a nonhomogeneous medium. Conventional array processing algorithms, such as matched field processing, require precise knowledge of the medium between the source and the receiving array. The holographic array processing method relaxes this stiff requirement and calibrates the integrated effect of the medium between a reference (virtual) array near the source, and the receiving array. Using conjugation and averaging techniques at the receiving array an image of the source wave front at the location of the virtual array is reconstructed without explicit knowledge of the characteristics of the medium. The target signal is measured at the receiving array while a reference source is deployed in the vicinity of the target and allowed to transmit at incremental depths. In this paper it is assumed that the ocean is to be horizontally stratified and range dependent. The target's signal is observed at the receiving array in the presence of additive, spatially correlated ambient noise. In the reconstruction step the volume of the ocean in the vicinity of the target is scanned. Both the maximum likelihood method (MLM) and the conventional Bartlett processor are used to estimate the reconstructed field near the target by comparing the virtual image of the wave front with a weighting function that depends on the replica field from the test source. In a noise-free situation, the magnitude of the reconstructed field is greatest at the location of the target. The detection criterion of the noisy signal is based on maximizing the intensity of the reconstructed field as a function of range and depth.