Mitigating ocean random medium effects in array processing by using a cross-correlator beamformer instrumentation

Abstract

Ocean random medium effects such as internal waves can cause distortions to signal wave fronts that are highly detrimental to detecting and localizing signals with acoustic arrays. Motivated by the lucky imaging techniques used in astronomy [1], we have previously found evidence in real array data that when the data are looked at a much finer time scales, there can be brief moments when the wave front has little distortion [2]. However, the challenge has been developing metrics for ranking the quality of the data frames in order to exploit these lucky scintillations. In this paper, we propose a simple frame quality metric based on a cross-correlator instrumentation of the delay-sum beamformer [3] evaluated by arithmetically averaging the higher-order spatial correlation lags along the time delay trajectory corresponding to the beamformer focal location and then utilizing only the highest ranked frames in the localization. We connect this metric to beamformer focus sharpness by showing that the main lobe curvature increases monotonically with the number of lags used and demonstrate the approach on real underwater acoustic array data. [1] https://www.ast.cam.ac. uk/research/lucky [2] I.P. Kirsteins, JASA 143, 1976 (2018). [3] E. Ruigrok et al., “Cross-correlation beamforming,” J. Seismol. 21, 495–508 (2017).