On the use of adaptive beam forming techniques for geoacoustic inversion of marine ambient noise

We provide an explanation for the poor performance of the minimum-variance distortionless-response (MVDR) beam former when applied to one particular technique for geoacoustic inversion of marine ambient noise. Previous studies have shown that using the Bartlett beam former one can estimate the reflection loss (RL) as a function of frequency and grazing angle, and use it to estimate the seabed geoacoustic parameters. Despite its superior performance in direction-of-arrival estimation, the MVDR beam former´s results for RL estimation are less reliable than those yielded by the Bartlett beam former. One important difference between the two beam formers lies in the fact that the MVDR beam patterns for the two look directions ±φ can be markedly different, whereas the Bartlett beam patterns are always symmetric with respect to the broadside look direction. By estimating the RL from simulated acoustic fields we first present a method of quantifying and visualizing the asymmetry, and then show that it is responsible for the anomalies in the MVDR-based RL estimate.