A forward-backward filter for estimating mud properties in SBCEX 17

The goal of SBCEX 17 was to investigate properties of the mud layer at the experiment site. We perform geoacoustic inversion for the mud site, integrating data and estimates obtained on a track of a moving source. The transmitted sound signals were LFM pulses in a mid-frequency range. The sound was received at a vertical array comprising 16 phones. Arrival times of multiple paths were extracted from the received signals using particle filtering and a backward-moving smoother: forward filtering and smoothing were applied to the received signals for the estimation of path arrival times generated by sound interacting with the waveguide. The smoothing step corrected for outliers that provided erroneous geoacoustic estimates for some source positions. The forward-backward filter provided accurate arrival times with a significant uncertainty reduction. The obtained arrival times were linked with a ray tracer and linearization for source localization and estimation of other parameters that affect sound propagation such as water column depth and sound speed. Sediment sound speed and thickness were then computed. Inversion was carried out at every source location within the track; the results were combined into a single set of estimates and corresponding probability density functions. [Work supported by ONR.]