Hybrid Seabed Parameterization to Investigate Geoacoustic Gradients at the New England Mud Patch

This article applies Bayesian geoacoustic inversion with a hybrid seabed-model parameterization to modal-dispersion data from the New England Mud Patch to estimate gradient structure in the upper mud layer. The hybrid parameterization comprises an upper layer with general smooth (continuous) gradients represented by Bernstein polynomials (BPs) for the mud, above an unknown number of discrete (uniform) layers. The Bayesian information criterion is applied to estimate BP orders for sound-speed and density profiles in the mud, and trans-dimensional (trans-D) inversion is applied for the underlying layered structure. The data, collected during the 2017 Seabed Characterization Experiment, include high-order modes (up to mode 21) extracted via warping time-frequency analysis from recordings of a combustive sound source at a vertical hydrophone array. Inversion results for the hybrid parameterization are compared to those from trans-D inversion with no gradient layer. Hybrid-inversion results indicate a nearly iso-speed mud layer with a rapidly increasing gradient near its base, consistent with increasing sand content in the mud above a sand interface, as indicated by cores. The sound-speed ratio of surficial sediments to bottom seawater is found to be $<$ 1 with high probability, which differs from trans-D inversion results, indicating the significance of the choice of parameterization in interpreting structure.