Hydroacoustic observations of the 15 January 2022 Hunga Tonga-Hunga Ha‘apai eruption: The role of bathymetry along the path

The 15 January 2022 submarine volcanic eruption of Hunga Tonga-Hunga Ha`apai (HTHH) released immense energy throughout the ocean, solid Earth, and atmosphere. We analyze mid-oceanic column acoustic pressure recordings from 24 freely drifting \textsc{Mermaid} sensors, and from 11 moored hydrophones in the International Monitoring System. We focus on the pulsed hydroacoustic phase which propagated horizontally through the water column as a T 30-minute \textit{T}~wave with energy around 2.5--10~Hz. The records show high correlation between some receivers, significant variation among others, and varying amplitudes that cannot be explained by distance alone. We investigate the origin of this heterogeneity via the influence of bathymetric features that may block, or occlude, \textit{T}-wave propagation, affecting both shape and amplitude of the records received. We count the number of seafloor obstacles within the horizontal plane of the first (ray-theoretical) Fresnel zone at a depth of 1350~m, where the fundamental-mode \textit{T}-wave eigenfunction is maximum. Adjusted for geometric spreading, the cross-correlations and sound pressure level differences between receivers systematically relate to differences in occlusion count. Our model of signal loss due to seafloor interactions predicts a 5.6~dB reduction in sound pressure level per log of occlusion count. Beyond helping us understand the particulars of the HTHH eruption, our findings aid in correcting signal amplitudes to constrain volcanic or explosive yield estimates and earthquake magnitudes, and are useful to model detectability through various oceanic corridors when designing hydroacoustic monitoring networks of the future.