MERMAID Tales: Travel-Times, Waveform Modeling, Infrasonic Spectral Densities, and Volcanic Eruptions

Since the launch of the first third-generation MERMAID in 2018, sixty-seven autonomous freely-drifting mid-column hydrophones have been afloat in the Pacific Ocean,  the South China Sea, and the Mediterranean. Over fifty-five instruments remain alive and well, and are continuing to report short, triggered, waveform segments of acoustic pressure variations, with six instruments directly reporting acoustic spectral densities. A new model equipped with a conductivity-temperature-depth (CTD) sensor is due for deployment (four new units). Over these last few years, many thousands of teleseismic arrival-times have been reported, and associated with global earthquake catalogs, so that their travel-time residuals with respect to global reference models can be determined in view of making tomographic models, especially of the area around French Polynesia, the original launch focus of the EarthScope-Oceans (ESO) fleet. The data stream has been flowing into the EarthScope (IRIS) data management center (DMC), and reporting short segments around first-arriving phases and the calculation of travel-times have become routine applications.

We briefly review those successes, but we focus on the latest data types (spectral densities computed in-situ rather than time-domain seismograms transmitted via satellite), on the codesign of MERMAID as an acoustic float with an environmental CTD sensor, on the latest modeling efforts at matching waveforms with synthetically computed pressure seismograms, and on the rich set of continuous records that were requested from MERMAID's one-year buffer in the hours and days immediately following the Hunga Tonga Hunga Ha'apai eruption, recorded by over twenty instruments over a wide epicentral distance and backazimuthal range.

Waveform modeling was not a design goal for MERMAID, but we discuss an innovative approach to circumvent the computational burden involved in matching the global earth ocean-bottom response to teleseismic earthquakes (computed using an axisymmetric spectral-element code) to the local oceanic mid-column pressure response, including the effects of bathymetry (computed using a local two-dimensional spectral-element modeling step). We applied our method, which is based on precomputed Green's functions via the publicly available code Instaseis and on a custom data base of ocean-floor-to-water-column response functions computed using SPECFEM-2D, to a set of over one thousand waveforms, after dynamically selecting the optimal bandwidth based on adaptive signal-to-noise considerations to steer clear of the noise generated by the ocean wave heave. The correlation between synthetics and observations is as high as 0.98, with a median of 0.72, and very coherent across the array, allowing for the determination of cross-correlation travel times and opening up MERMAID seismograms to conduct full-waveform tomography of Earth's mantle.

Similarly, volcanic monitoring was not a design goal for MERMAID, but in recovering, on-demand, the many hours of continuous records of the Hunga Tonga Hunga-Ha`apai eruption, we have obtained a unique data set from which are piecing together a detailed picture of the eruption sequence. We discuss signal correlations and disparities within and across the South Pacific Plume Imaging and Modeling (SPPIM) array, and address, in particular, path-dependent effects due to "bathymetric occlusion", the influence of seafloor topography on the coherent propagation of hydroacoustic energy over large distance ranges.