We describe an algorithm to pick event onsets in noisy records, characterize their error distributions, and derive confidence intervals on their timing. Our method is based on an Akaike information criterion that identifies the partition of a time series into a noise and a signal segment that maximizes the signal-to-noise ratio. The distinctive feature of our approach lies in the timing uncertainty analysis, and in its application in the time domain and in the wavelet timescale domain. Our novel data are records collected by freely floating Mobile Earthquake Recording in Marine Areas by Independent Divers (MERMAID) instruments, midcolumn hydrophones that report triggered segments of ocean-acoustic time series.

Mobile Earthquake Recorder in Marine Areas by Independent Divers (MERMAID) is a passively drifting oceanic diving float that transmits acoustic pressure records from global earthquakes within hours or days of their rupture. The onboard algorithm used for the detection and identification of signals from the hydrophone prioritizes the recovery of ∼1 Hz teleseismic P waves, which are useful for seismic imaging of Earth’s mantle. Two years into a mission that launched 50 MERMAIDs to map 3D mantle wavespeed anomalies with high resolution under the Pacific in French Polynesia, it is clear that the data returned contain much information beyond the first-arriving seismic P phases. These include acoustic conversions from S waves, surface waves, T waves, and inner- and outer-core phases, generated by earthquakes heard across the globe—and sounds from otherwise unidentified events occurring in remote and uninstrumented parts of the world’s oceans. Our growing database of automatically accumulating ∼240 s long-triggered segments contains a treasure trove for geophysicists interested in seismology beyond P-wave tomography. Furthermore, equipped with two-way communication capabilities, MERMAID can entertain requests to deliver data from its 1 yr buffer. In this article, we highlight the data classes and categories in MERMAID’s “extended-utility” catalog.

Summary We present the first 16 months of data returned from a mobile array of 16 freely-floating diving instruments, named MERMAID for Mobile Earthquake Recording in Marine Areas by Independent Divers, launched in French Polynesia in late 2018. Our 16 are a subset of the 50 MERMAIDs deployed over a number of cruises in this vast and understudied oceanic province as part of the collaborative South Pacific Plume Imaging and Modeling (SPPIM) project, under the aegis of the international EarthScope-Oceans consortium. Our objective is the hydroacoustic recording, from within the oceanic water column, of the seismic wavefield generated by earthquakes worldwide, and the nearly real-time transmission by satellite of these data, collected above and in the periphery of the South Pacific Superswell. This region, characterized by anomalously elevated oceanic crust and myriad seamounts, is believed to be the surface expression of deeply-rooted mantle upwellings. Tomographically imaging Earth’s mantle under the South Pacific with data from these novel instruments requires a careful examination of the earthquake-to-MERMAID travel times of the high-frequency P-wave detections within the windows selected for reporting by the discrimination algorithms on board. We discuss a workflow suitable for a fast-growing mobile sensor database to pick the relevant arrivals, match them to known earthquakes in global earthquake catalogs, calculate their travel-time residuals with respect to global seismic reference models, characterize their quality, and estimate their uncertainty. We detail seismicity rates as recorded by MERMAID over 16 months, quantify the completeness of our catalog, and discuss magnitude-distance relations of detectability for our network. The projected lifespan of an individual MERMAID is five years, allowing us to estimate the final size of the data set that will be available for future study. To prove their utility for seismic tomography we compare MERMAID data quality against “traditional” land seismometers and their low-cost Raspberry Shake counterparts, using waveforms recovered from instrumented island stations in the geographic neighborhood of our floats. Finally, we provide the first analyses of travel-time anomalies for the new ray paths sampling the mantle under the South Pacific.

Mapping the Earth's uncharted interior through global seismic tomography is dependent on increasing the number of seismic stations in the oceans. We have developed a low-cost, autonomously floating hydrophone to capture earthquake signals suitable for the study of the interior of the Earth and the tectonically and magmatically active underwater realm, while it maintains its potential to be an environmental monitoring device. MERMAID is a freely drifting diver that combines (1) a hydrophone, (2) GPS, (3) an on-board digitizing and processing unit that uses wavelet detection and discrimination algorithms, and (4) an iridium unit for near real-time data transfer with two-way communication. The instrument, 50 kg in air, submersible to 3000 m water depth, with a projected lifetime of up to 5 years, is commercially available from OSEAN SAS. Some 60 units will have been deployed in the Pacific Ocean by the Fall of 2019. With up to 7 kg of sensor payload, additional configurable sensors today include a Sea-Bird SBE 41 CTD, and, in the future, a suite of other instruments with utility in bioacoustics, environmental monitoring, meteorology, bathymetric determination, and chemical and physical oceanography. We report on the performance of our instruments as regards teleseismic event recovery, and on the development of a new method for extracting high-resolution travel times from the first ~1500 seismograms reported live from the Pacific Ocean.

<p>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.</p> <p>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.</p> <p>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.</p> <p>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.</p>