Data and Analysis Methods of the Son-O-Mermaid and MERMAID Experiments

Abstract:

Here we present, for the first time, the results from pilot deployments of the Son-O-Mermaid project, an autonomously drifting, oceangoing data-collection platform that can be cheaply deployed without the need for a costly research vessel, and that records and transmits hydroacoustic signals (with a target of those generated by teleseismic earthquakes) in near real-time. Both deployments employ three identical hydrophones spaced about 70 cm apart at the end of a 700 m long cable attached to a surface buoy that houses electronics, communications, and a GPS location package. The maiden voyage occurred October 8, 2012, in Exuma Bay, Bahamas and returned 25 hours of acoustic data. A second deployment is to take place in September 2014, with near-immediate data relay via Iridium modem. The experimental setup is different from the MERMAID instrument, which sinks to a midcolumn depth and surfaces to transmit waveforms when an event detection algorithm is triggered. MERMAIDs have recorded hundreds of seismic traces from 18 active floats in the Indian and Mediterranean Oceans with a signal-to-noise ratio suitable for global tomography. As with earthquake early-warning studies, both Son-O-Mermaid and MERMAID benefit from rapid and accurate event detection, discrimination, and measurement technology. With this triple purpose in mind we continue to improve methods (in the time-domain, via spectrogram analysis and using wavelets) that we illustrate on the latest and some of the older data, as well. While hydroacoustic data have numerous other applications beyond seismology (whale calls, ice calving, weather patterns, and so on) the Son-O-Mermaid and MERMAID instruments may potentially revolutionize seismic data collection in the oceans. Moreover, in the near future they will be fitted with more or different instruments, becoming multipurpose and multidisciplinary platforms for all types of scientific research in and of the oceans. In seismology we envision a future when the world’s oceans are populated by a permanent array of hundreds of passively drifting Son-O-Mermaid and MERMAID floats, recording and transmitting data, being to ocean-bottom seismometers what a US Flexible Array is to the Backbone Array: lesser data in some sense, but much more abundant, more reliably recovered - and all that at a much lower-cost.