Floating seismographs (MERMAIDS)

International audienc

Modern Mermaids: new floats image the deep Earth

International audienceScientists get a window into deep Earth structures by using a method called seismic tomography. Similar to computed tomography (CT) scans of the brain, seismic tomography uses delays in the arrival times of seismic P waves to make scans and three-dimensional images of the variations in seismic wave speed in the Earth's interior. Patterns in the delays indicate thermal or compositional anomalies in the Earth's mantle and core, such as those caused by sinking cold oceanic lithosphere or rising hot thermal plumes. A large number of observations of such delays are available for continental regions; the number of observations for the United States is especially high due to a dense deployment of stations currently being installed temporarily and moved across a large area as part of the USArray project, a branch of the U.S. National Science Foundation's EarthScope program. In contrast, no comparable sensor density has been available in the ocean

Automatic discrimination of underwater acoustic signals generated by teleseismic P-waves : a probabilistic approach

We propose a new probabilistic scheme for the automatic recognition of underwater acoustic signals generated by teleseismic P-waves recorded by hydrophones in the ocean. The recognition of a given signal is based on the relative distribution of its power among different frequency bands. The signal's power distribution is compared with a statistical model developed by analyzing relative power distributions of many signals of the same origin and a numerical criterion is calculated, which can serve as a measure of the probability for the signal to belong to the statistical model. Our recognition scheme was applied to 6-month-long continuous records of seven ocean bottom hydrophones (OBH) deployed in the Ligurian Sea. A maximum of 94% of all detectable teleseismic P-waves recorded during the deployment of the OBHs were recognized correctly with no false recognitions. The proposed recognition method will be implemented in autonomous underwater robots dedicated to detect and transmit acoustic signals generated by teleseismic P-waves. Citation: Sukhovich, A., J.-O. Irisson, F. J. Simons, A. Oge, Y. Hello, A. Deschamps, and G. Nolet (2011), Automatic discrimination of underwater acoustic signals generated by teleseismic P-waves: A probabilistic approach, Geophys. Res. Lett., 38, L18605, doi:10.1029/2011GL048474

Seismic monitoring in the oceans by autonomous floats

Our understanding of the internal dynamics of the Earth is largely based on images of seismic velocity variations in the mantle obtained with global tomography. However, our ability to image the mantle is severely hampered by a lack of seismic data collected in marine areas. Here we report observations made under different noise conditions (in the Mediterranean Sea, the Indian and Pacific Oceans) by a submarine floating seismograph, and show that such floats are able to fill the oceanic data gap. Depending on the ambient noise level, the floats can record between 35 and 63% of distant earthquakes with a moment magnitude M >= 6.5. Even magnitudes <6.0 can be successfully observed under favourable noise conditions. The serendipitous recording of an earthquake swarm near the Indian Ocean triple junction enabled us to establish a threshold magnitude between 2.7 and 3.4 for local earthquakes in the noisiest of the three environments

A probabilistic approach to the discrimination of underwater acoustic signals generated by teleseismic P-waves

International audienc

MERMAIDs: First observations of teleseismic P-waves with freely floating submarine robots

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