Mermaid: evaluation of its capabilities to observe P-delays in the oceans

Robots, floating freely at depths of up to 2 km are able to observe the acoustic waves in the water column created by teleseismic P waves. In a first experiment, Simons et al. (JGR 2009) reported a clear observation of the P arrival from a Mw=6.0 earthquake at a distance of 46 degrees in the Pacific Ocean. After completion of a prototype Mermaid based on a TWR Apex float equipped with our own electronics, we are now able to report on the performance of such floats under different noise conditions after continuous deployment of four Mermaids since early 2013 in the Mediterranean and Indian Ocean. P wave signals are identified and transmitted using the detection algorithm from Sukhovich et al. (GRL, 2011). At the time of writing of this abstract, five earthquakes of magnitude below 6.0 had been succesfully observed by at least one of two floats in the Mediterranean, of which one Mw=5.5 at a distance of 90 degrees. However, the rate of succesful detection for magnitudes in the 5.7-6.0 range is well below 10%. It approaches 50% for Mw>6.5 in the Mediterranean, with so far only one event missed above magnitude 7. That noise conditions play an important role becomes evident when this detection rate is compared with that in the much noisier environment of the Indian Ocean. So far we detected only 4 out of 16 earthquakes with magnitude 6.5 or higher (using detection parameters originally designed for the Mediterranean), and only one of magnitued lower than 6.5 (see figure). This is a preliminary result and likely to improve once we adapt detection parameters to the observed higher noise at low frequency in the Indian Ocean. The seasonal variations of the noise require at least a full year of observation to obtain representative statistics. We shall present updated results at the AGU Meeting. However, from the data available so far we may already conclude that Mermaids are able to fill the tomography data gap in the oceanic domain at a fraction of the cost of deploying OBSs. Examples of signals detected by the Mermaids with the Sukhovich discriminator under high noise conditions in the Indian Ocean.