RECORDING P WAVES (AND MUCH MORE) EVERYWHERE IN THE OCEANS

The lack of stations on islands or the ocean bottom deprives seismic tomographers of almost 2/3 of the information potentially available for global seismic tomography. Before mounting an expensive global array of OBS's that can record the full wavefield, I propose to use floats to record onset times of P waves everywhere 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. In the ERC-funded Globalseis project we developed the "Mermaid", based on a TWR Apex float equipped with our own electronics. We have studied the performance of such floats under different noise conditions in the Mediterranean and Indian Ocean. P wave signals are identified and transmitted using the detection algorithm from Sukhovich et al. (GRL, 2011). In the Mediterranean, we regularly detect P waves at teleseismic distances of earthquakes with magnitude 6, occasionally below that. 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, where the threshold seems to be closer to 7. Yet we were also able to record many low magnitude events (not recorded on land stations) when close to a swarm near the triple junction. A new prototype of a spherical Mermaid is currently being tested. It allows for multidisciplinary observations (seismic and kHz acoustics, magnetic field, temperature) and will function five years with its lithium batteries. A global deployment of such instruments in a five-year program is affordable.