A future for drifting seismic networks

One-dimensional, radial Earth models are sufficiently well constrained to accurately locate earthquakes and calculate the paths followed by seismic rays. The differences between observations and theoretical predictions of seismograms in such Earth models can be used to reconstruct the three-dimensional wave speed distribution in the regions sampled by the seismic waves, by the technique of seismic tomography. Caused by thermal, compositional, and textural variations, wave speed anomalies remain the premier data source to fully understand the structure and evolution of our planet, from the scale of mantle convection and the mechanisms of heat transfer from core to surface to the international between the deep Earth and surface processes such as plate motion and crustal deformation. Unequal geographical data coverage continues to fundamentally limit the quality of tomographic reconstructions of seismic wave speeds in the interior of the Earth. Only at great cost can geophysicists overcome the difficulties of placing seismographs on the two thirds of the Earth's surface that is covered by oceans. The lack of spatial data coverage strongly hampers the determination of the structure of the Earth in the uncovered regions: all 3-D Earth models are marked by blank spots in areas, distributed throughout the Earth, where little or no information can be obtained. As a possible solution to gaining equal geographic data coverage, we have developed MERMAID, a prototype mobile receiver that could provide an easy, cost-effective way to collect seismic data in the ocean. It is a modification of the robotic floating instruments designed and used by oceanographers. Like them, MERMAID spends its life at depth but is capable of surfacing using a pump and bladder. We have equipped it with a hydrophone to record water pressure variations induced by compressional (P) waves. Untethered and passively drifting, such a floating seismometer will surface upon detection of a "useful" seismic event (for seismic tomography, that is), determine a GPS location, and transmit the waveforms to a satellite. In this presentation we discuss the progress made in this field by our group. More specifically, we discuss the results of preliminary tests conducted off-shore La Jolla in 2003 and 2004, as well as just-in results from a third successful, in situ, test completed in August 2007. We will draw attention to design issues and bottlenecks and the need for and features of sophisticated onboard data analysis software which we have developed and tested. We will chart a road map of the way to our ultimate goal: a worldwide array of MERMAID floating hydrophones, on the scale of the current international land-based seismic arrays. This, we believe, has the potential to progressively eliminate the discrepancies in spatial coverage that currently result in seismic Earth models that are very poorly resolved in places.