The MERMAID Project

In the last few decades seismologists have mapped out elastic wavespeeds of the Earth's interior with often perplexing if not always uncontested detail. Earthquake sources used in seismic tomography lie mostly on plate boundaries; receivers mostly on dry land. The uneven coverage resulting from this fundamentally inadequate source-station distribution leaves large volumes inside the Earth entirely unsampled. Placing seismic stations on the ocean bottom is often touted as the only solution. The MERMAID project (Mobile Earthquake Recorder in Marine Areas by Independent Divers) is a radical low-cost alternative that uses passively drifting autonomous hydrophones with a now proven potential to record hundreds of distant earthquakes over their projected life span.

Introduction

Earth models in which seismic wavespeeds vary only with depth are sufficiently well constrained to accurately locate earthquakes and calculate the paths followed by seismic rays. The differences between theoretical predictions of seismograms in such one-dimensional Earth models and observations can be used to reconstruct the three-dimensional (3D) wavespeed distribution of the Earth in the regions sampled by the seismic waves.

Caused by thermal and compositional variations, wavespeed anomalies remain the premier observable 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 interaction between the deep Earth and surface processes such as plate motion and crustal deformation.

Limitations of Global Tomography

Unequal geographical data coverage remains the fundamental limiting factor in seismic tomography. 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, which strongly hampers the determination of the structure of the Earth in the uncovered regions. Thus, all 3D Earth models are marked by blank spots in areas where little or no information can be obtained.


Remediating this problem requires the observation of seismic waves in the oceans. Sonobuoys have had success in the past in recording local earthquake signals, but have been too noisy to give an acceptable signal-to-noise ratio for all but the strongest earthquakes. Ocean bottom seismometers (OBS) and moored hydrophones are capable of addressing the coverage gap, but they are expensive to manufacture (about $50,000 for a three-component OBS) and deploy (about $20,000 per day of ship time). Unable to communicate, stationary underwater devices have to be retrieved at regular intervals.

Design of a mobile receiver

Oceanographers have designed robotic floats that spend their life at depth but surface periodically -- using a pump and bladder -- to make temperature and salinity profile measurements. Such low-cost (about $15,000) Sounding Oceanographic Lagrangian Observers (SOLO) can be equipped 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 (for global tomography) seismic event, determine a GPS location and transmit the waveforms to a satellite. The surfacing speed guarantees a location accuracy of the float at depth to within a few hundred meters. Operating costs are minimal: their autonomy and low weight guarantee easy deployment from any vessel, and the data will be available in real time; what's left is the price of a satellite subscription.

The design challenges are formidable because, pending alternative means of power generation, the success of the device depends on how long it can last before its batteries run out or corrosion and barnacles take over. Lifespan is critically dependent on limiting power consumption by using a minimum of numerical operations to perform the detection and identification of the waveforms. Recent advances in signal processing have allowed us to address this bottleneck: our tests have demonstrated the success of so-called second-generation wavelets to provide useful sensitivity and discriminating power, even in the presence of high levels of contaminating noise.

Proof of concept

Our prototype is nicknamed MERMAID, for Mobile Earthquake Recorder in Marine Areas by Independent Divers.



The great promise of this technology was demonstrated by the prototype on its maiden voyage (November 4-6, 2003. A second test was conducted September 10-12, 2004, and a third test on August 9-11, 2007.). Submerged, and freely drifting for about 30 hours, 700 m below the sea surface, in a canyon off the coast of La Jolla, California, MERMAID recorded a very promising signal, coming from a relatively faint (in global seismological terms) magnitude 6 earthquake near the west coast of Colombia, about 5,000 km away. Earthquakes of a magnitude larger than this occur at a rate of about 200 per year. The recording shows a clear incoming P wave whose precise arrival time can be determined to within a fraction of a second.The demonstrated high sensitivity of the MERMAID platform clearly illustrates its likely contributions to global seismic tomography.



In addition to recording teleseismic P waves, such a system will pick up trapped T waves propagating in the SOFAR (Sound Fixing And Ranging) channel. Although noise for the purposes of global tomography, recent hydroacoustic studies have shown their utility in studying the mechanisms of large, tsunamigenic earthquakes such as the Dec. 26, 2004, Sumatran earthquake.

The future

A worldwide array of MERMAID floating hydrophones, on the scale of the current international land-based seismic arrays, has the potential to progressively eliminate the discrepancies in spatial coverage resulting in poorly resolved seismic Earth models. Oceanographers have already pointed the way with the large-scale international Argo project. There are currently over 3,000 SOLO floats measuring conductivity, temperature and depth throughout the Earth's oceans, to understand and forecast climate. Added to future generations of the Argo project, MERMAID's regular resurfacings will provide useful corollaries to other disciplines, such as average current speeds at depth, spot depth soundings, and, with the ongoing miniaturization of marine technology, an additional payload of low-power instruments only limited by the imagination.

Many of the important dynamic processes in the deep Earth seem now located beneath the larger oceans in the southern hemisphere. This may not be accidental, but the absence of seismic observations in the southern oceans severely limits our ability to study these processes. Does the Earth's mantle convect as a whole or is it layered? What is the contribution of mantle plumes to the transport of heat to the Earth's surface? What is the scales of mantle heterogeneity and how does it originate? What are the nature and role of geochemical reservoirs? Is there an undifferentiated reservoir in the lowermost mantle?

Seismic tomography will provide the primary models in an Earth systems framework ultimately involving geodynamics and geochemistry. But, first and foremost, definitive answers will lie in the data: currently hidden in plain sight, out of reach of more conventional approaches.