On the potential of recording earthquakes for global seismic tomography
by low-cost autonomous instruments in the oceans

Frederik J Simons¹, Guust Nolet¹, Paul Georgief², Jeff M. Babcock², Lloyd A. Regier² and Russ E. Davis²

1 Geosciences Department
Princeton University
Princeton NJ 08544, USA

2 Institute of Geophysics and Planetary Physics
Scripps Institution of Oceanography
La Jolla, CA 92093, USA

Abstract

We describe the design, development, and testing of an autonomous oceanic device that should revolutionize earth structure determination via global seismic tomography by detecting earthquakes teleseismically, in the oceans. One prototype of the Mermaid, as our instrument is called, for Mobile Earthquake Recording in Marine Areas by Independent Divers, was constructed and tested in situ. It consists of the combination of two readily available, relatively low-cost, but state-of-the-art components, namely a Sounding Oceanographic Lagrangian Observer, or Solo float, and an off-the-shelf hydrophone, interacting by custom-built data logging hardware. The final design, which is yet to be completed, must have, in addition, a depth sounder, a Global Position System receiver, and onboard detection and discrimination software operating on a low-power processing platform, as well as be endowed with satellite communication capabilities for telemetered data transfer. In this paper, we describe the necessary, and successful, seismological tests required to justify and prepare for these improvements, and to make the vision of a floating global array of seismic sensors a reality. We report on three pilot experiments conducted at a depth of 700 m offshore La Jolla, during which over 120 hours of data were gathered; on the development of efficient wavelet-based signal processing algorithms; and on the analysis of the actual pressure time series collected using them. Five signals from earthquakes, of which one teleseismic, were successfully recorded and identified, and this information is sufficient to allow quantitative estimates of the likely success of our instrument in collecting data useful for seismic tomography during dedicated campaigns, as will be planned for the future.

Figures

1. Figure 01 Pictures and design schematic of the MERMAID-001 prototype
   
2. Figure 02 Scences from the life: The MERMAID experiments
   
3. Figure 03 Raw data from the three MERMAID experiments
   
4. Figure 04 Time-, time-frequency-, wavelet-, and frequency-domain representations of the "noises" detected by non-MERMAID, tethered, floats
   
5. Figure 05 Time-, time-frequency-, wavelet-, and frequency-domain representations of the signals detected by the MERMAID float deployed in situ
   
6. Figure 06 Relax! It's just noise. Noise spectra for the three experiments.
   
7. Figure 07 Onsets of the detected events, their wavelet-thresholded reconstruction and possible compression
   
8. Figure 08 Detected events and likely detectability thresholds.
   
9. Figure 09 Likely numbers of event detection as a function of campaign duration.
   

1. Figure X1 Situation map of the MERMAID pilot experiment, November 4-6, 2003. (Not included in paper.)
2. Figure X2 A time-series day in the life of MERMAID, November 4-6, 2003. (Not included in paper.)
3. Figure X3 A time-series day in the life of MERMAID, September 10-12, 2004. (Not included in paper.)
4. Figure X4 A spectral day in the life of MERMAID, November 4-6, 2003. (Not included in paper.)
5. Figure X5 A spectral day in the life of MERMAID, September 10-12, 2004. (Not included in paper.)
6. Figure X6 A seismic day in the life of MERMAID, November 4-6, 2003. (Not included in paper.)
7. Figure X7 A seismic day in the life of MERMAID, September 10-12, 2004. (Not included in paper.)

Frederik Simons