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Journal of the Acoustical Society of America / Volume 124 / Issue 3 / JASA EXPRESS LETTERS

Shallow-water seismoacoustic noise generated by tropical storms Ernesto and Florence

J. Acoust. Soc. Am. Volume 124, Issue 3, pp. EL170-EL176 (2008); (7 pages)

James Traer, Peter Gerstoft, Peter D. Bromirski, William S. Hodgkiss, and Laura A. Brooks

Scripps Institution of Oceanography, La Jolla, California 92093-0238

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Land-based seismic observations of double frequency (DF) microseisms generated during tropical storms Ernesto and Florence are dominated by signals in the 0.15–0.5 Hz band. In contrast, data from sea floor hydrophones in shallow water (70 m depth, 130 km off the New Jersey coast) show dominant signals in the ocean gravity-wave frequency band, 0.02–0.18 Hz, and low amplitudes from 0.18 to 0.3 Hz, suggesting significant opposing wave components necessary for DF microseism generation were negligible at the site. Florence produced large waves over deep water while Ernesto only generated waves in coastal regions, yet both storms produced similar spectra. This suggests near-coastal shallow water as the dominant region for observed microseism generation.

© 2008 Acoustical Society of America

Acknowledgments

This project was funded by: the Office of Naval Research, the Department of Energy National Energy Technology Laboratory via the Gulf of Mexico Hydrates Research Consortium, University of Mississippi, and the California Department of Boating and Waterways (to P.D.B.). Data assistance provided by Neil Williams and Hans Graber, University of Miami (local wind and wave data); Arthur Newhall, Woods Hole Oceanographic Institute (SHARK); and Applied Research Laboratories, University of Texas (SWAMI) are appreciated.

Article Outline

  1. Introduction
  2. Array environment
  3. Acoustic spectrograms
  4. Seismic spectrograms
  5. Discussion

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KEYWORDS and PACS

Keywords

hydrophones, seismology, storms, underwater sound

PACS

  • 43.30.Nb

    Noise in water; generation mechanisms and characteristics of the field

  • 43.30.Pc

    Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography

  • 91.30.Ye

    Oceanic crust seismology

ARTICLE DATA

History
Received 01 Apr 2008
Accepted 19 Jun 2008
Revised 18 Jun 2008
Published online 28 Aug 2008
Digital Object Identifier
http://dx.doi.org/10.1121/1.2968296

PUBLICATION DATA

ISSN

0001-4966 (print)  

Publisher

$abstract.society.value is a Member of CrossRef Acoustical Society of America

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Figures (click on thumbnails to view enlargements)

FIG.1
(Color online) The experiment environment. (a) Experiment location (rectangles) and the recorded path of the storm centers. Triangles mark the storm center for Ernesto and circles the center for Florence every 24 h starting 0 Z 30 August and 9 September, respectively. (b) Bathymetry contours from 100 to 5000 m depth. Water depth less than 100 m is white. (c) Wind direction. (d) Wind speed. (e) The surface wave spectra (dB) from 0.02 to 0.5 Hz for 30 August–3 September. The wave spectral energy is normalized with respect to the highest observed signal. Wind and wave data from the ASIS buoys are averaged over ½ h periods. (f), (g) Significant wave heights (Hs) from Ernesto (9 Z 2 September) and Florence (9 Z 12 September), respectively.

FIG.1 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.2
(Color online) Normalized spectrograms (dB) of the acoustic data at three frequency scales [(a), (b) 30–220 Hz, (c), (d) 0.2–5 Hz, and (e), (f) 0.02–0.5 Hz] obtained over a 5 day period (31 August–4 September) from the SWAMI32 [(a), (c), and (e)] and SWAMI52 [(b), (d), and (f)] arrays. The wind velocity trace from Fig. 1d is superimposed in (a) and (b) and the wind direction from Fig. 1c is superimposed in (c) and (d). The spectrograms are averaged over five hydrophones and normalized with respect to the highest power spectral density in the observed range.

FIG.2 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.3
(Color online) Normalized spectrograms (dB) of the acoustic data between 0.01 and 0.5 Hz from the (a) SWAMI52 array, (b) SHARK array, and (c) HRV seismic station. SWAMI52 data were available from 24 August to 6 September, SHARK data from 25 August to 19 September, and HRV data from 23 August to 20 September.

FIG.3 Download High Resolution Image (.zip file) | Export Figure to PowerPoint



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