Predictions and observations of seafloor infrasonic noise generated by sea surface orbital motion. PDF
Preview Predictions and observations of seafloor infrasonic noise generated by sea surface orbital motion.
Predictions and Observations of Seafloor Infrasonic Noise Generated by Sea Surface Orbital Motion Timothy Edward Lindstrom B.S., Electrical Engineering Rochester Institute of Technology (1978) Submitted in partial fulfillment of the requirements for the degree of OCEAN ENGINEER at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY and the WOODS HOLE OCEANOGRAPHIC INSTITUTION September 1991 © Tunothy Edward Lmdstrom, 1991 The author hereby grants to MIT, WHOI and the U.S. Government pennission to reproduce and to distribute copies of this thesis document in whole or in part. /v Predictions and Observations of Seafioor Infrasonic Noise Motion Generated by Sea Surface Orbital by Timothy Edward Lindstrom Submitted to the Massachusetts Institute of Technology/ Woods Hole Oceanographic Institution Joint Program in Oceanographic Engmeermg on August 9, 1991, in partial fulfillment of the requirements for the degree of Ocean Engineer Abstract A model is developed for the prediction of the seismo-acoustic noise spectrum in the microseism peak region (0.1 to 0.7 Hz). The model uses a theory devel- oped by Cato [J. Acoust. Soc. Am., 89 , 1096-1112 (1991)] for an mfimte depth ocean in which the surface orbital motion caused by gravity waves may produce acoustic waves at twice the gravity wave frequency. Usmg dh-ectional wave spec- tra as inputs, acoustic source levels are computed and incorporated mto a more envbonment consisting of a horizontally stratified ocean with an elastic realistic bottom. Noise predictions are made using directional wave spectra obtamed from the SWADE surface buoys moored offthe coast ofVirginia and the SAFARIsounmd propagation code, with a bottom model derived using wave speeds measured the EDGE deep seismic reflection survey. The predictions are analyzed for noise level variations with frequency, wave height, wind direction, anEdCreOcNeiOveMrEdeXpth. These predictions are compared to noise measurements made in us- ing near-bottom receivers located close to the surface buoys. Good agreement is found between the predictions and observations under a variety of environmental conditions. Thesis Supervisor: Dr. George V. Frisk Senior Scientist Woods Hole Oceanographic Institution Acknowledgements I would like to thank a number of people whose help made this work possi- ble. Foremost on this list are my thesis advisor, George Frisk, my co-advisor, Subramaniam Rajan, and my MIT advisor, Henrik Schmidt. These three were truly instrumental in the successful completion of both my course of study and my research, and are greatly deserving of my gratitude. ECONOMEX This particular thesis would not have been possible without the SWADE and program support provided by the Office of Naval Research. Addi- tionally, a great many individuals contributed directly to the sucess of these two experiments ajid to my understanding and data acquisition efforts. Among those at WHOI, I would like to especially thank Paul Boutin, John ColHns, Jim Doutt, John Hallinan, and John Kemp. Those at other institutions who greatly aided my work include Steve Elgaj, Hans Graber, and John Schneider. I would also like to thank Mike Purdy and Steve Holbrook for providing me with the EDGE data so soon after it was analyzed. I will also be forever indebted to Eddie Scheer and Randy Richards for their help in my titanic struggle with the computer. Finally I would like to acknowledge Captain Frank Lacroix, USN, whose pro- fessional guidance made it possible for me to become part of the Joint Program; and my wife Cindy, whose love and support made it all worthwhile. Contents 1 Introduction 6 2 Basic Equations 9 3 Solution to the Inhomogeneous Wave Ekjuation 13 4 Application to Orbital Motion in the Deep Ocean 21 5 Prediction Techniques 32 SWADE 5.1 data 32 5.2 Coupling fax:tors and bottom gain 38 6 Predictions for Receivers at 450 meters and 2500 meters 42 6.1 Variation of spectral level with frequency (spectral shape) 42 6.2 Variation of spectral level with wave height 44 6.3 Variation of spectral level with changing wind direction 46 6.4 Variation of spectral level with receiver depth 47 7 Experimental Noise Measurements 49 7.1 Experimental description 49 7.2 Data selection and processing 51 7.3 Observed results and comparison with predictions 52 7.3.1 Spectral shape and overall spectral noise levels 52 7.3.2 Variation with wave height 52 7.3.3 Variation with changes in wind direction 54 8 Conclusions and Recommendations for Future Research 55 8.1 Conclusions 55 8.2 Recommendations for future research 56 Bibliography 58 Figures 61 Chapter 1 Introduction Reseaxchers have long been aware of a peak in both the seismic ambient groimd motion spectrum and the ocean bottom pressure spectrum which occurs with a period of about three to four seconds. This peak has historically been called the microseism peak. It has been noted that the frequency ofthis peak seems to occur at twice the frequency ofthe peak in the surface gravity wave spectnim. Longuet- Higgins [l] was the first to develop a comprehensive theory which proposed the interaction of opposing sxirface waves, which closely approximate standing waves, as the source ofthe microseismpeak. He showedthat while the first-orderpressure fluctuations caused by the gravity waves exhibit an exponential decay in depth, the second-order fluctuations caused by the nonlinear interaction of two opposing waves does not attenuate. Brekhovskikh [2] was the first to develop this idea into a prediction of noise in the ocean. He has been followed by others, most recently Kibblewhite and Wu [3], who all used a perturbation expansion as the solution to the wave equation with the resulting second order solution as the acoustic field. Cato [4] has developed a model which does not rely on a perturbation expansion, and does not require that a standing wave approximation be made. His theory predicts somewhat higher noise levels for a shallow receiver than those using the perturbation expansion, and a different directionality. In contrast to
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