The Ocean as an Acoustic System 



111 



any significant distances due to downward leak- 

 age of energy out of the system as body waves. 



Recently it has been shown that under cer- 

 tain circumstances a resonant transfer of ener- 

 gy from the atmosphere to the earth's surface 

 can occur despite the tremendous impedance 

 mismatch between the two media (Haskell 

 1951). This phenomenon has now been ob- 

 served for coupling between compressional 

 waves in the atmosphere and Rayleigh waves 

 on the earth's surface (Press and Ewing 

 1951a), flexural waves on floating ice and 

 tsunami (Press et al. 1951b, Press and 

 Ewing 1951c). When viewed from the ele- 

 mentary standpoint of the theory of travelling 

 disturbances, resonant coupling occurs when a 

 disturbance travels along the surface of a me- 

 dium at a velocity close to that of a free wave 

 in the medium. If the free wave is dispersive 

 the energy from the disturbance goes into those 

 waves whose periods are such that the phase 

 velocity is close to the velocity of the disturb- 

 ance. The resonance is especially sharp for 

 large density contrasts between the two media 

 as is the case with the atmosphere and the 

 earth. The possible connection between this 

 mode of coupling of atmosphere to ocean and 

 microseisms is being investigated. One obvi- 

 ous feature is that pressure oscillations in the 

 atmosphere striking the sea surface at an al- 

 most vertical angle and maintaining coherence 

 over a large area do not fully satisfy the con- 

 ditions for resonant coupling since "organ 

 pipe" oscillations in the sea column are not 

 free due to the small leakage at each boundary. 



REFERENCES 



Banerji, S. K., Proc. Ind. Acad. Sci. 1, 727-53, 1935. 

 For a discussion see F. J. W. Whipple and A. W. 

 Lee, "Notes on the Theory of Microseisms," Mori. 

 No. Roy. Astron. Soc. Geophys. Suppl., 3, 287-297, 

 1935. 



Burg, K. E., Ewing, M., Press, F., and Stulken, E. J., 

 "A Seismic Wave Guide Phenomenon," Geophysics, 

 16, 594-612, 1951. 



Donn, W. L., "Cyclonic Microseisms Generated in the 

 Western North Atlantic Ocean," Journ. Met. 9. 

 61-71, 1952a. 



"Frontal Microseisms Generated in the 



Western North Atlantic Ocean," Journ. Met. 8, 406- 

 415, 1951a. 



"An Investigation of Swell and Micro- 

 seisms from the Hurricane of Sept. 13-16, 1946" 

 Trans. Amer. Geophys. Union 33, 341-344, 1952a. 



"A Comparison of Microseisms and Ocean 



Waves Recorded in Southern New England" Col. 

 Univ. Techn. Report on Seismology No. 21, 1951b. 



Evernden, J. F., "Direction of Approach of Rayleigh 

 Waves and Related Problems," Abstr. 1952 Pro- 

 gram Geol. Soc. Amer. Cordill Sect. mtg. 



Ewing, M., Worzel, J. L., Steenland, N. C, and Press, 

 F., "Geophysical Investigations in the Emerged 

 and Submerged Atlantic Coastal Plain" Bull. Geol. 

 Soc. Amer. 61, 877-892, 1950. 



Ewing, M., Worzel, J. L., Hersey, J. B., Press, F., 

 and Hamilton, G. R., "Seismic Refraction Meas- 

 urements in the Atlantic Ocean Basin," Bull. 

 Seism. Soc. Amer. 4, 233-242, 1952. 



Ewing, M., and Press, F., "Crustal Structure and Sur- 

 face Wave Dispersion," Bull. Seism. Soc. Amer.. 

 40, 271-280, 1950. 



'Crustal Structure and Surface Wave Dis- 



persion," Part II, Bull. Seism. Soc. Amer., in press. 



Ewing, M., and Press, F., "An Investigation of Mantle 

 Rayleigh Waves," in press, Bull. Seism. Soc. Amer., 

 1953. 



Haskell, N. A., "A Note on Air Coupled Surface 

 Waves," Bull. Seism. Soc. Amer., 41, 295-300, 1951. 



Officer, C. B., Ewing, M., and Wuenschel, P. C, 

 "Seismic Refraction Measurements in the Atlantic 

 Ocean," Part IV, Geol. Soc. Amer. Bull, 63, 777- 

 808, 1952. 



Press, F., and Ewing, M., "A Theory of Microseisms 

 with Geologic Applications," Trans. Amer. Geophys. 

 Union, 29, 163-174, 1948. 



Press, F., and Ewing, M., "Ground Roll Coupling to 

 Atmospheric Compressional Waves," Geophysics. 

 16, 416-430, 1951a. 



Press, F., Crary, A. P., Oliver, J., and Katz, S., "Air 

 Coupled Flexural Waves in Floating Ice," Trans. 

 Amer. Geophys. Union, 32, 166-172, 1951b. 



Press, F., and Ewing, M., "Theory of Air Coupled 

 Flexural Waves," Journ. Appl. Phys., 22, 892-899, 

 1951c. 



Roschke, W. H., Jr., "The Relation Between Air Pres- 

 sure Micro Oscillations and Concurrent Synoptic 

 Patterns," Journ. Met., 9, 213-219, 1952. 



Discussion 



N. A. Haskell 

 Air Force Cambridge Research Center 



I have tried to make a crude order of mag- 

 nitude estimate of the amplitudes to be ex- 

 pected from the generation of microseisms via 

 excitation of the "organ pipe" modes in the 

 ocean by atmospheric pressure oscillations. The 

 results seem to me to indicate that this mech- 

 anism is of questionable quantitative signifi- 

 cance. In Roschke's (1952) study of at- 

 mospheric pressure oscillations he classifies os- 

 cillations of periods less than 1 minute as 

 "large" when the double amplitude is greater 

 than_ about 2 dynes/cm 2 . In the illustrations 

 he gives of typical high microbarometric activ- 

 ity immediately following the passage of a cold 

 front the double amplitude appear to run 

 around 6 dynes/cm 2 . The same order of 

 magnitude has been quoted for pressure fluctua- 

 tions having periods in the neighborhood of 5 

 sec. observed at the Signal Corps Engineering 

 Laboratories (Daniels. 1952). 



Now if the ocean is excited in one of the 

 vertical "organ pipe" compressional modes by 

 a pressure oscillation of amplitude P at the 



