Acoustical Measurement of Nonlinear Internal Waves Using the Inverted Echo Sounder

The performance of pressure sensor‐equipped inverted echo sounders for monitoring nonlinear internal waves is examined. The inverted echo sounder measures the round-trip acoustic travel time from the sea floor to the sea surface andthus acquiresverticallyintegratedinformationon the thermal structure,from which the first baroclinic mode of thermocline motion may be inferred. This application of the technology differs from previous uses in that the wave period (;30 min) is short, requiring a more rapid transmission rate and a different approach to the analysis. Sources of error affecting instrument performance include tidal effects, barotropic adjustment to internal waves, ambient acoustic noise, and sea surface roughness. The latter two effects are explored with a simulation that includes surface wave reconstruction, acoustic scattering based on the Kirchhoff approximation, wind-generated noise, sound propagation, and the instrument’s signal processing circuitry. Bias is introduced as a function of wind speed, but the simulation provides a basis for bias correction. The assumption that the waves do not significantly affect the mean stratification allows for a focus on the dynamic response. Model calculations are compared with observations in the South China Sea by using nearby temperature measurements to provide a test of instrument performance. After applying corrections for ambient noise and surface roughness effects, the inverted echo sounder exhibits an RMS variability of approximately 4 m in the estimated depth of the eigenfunction maximum in the wind speed range 0 # U10 # 10 m s 21 . This uncertainty may be compared with isopycnal excursions for nonlinear internal waves of 100 m, showing that the observational approach is effective for measurements of nonlinear internal waves in this environment.

[1]  Temporal and cross-range coherence of sound traveling through shallow-water nonlinear internal wave packets , 2006 .

[2]  W. Pierson,et al.  A proposed spectral form for fully developed wind seas based on the similarity theory of S , 1964 .

[3]  B. Larock Topographic Effects in Stratified Flows , 1996 .

[4]  Rex K. Andrew,et al.  Underwater Ambient Noise , 2007 .

[5]  P. Beckmann,et al.  The scattering of electromagnetic waves from rough surfaces , 1963 .

[6]  D. Farmer,et al.  The Influence of Bubbles on Ambient Noise in the Ocean at High Wind Speeds , 1984 .

[7]  S. Kitaigorodskii The Dissipation Subrange of Wind Wave Spectra , 1992 .

[8]  Robert J. Urick,et al.  Principles of underwater sound , 1975 .

[9]  R. L. Holford Scattering of sound waves at a periodic, pressure‐release surface: An exact solution , 1981 .

[10]  S. Rice Reflection of electromagnetic waves from slightly rough surfaces , 1951 .

[11]  W. Melville,et al.  Sound-speed measurements in the surface-wave layer , 1997 .

[12]  T. Rossby,et al.  On monitoring depth variations of the main thermocline acoustically , 1969 .

[13]  Timothy F. Duda Initial Results from a Cartesian Three-Dimensional Parabolic Equation Acoustical Propagation Code , 2006 .

[14]  O. Phillips Spectral and statistical properties of the equilibrium range in wind-generated gravity waves , 1985, Journal of Fluid Mechanics.

[15]  S. McDaniel Sea surface reverberation: A review , 1993 .

[16]  Carl Eckart,et al.  The Scattering of Sound from the Sea Surface , 1953 .

[17]  Leonard Fortuin,et al.  Survey of Literature on Reflection and Scattering of Sound Waves at the Sea Surface , 1970 .

[18]  Suzanne T. McDaniel,et al.  An examination of the composite-roughness scattering model , 1983 .

[19]  G. M. Wenz Acoustic Ambient Noise in the Ocean: Spectra and Sources , 1962 .

[20]  H. Charnock Wind stress on a water surface , 1955 .

[21]  Christopher R. Jackson,et al.  Internal wave detection using the Moderate Resolution Imaging Spectroradiometer (MODIS) , 2007 .

[22]  J. P. Hansen,et al.  High Range Resolution Radar Measurements of the Speed Distribution of Breaking Events in Wind-Generated Ocean Waves: Surface Impulse and Wave Energy Dissipation Rates , 2001 .

[23]  C. S. Clay Fluctuations of Sound Reflected from the Sea Surface , 1960 .

[24]  The 1998 WHOI/IOS/ONR internal solitary wave workshop : contributed papers , 1999 .

[25]  L. Rayleigh,et al.  The theory of sound , 1894 .

[26]  Robert R. Long,et al.  Some Aspects of the Flow of Stratified Fluids: I. A Theoretical Investigation , 1953 .

[27]  Y. Stepanyants,et al.  Internal solitons in the ocean and their effect on underwater sound. , 2007, The Journal of the Acoustical Society of America.

[28]  J. Moum,et al.  The pressure disturbance of a nonlinear internal wave train , 2006, Journal of Fluid Mechanics.

[29]  Suzanne T. McDaniel Diffractive corrections to the high‐frequency Kirchhoff approximation , 1986 .

[30]  D. Farmer,et al.  Wave Kinematics at High Sea States , 2002 .

[31]  K. Helfrich,et al.  Long Nonlinear Internal Waves , 2006 .

[32]  S. Rintoul,et al.  A Two-Dimensional Gravest Empirical Mode Determined from Hydrographic Observations in the Subantarctic Front , 2001 .

[33]  N. R. Chapman,et al.  A wide‐angle split‐step algorithm for the parabolic equation , 1983 .

[34]  A Computational Method for Solitary Internal Waves in a Continuously Stratified Fluid , 1991 .

[35]  R. R. Long Some Aspects of the Flow of Stratified Fluids , 1955 .