The Journal of the Acoustical Society of America, Nov 1, 2016
A mechanism is presented by which the observed acoustic intensity is made to vary due to changes ... more A mechanism is presented by which the observed acoustic intensity is made to vary due to changes in the acoustic path that are caused by internal-tide vertical fluid displacements. The position in range and depth of large-scale caustic structure is determined by the background sound-speed profile. Internal tides cause a deformation of the background profile, changing the positions of the caustic structures-which can introduce intensity changes at a distant receiver. Gradual fades in the acoustic intensity occurring over timescales similar to those of the tides were measured during a low-frequency (284-Hz) acoustic scattering experiment in the Philippine Sea in 2009 [White et al., J. Acoust. Soc. Am. 134(4), 3347-3358 (2013)]. Parabolic equation and Hamiltonian ray-tracing calculations of acoustic propagation through a plane-wave internal tide environmental model employing sound-speed profiles taken during the experiment indicate that internal tides could cause significant gradual ch...
The Journal of the Acoustical Society of America, 2016
Observations of the spread of wander-corrected averaged pulses propagated over 510 km for 54 h in... more Observations of the spread of wander-corrected averaged pulses propagated over 510 km for 54 h in the Philippine Sea are compared to Monte Carlo predictions using a parabolic equation and path-integral predictions. Two simultaneous m-sequence signals are used, one centered at 200 Hz, the other at 300 Hz; both have a bandwidth of 50 Hz. The internal wave field is estimated at slightly less than unity Garrett-Munk strength. The observed spreads in all the early ray-like arrivals are very small, <1 ms (for pulse widths of 17 and 14 ms), which are on the order of the sampling period. Monte Carlo predictions show similar very small spreads. Pulse spread is one consequence of scattering, which is assumed to occur primarily at upper ocean depths where scattering processes are strongest and upward propagating rays refract downward. If scattering effects in early ray-like arrivals accumulate with increasing upper turning points, spread might show a similar dependence. Real and simulation results show no such dependence. Path-integral theory prediction of spread is accurate for the earliest ray-like arrivals, but appears to be increasingly biased high for later ray-like arrivals, which have more upper turning points.
Abstract : Ray equations based on an acoustic Hamiltonian, and formulated by Jones et al. (1986) ... more Abstract : Ray equations based on an acoustic Hamiltonian, and formulated by Jones et al. (1986) for spherical coordinates in NOAA's three dimensional ray tracer HARPO, are adapted to ellipsoidal coordinates in the oceanic waveguide. The ensuing modified HARPO is used to model very long range (up to antipodal) acoustic paths in which the difference between geodesics and great circles is measurable. The eventual objective of this modeling is to extract the predictable part of the travel time trend and fluctuations along several long paths that will be used to monitor hypothetical global warming effects. The requirements for easy assimilation and representation of realistic environmental inputs are discussed. These requirements, when coupled with the possibility of classical chaos in the ray paths, dictate a new software architecture. We use the existing software, however, to breadboard and test features of new ray tracers in the global boundary layer, and to support the experimental design of a forthcoming pilot experiment that will use a transmitter located at Heard Island in the Indian Ocean near Antarctica.
Abstract : Characterization of sound propagation in shallow water requires a detailed understandi... more Abstract : Characterization of sound propagation in shallow water requires a detailed understanding of the propagation processes and environmental properties, and how they are linked. This report contains three separate sections that address the effects of range dependence, sediment transverse isotropy, and high attenuation on acoustic propagation in shallow water. The investigations described are theoretical and numerical, and are based on the method of coupled local modes. (AN)
Acoustic travel times in the ocean are sensitive both to the mean sound speed field and to high-w... more Acoustic travel times in the ocean are sensitive both to the mean sound speed field and to high-wavenumber ocean features with wavelengths corresponding to ray loop “wavelengths” and their harmonics (70 km and smaller). The quasi-sinusoidal rays can be thought of as taking a partial Fourier transform of the sound speed field (Cornuelle and Howe, 1987). The SLICE89 experiment was conducted in the northeast Pacific during summer 1989 to determine how well high-wavenumber information can be resolved in practice. A broadband acoustic source near the sound channel axis transmitted to hydrophones 1000 km away. Preliminary travel time data from multiple receivers distributed through the water column are combined to construct time fronts. The measured time fronts are compared with predictions based on the sound speed field as calculated from CTD and XBT data taken concurrently. Although inversions of the measured travel time data are not yet complete, simulations are presented to show the horizontal resolution that can be attained.
Absrracf The Acoustic Thermometry of Ocean Climate project (ATOC) Is just beginning. The goal of ... more Absrracf The Acoustic Thermometry of Ocean Climate project (ATOC) Is just beginning. The goal of the project is to deter-mine over the course d a decade U the oceans are warming or cooling and if numerical models can be validated. Long range acoustic transmissions between ...
The Journal of the Acoustical Society of America, 2009
An unexplained set of arrivals has been observed on ocean bottom seismometers (OBSs) during the N... more An unexplained set of arrivals has been observed on ocean bottom seismometers (OBSs) during the NPAL04 long‐range ocean acoustic propagation experiment in the North Pacific. The observed intensity pattern of the OBS arrivals is significantly more complex than the waterborne arrivals seen on the deep vertical line array (DVLA). These “deep seafloor” arrivals occur later than the first PE predicted arrival; their arrival time is not predicted by acoustic PE propagation models, they do not correspond to decay from shallower turning points (as is the case for deep shadow zone arrivals), and they are not readily observed on the DVLA hydrophone just 750 m above the seafloor. The arrival structure in the observed data, in time and amplitude, varies substantially between three OBSs that are separated by less than 4 km. Could these unexplained arrivals be scattering or horizontal multi‐path from persistent ocean thermal structure?
The Journal of the Acoustical Society of America, 2010
The 2010 Philippine Sea Experiment cruise included two tows of the “towed CTD chain”, an 800‐m‐lo... more The 2010 Philippine Sea Experiment cruise included two tows of the “towed CTD chain”, an 800‐m‐long, ship‐towed cable with 88 CTD sensor fins distributed along its length. The goal of this instrument’s use in the experiment was to obtain high‐resolution (in space and time) measurements of ocean conductivity and temperature in the upper 500–700 m of the deep‐ocean water column, in order to characterize the horizontal spatial statistics of internal waves and “spice” (density‐compensated sound‐speed fluctuations). The effects of such ocean variability upon acoustic variability in long‐range ocean acoustic propagation is an active area of current research. Despite technical difficulties with the system, measurements at 3–5 s sampling periods were obtained on one to three dozen sensors distributed over 700 m depth. The first tow was 93 km for about 39 h and the second tow was 124 km for about 30 h. These datasets will be validated and/or augmented by measurements from a Microcat CTD mounted on the bottom of th...
The Journal of the Acoustical Society of America, Nov 1, 2016
A mechanism is presented by which the observed acoustic intensity is made to vary due to changes ... more A mechanism is presented by which the observed acoustic intensity is made to vary due to changes in the acoustic path that are caused by internal-tide vertical fluid displacements. The position in range and depth of large-scale caustic structure is determined by the background sound-speed profile. Internal tides cause a deformation of the background profile, changing the positions of the caustic structures-which can introduce intensity changes at a distant receiver. Gradual fades in the acoustic intensity occurring over timescales similar to those of the tides were measured during a low-frequency (284-Hz) acoustic scattering experiment in the Philippine Sea in 2009 [White et al., J. Acoust. Soc. Am. 134(4), 3347-3358 (2013)]. Parabolic equation and Hamiltonian ray-tracing calculations of acoustic propagation through a plane-wave internal tide environmental model employing sound-speed profiles taken during the experiment indicate that internal tides could cause significant gradual ch...
The Journal of the Acoustical Society of America, 2016
Observations of the spread of wander-corrected averaged pulses propagated over 510 km for 54 h in... more Observations of the spread of wander-corrected averaged pulses propagated over 510 km for 54 h in the Philippine Sea are compared to Monte Carlo predictions using a parabolic equation and path-integral predictions. Two simultaneous m-sequence signals are used, one centered at 200 Hz, the other at 300 Hz; both have a bandwidth of 50 Hz. The internal wave field is estimated at slightly less than unity Garrett-Munk strength. The observed spreads in all the early ray-like arrivals are very small, <1 ms (for pulse widths of 17 and 14 ms), which are on the order of the sampling period. Monte Carlo predictions show similar very small spreads. Pulse spread is one consequence of scattering, which is assumed to occur primarily at upper ocean depths where scattering processes are strongest and upward propagating rays refract downward. If scattering effects in early ray-like arrivals accumulate with increasing upper turning points, spread might show a similar dependence. Real and simulation results show no such dependence. Path-integral theory prediction of spread is accurate for the earliest ray-like arrivals, but appears to be increasingly biased high for later ray-like arrivals, which have more upper turning points.
Abstract : Ray equations based on an acoustic Hamiltonian, and formulated by Jones et al. (1986) ... more Abstract : Ray equations based on an acoustic Hamiltonian, and formulated by Jones et al. (1986) for spherical coordinates in NOAA's three dimensional ray tracer HARPO, are adapted to ellipsoidal coordinates in the oceanic waveguide. The ensuing modified HARPO is used to model very long range (up to antipodal) acoustic paths in which the difference between geodesics and great circles is measurable. The eventual objective of this modeling is to extract the predictable part of the travel time trend and fluctuations along several long paths that will be used to monitor hypothetical global warming effects. The requirements for easy assimilation and representation of realistic environmental inputs are discussed. These requirements, when coupled with the possibility of classical chaos in the ray paths, dictate a new software architecture. We use the existing software, however, to breadboard and test features of new ray tracers in the global boundary layer, and to support the experimental design of a forthcoming pilot experiment that will use a transmitter located at Heard Island in the Indian Ocean near Antarctica.
Abstract : Characterization of sound propagation in shallow water requires a detailed understandi... more Abstract : Characterization of sound propagation in shallow water requires a detailed understanding of the propagation processes and environmental properties, and how they are linked. This report contains three separate sections that address the effects of range dependence, sediment transverse isotropy, and high attenuation on acoustic propagation in shallow water. The investigations described are theoretical and numerical, and are based on the method of coupled local modes. (AN)
Acoustic travel times in the ocean are sensitive both to the mean sound speed field and to high-w... more Acoustic travel times in the ocean are sensitive both to the mean sound speed field and to high-wavenumber ocean features with wavelengths corresponding to ray loop “wavelengths” and their harmonics (70 km and smaller). The quasi-sinusoidal rays can be thought of as taking a partial Fourier transform of the sound speed field (Cornuelle and Howe, 1987). The SLICE89 experiment was conducted in the northeast Pacific during summer 1989 to determine how well high-wavenumber information can be resolved in practice. A broadband acoustic source near the sound channel axis transmitted to hydrophones 1000 km away. Preliminary travel time data from multiple receivers distributed through the water column are combined to construct time fronts. The measured time fronts are compared with predictions based on the sound speed field as calculated from CTD and XBT data taken concurrently. Although inversions of the measured travel time data are not yet complete, simulations are presented to show the horizontal resolution that can be attained.
Absrracf The Acoustic Thermometry of Ocean Climate project (ATOC) Is just beginning. The goal of ... more Absrracf The Acoustic Thermometry of Ocean Climate project (ATOC) Is just beginning. The goal of the project is to deter-mine over the course d a decade U the oceans are warming or cooling and if numerical models can be validated. Long range acoustic transmissions between ...
The Journal of the Acoustical Society of America, 2009
An unexplained set of arrivals has been observed on ocean bottom seismometers (OBSs) during the N... more An unexplained set of arrivals has been observed on ocean bottom seismometers (OBSs) during the NPAL04 long‐range ocean acoustic propagation experiment in the North Pacific. The observed intensity pattern of the OBS arrivals is significantly more complex than the waterborne arrivals seen on the deep vertical line array (DVLA). These “deep seafloor” arrivals occur later than the first PE predicted arrival; their arrival time is not predicted by acoustic PE propagation models, they do not correspond to decay from shallower turning points (as is the case for deep shadow zone arrivals), and they are not readily observed on the DVLA hydrophone just 750 m above the seafloor. The arrival structure in the observed data, in time and amplitude, varies substantially between three OBSs that are separated by less than 4 km. Could these unexplained arrivals be scattering or horizontal multi‐path from persistent ocean thermal structure?
The Journal of the Acoustical Society of America, 2010
The 2010 Philippine Sea Experiment cruise included two tows of the “towed CTD chain”, an 800‐m‐lo... more The 2010 Philippine Sea Experiment cruise included two tows of the “towed CTD chain”, an 800‐m‐long, ship‐towed cable with 88 CTD sensor fins distributed along its length. The goal of this instrument’s use in the experiment was to obtain high‐resolution (in space and time) measurements of ocean conductivity and temperature in the upper 500–700 m of the deep‐ocean water column, in order to characterize the horizontal spatial statistics of internal waves and “spice” (density‐compensated sound‐speed fluctuations). The effects of such ocean variability upon acoustic variability in long‐range ocean acoustic propagation is an active area of current research. Despite technical difficulties with the system, measurements at 3–5 s sampling periods were obtained on one to three dozen sensors distributed over 700 m depth. The first tow was 93 km for about 39 h and the second tow was 124 km for about 30 h. These datasets will be validated and/or augmented by measurements from a Microcat CTD mounted on the bottom of th...
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