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Application of the PE method to up‐slope sound propagation

Marta Galindo, Karsten Bo Rasmussen

1995
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Journal of the Acoustical Society of America
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The rocky road followed in the 1990s leading to the present status of acoustic thermometry of ocean climate (ATOC) will be reviewed. Contributed Papers 8:30 laAO2. A comparison of measured and predicted broadband acoustic arrival patterus out to 10-Mm range during the ATOC Acoustic Engineering Test. Brace A low-frequency acoustic source suspended from R/P FLIP approximately 340 nautical miles WSW of San Diego transmitted to receivers 90 to 10 000 km distant during the Acoustic Engineering Test
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... f the Acoustic Thermometry of Ocean Climate (ATOC) Program. The source was suspended for 7 days during November 1994 near the depth of the sound channel axis (about 650 m) in water over 4000 m deep, in order to avoid near-source bottom interactions. The source transmitted a phase-coded m-sequence with a center frequency of 75 Hz and a digit length of 27 ms [Metzget et al., this m•eting]. Measured receptions on five bottommounted SOSUS receivers at ranges from 300-4000 kin, on two vertical line array receivers at ranges of 90 and 3300 km, and on a Sohobuoy modified to have the hydrophone on the sound channel axis at about 10 000-kin range, are compared with ray theoretic, adiabatic normal mode, and broadband parabolic equation predictions. The waveforms transmitted during the November 1994 Acoustic Thermometry of Ocean Climate (ATOC) project R/P Flip measu,rement used an m-sequence phase modulated 75-Hz carrier, two carder cycles per digit. Because the soume had an inherent Q of 5, the drive waveform was modified in order to produce a two cycle. per digit outputß This paper describes the basic characteristics of the soume, the procedure used to broaden the source bandwidth, the enforcement of a soume peak power limit, and shows the resulting waveforms. The received waveforms possessed small amounts of Doppler "shift" caused by'the motion of Flip on its mooring. Doppler limits the amount of time over which receptions can be integrated. Doppler normally must be treated as time compression/ expansion. However, if the Doppler is sufficiently small it can be treated as simple time shift. This paper compares the results of using both approaches to account for the motion of Flip. [Work supPorted by the Strategic Environmental Research and Development Program through ARPA.] 9:00 laAO4. The respons e of acoustic meaSurements to anthropogenic climate change. Matthew A. Dzieciuch (Scripps Inst. of Oceanog., IGPP-0225, UCSD, La Jolla, CA 92093) Anthropogenic CO2 in the atmosphere is expected to result in increased ocean heating. The output of a coupled ocean-atmospher• general circulation model was used tO simulate to expected climate change scenario. The model was mn with no CO2 increase and with CO•2 doubling. The spatial properties of the CO2 signal are nonuniform and show that heating is not confined to the ocean surface. The ambient climate variability shows a different spatial structure that is more emphasized at the surface. The GCM output was then used as an input to an acoustic propagation model. The acoustic time series can then be analyzed for climatic trends. The control mn is used to estimate the ambient noise processes on long-term scales. Maps of the anthropogenic signal to the ambient noise ratio of the climate system can then be constructed. Acoustic thermometry measures integrals of heat content through these maps. A network of sources and receivers can then be designed to efficiently monitor anthropogenic climate change. Work is in progress to compare the efficiency of different measurement systems such as satellite altimeters, drifters, and traditional measurements to that of acoustic thermometry. 9:30 laAO6. Are faster than predicted arrival times seeing Arctic Ocean warming? Peter N. Mikhalevsky (Sci. Appl. Intl. Corp., McLean, VA 22102), Alexander Gayalloy (Andreev Inst. of Acoust., Moscow, Russia), and Arthur B. Baggeroer (MIT, Cambridge, MA 02139) Arrival times of M sequences transmitted across the Arctic in the spring of 1994 during the trans-Arctic acoustic propagation (TAP) experiment [P. N. Mikhalevsky, J. Acoust. Soc. Am. 95, 2851(A) (1994)] are faster than modeled arrival times using historical climatology. The modal dependence of the travel times appears to be consistent with a warming of the Atlantic intermediate water (AIW) in the Arctic Ocean. Calculations of the effects of this type of climate change signal, as well as ambient variability on the modal arrival times, will be presented. The possibility that the TAP results are consistent with new reports of AIW warming in the Arctic [Carmack et al., Geophys. Res. Lett. (in press) and K. Aagaard and E. C. Carmack, Science 266 (23 December 1994)] will be discussed. [Work supported by ONR, ARPA, and the Ministry of Science, Russian Federation.] 9:45 laAO7. Long-distance low-frequency modal propagation into the Lincoln Sea. Rich Pawlowicz (Instl of Ocean Sci., One-Mm transmissions from the April 1994 trans-Arctic acoustic propagation (TAp) experiment recorded by a vertical line array deployed from an icecamp at the edge of the continental shelf in the Lincoln Sea are analyzed. The [eceived phase is astonishingly stable, and appears to vary mainly with source/receiver motions. Travel times determined from M-sequence transmissions are less stable and are not consistent with the phase measurements. Modal decomposition shows that bottom effects strip out the higher-order modes as sound propagates onto the shelf. The amplitude of the surface4rapped first mode is also weaker than predicted by standard loss mechanisms. Some implications of this analysis for the design of future Arctic low-frequency tomography experiments are discussed. 10:00-10:15 Break 9:15 laAOS. Stochastic modeling and global warming trend extraction for ocean acoustic travel times. A possible indication of the existence of global climate warming is a negative trend for the travel time of an acoustic pulse along a fixed long path, or paths, in the ocean over a period of many years. Statistical methods for determining whether a significant long term trend is present in a given set of time series data of acoustic propagation times have been developed and, for illustration, applied to some specific travel-time time series generated by ih• MASiG m,d GFDL oceas• modds. Fo, the multiple path case, line + noise models, where the noise is modeled as a multivariate autoregressive (AR) process are considered. It is shown that the time necessary to detect the presence of a warming trend in a time series for a single path is reduced by considering time series for multiple paths. [Work sponsored by ARPA.] 10:15 laAO8. Use of simplified coupled mode propagation for the prediction of impulse responses of megameter trans-Arctic propagation paths. Herbert A. Freese (Sci. Applic. Int. Corporation, McLean, VA 22102) In the spring of 1994, a long-range propagation experiment was conducted in the Arctic during which large time-bandwidth product signals (M sequences) were transmitted to two receiver sites. These data were processed to extract the pulse response of the propagation channel at the vertical arrays at each site and a horizontal array at one of the sites. These data are being used for an assessment of the utility of Arctic acoustic measurements for global warming signature detection and monitoring. A critical part of this measurement is the identification of the observed paths with specific propagating modes. A method has been developed based on coupled normal modes which allows us to estimate pulse responses quickly. The results agree favorably with the experimental data when historical environment data are used as the model inputs. The modeling methods used, comparisons with the measurements and implications, as well as the impact on sound speed accuracy required, will be discussed. The modal description of sound propagation in deep ocean environments is considered. Several recently published inversion algorithms have assumed that modal group time delays can be measured. Such a measurement requires that the frequency bandwidth: (l) be sufficiently broad that successive modal arrivals are resolved in time, and (2) be sufficiently narrow that, across the band, the group slownesses of neighboring mode numbers do not overlap. These conflicting requirements can be reconciled only at long range. In the deep ocean, the late arriving, low-order modes are most likely to satisfy condition (2}. Unfortunately, recent measurements and numerical simulations suggest that these modes are susceptible to internal wave-induced mode coupling---especially at long range. The following pessimistic conclusion is made: Modal group delay-based inversion schemes can be applied to measurements of acoustic wavefields in the deep ocean only for a carefully selected choice of experimental parameters (range, center frequency, and bandwidth) which may not be experimentally accessible. [Work supported by ONR.] 11:15 laAO12. Internal wave effects on long-range ocean acoustic tomography. M. A. Wolfson, J. L. Spiesberger (Dept. Meteorol. and ARL, Penn State Univ., 602 Walker Building, University Park, PA 16802), and ED. Tappert (Univ. of Miami, Miami, FL 33149) Full-wave numerical simulations of internal wave scattering of lowfrequency sound in the deep ocean are performed with an efficient broadband PE model based on a new parabolic approximation that is secondorder accurate, and an internal wave model based on a new representation of the GM spectrum that allows efficient generation of internal wave fields that evolve in geophysical time. Results are displayed as plots of acoustic travel time versus geotime for various source-receiver separations (up to 8 Mm), and various center frequencies and bandwidths. Quantitative examination (coherent and incoherent geotime averages) yields information about the temporal coherence and stability of each multipath. It is found that the later near-axial arrivals are unstable and unresolvable, and therefore not useful for tomography, at ranges and frequencies that are consistent with the predictions of Dozier and Tappert [J. Acoust. Soe. Am. 64, 533-547 (1978)] who used a coupled mode model. [Work supported by ON'P, and ARPA.] 10:45 laAO10. Ray identification theory in ocean acoustic tomography. D. Manuary (Inst. Fiir Meereskunde, The identification problem in multipath ocean acoustic tomography is not only one of the more crucial but also one of the more difficult signal processing problems to solve for further inverse studies. It is only recently that a tool, based on the Bayesian decision theory and close to the data association problem in RADAR, has been proposed by Mauuary and Moura. It fundamentally uses prior information ocean variability which transits through the ray acoustic model. It also statistically solves the ray identification problem with a Bayesian strategy. Despite the inherent complexity of resulting algorithms, a first successful attempt has been made on a French tomographic set of data (GASTOM). Some simplifications and further experimental use are now being investigated on the Mediteranean set of data (THETIS). With another approach given by Send, those are the only practical tools, but, they are sufficiently general to solve the identification problem in the most difficult conditions given by unresolved and unstable data. Both solutions are very close to the generalized Kalman filter theory and joint use of these algorithms in data assimilation models can be expected. 11:00 laAOll. Frequency interpolation method for increasing speed of long-range broadband parabolic equation (PE) calculation, An interpolation method that increases the speed of broadband rangedependent calculations by an order of magnitude using the parabolic equation has been developed. By decomposing the vertical field into complex modal amplitudes, interpolation across frequency is possible. The method was designed to be used in Monte-Carlo studies of the effects of internal waves. Signal processing algorithms are being developed to extract the deterministic signal from the internal wave noise. The method has been benchmarked against the fully broadband PE for several ocean environments. The results are in good agreement with the PE for 1000-kin propagation through internal waves and for 3000-km mildly varying ocean environments. The reproduction of the internal wave effects in the SLICE89 experiment reported by Colosi [Colosi et at., J. Acoust. Soc. Am. 96, 452-68 (1994)] has also been possible. Frequency interpolation predictions of the ATOC Acoustic Engineering Test will be presented and compared with preliminary results from the experiment. In particular, simulation of the statistics of the internal wave effects as measured in the Acoustic Engineering Test is of main concern. 11:30 laAO13. Horizontal multipaths caused by mesoscale structure and their effects on global tomography with near-axial paths. ED. Tapper (Appl. Marine Phys., Univ. Miami, RSMAS, 4600 Rickenbacker Cswy., Miami, FL 33149) and M. A. Wolfson (Penn State Univ., The optimistic and questionable assumption that the lowest acoustic normal modes are adiabatic is used. Then, for each low mode a parabolic wave equation in the horizontal plane is derived that contains lateral vailability of sound speed near the sound channel axis caused by mesoscale structure that is modeled by homogeneous and isotropic fluctuations having a single scale length, L•100 kin. In the geometrical acoustics limit, horizontal rays are found to be chaotic with the growth rate (Lyapunov exponent) given by v•e"2131L, where •-5x 10 -3 is the rms sound speed fluctuation. Horizontal multipaths begin at the focal range, rf•v-•3 Mm, and thereafter the number of horizontal multipaths per mode increases exponentially at the rate v until diffraction effects limit this ray chaos. Full saturation is found to occur at the "log range," rs•r f ln(L2/hrl)•14 Mm, for the acoustic wavelength h-30 m. Since mode coupling is neglected, this saturation range is a hard upper bound for doing global ocean acoustic tomography with near-axial paths. [Work supported by ONR and ARPA.] 11:45 laAO14. Travel time effects of mesoscale structure on rays and waves at global ranges. F. D Tappert and Xin Tang (Appl. Marine Phys., Univ. Miami, RSMAS, 4600 Rickenbacker Cswy., Miami, FL 33149) A range-dependent ray trace model and a broadband PE model are used to model sound propagation at fixed bearing through a field of mesoscale baroclinic modes in order to study the effects of mesoscale structure on travel time at long ranges. The ray model exhibits chaos at ranges beyond a few Mm as manifested by an exponentially increasing number of eigenrays and triplications, especially in the late-time near-axial arrivals. In addition, the ray model predicts a mesoscale travel time bias, in the direction of later time, of the last axial arrival amounting to about 100-200 ms/Mm. At center frequency 75 Hz and bandwidth 50 Hz, the full-wave PE model qualitatively confirms the ray trace predictions of chaos, and shows that the later near-axial amvals are smeared out into a continuum of unresolvable multipaths characterized by saturated (Gaussian) statistics at ranges of a few Mm, and also quantitatively confirms the mesoscale bias of 100-200 ms/Mm of the last axial arrival. The steeper, early raylike arrivals are relatively stable in the presence of mesoscale structure and may be useful for long-range tomogmphy. [Work supported by ONR.] 3235

doi:10.1121/1.411728
fatcat:bvxtg4xt7zcljo4rpxldvug444