Charles L. Monjo

Analysis of pulse propagation in a bottom‐limited sound channel with a surface duct

Measurements of acoustic transmission in a bottom‐limited sound channel with a surface duct, are compared with model predictions using broadband ray, normal mode, fast‐field, and parabolic approximation methods. At a range of 42 km, six surface‐ducted arrivals are evident in the data set and are predicted by all models except simple ray theory. Surface‐ducted propagation is compared to propagation without a surface duct. The surface duct is found to generate a series of modes which display a resonance between the highly dispersive surface reflected bottom reflected (SRBR) propagation and less dispersive refracted bottom reflected (RBR) propagation. The six arrivals contain energy from three types of propagation paths. Faster precursors are purely diffracted energy, while slower precursors have RBR and SRBR contributions. Ray theory predicts the SRBR contribution, but not the diffracted energy. The precursors are generated at discrete ranges where RBR and SRBR phase fronts exchange energy with the RSR phase front in the duct. Energy leaking from the duct also reenforces the slower precursors. Since range‐independent models were used, mode coupling cannot be the cause of energy exchange.

Modulations of detectable pulse response time spread in shallow water resulting from a combination of sound-speed variability and bottom loss

Reciprocal transmission tomography experiments in the Florida Straits have yielded a month-long time history of reciprocal pulse responses over three ranges approximately 22 km in length and 250 m in depth. A transmitted pulse of 8.75 ms in duration is spread out approximately 150 ms. The detectable time spread, or pulse width, is observed to vary by as much as 120 ms over time scales on the order of days. In a previous paper a strong correlation was reported between the sound-speed gradient at mid-depth and the total pulse width. A plausible explanation that changes in the sound-speed profile shape was causing the ray arrivals to focus and defocus in time was set forth as a hypothesis for this study, to be examined with range-dependent ray and PE models. The models predict a different and unexpected relation. Variations in the sound-speed profile from the average profile shape to a more convex profile increase the bottom grazing angle to the early arrivals. This results in an attenuation of the early arr...

Backscattering from the ocean surface: Observations from the acoustic surface reverberation experiment (ASREX) December ’93–March ’94

Preliminary results from a surface reverberation experiment that was performed in the North Atlantic Ocean during the winter of 1993–94 will be presented. Observations of acoustic backscatter from the ocean surface were made from a moored vertical array at frequencies ranging from 100 to 800 Hz at 12‐min intervals during a 3‐month period. Simultaneous measurements were made by other investigators (from SIO, WHOI, and IOS‐BC, Canada) of a variety of environmental parameters, including wind speed and direction, wave spectra, air and sea temperature and current fields, and the presence and nature of bubble clouds were observed with devices that detected anomalies in the near surface sound speed and using high‐frequency side scan sonars. Variation of backscattering strengths with changes in the environment will be presented and comparisons with results from other recent observations [P. M. Ogden and F. T. Erskine, 746–761 (1994)] will be made. [Work supported by ONR.]

Modeling issues for predicting sound propagation at moderate frequencies in bottom‐limited sound channels

Consider sound propagation in the moderate frequency range (depth to wavelength ratio, H/λ, between 20 and 100) in shallow water environments with bottom roughness. Measurements in the Straits of Florida have shown remarkable phase stability together with rapid (minutes‐to‐hours) unstable amplitude fluctuations that nevertheless yield stable envelopes when averaged over many hours. What physical effects should be contained in an acoustic model in order to predict the observed behavior? It is suggested that a full‐wave model that is capable of multiple rough bottom forward scattering and includes temporally slowly varying range‐dependent sound‐speed profiles will yield observed results, when used together with coherent signal processing techniques. A broadband PE model will illustrate the assertion, at least qualitatively. Absolute level prediction is quite another matter that requires more extensive environmental inputs.

Validating physics‐based clutter from seafloor roughness and biologic scattering

Two physics‐based clutter models were developed for comparison and to study the merits and limitations of both approaches. The stochastic approach [Newhall and Arvelo, J. Acoust. Soc. Am. 118, 2041 (2005)] makes use of perturbation theory, to relate the mean seafloor scattering strength to the mean surface wavenumber spectrum, and the extrapolation of this spectrum to the Bragg wavenumber to generate the clutter distribution from the seafloor roughness. The semi‐deterministic approach [Monjo and Arvelo, J. Acoust. Soc. Am. 118, 2041 (2005)] is based on 2‐D power‐law fractal realizations of the ocean floor, coherent modal addition, and the microscale bathymetric slopes. Biologic scattering was later included where spatial distribution and nonstationarity of schools of fish and individual marine mammals are realistically represented [Frankel et al. J. Acoust. Soc. Am. 119, 3437 (2006)] through individual‐based modeling of their movements, school size, and target strengths. Clutter distributions from both ap...

Physics‐based volume clutter from GeoClutter biological distributions

Empirical fish species composition and distribution data obtained during a GeoClutter survey were input into scattering models for marine mammals [Love, J. Acoust. Soc. Am. 49, 816–823 (1971)] and for schools of fish [Feuillade et al., J. Acoust. Soc. Am. 99, 196–208 (1996)]. These scattering models drew additional inputs from a database of biological parameters to generate volume clutter simulations. Model inputs included distribution of fish size, school population size, depth, orientation, and location. The volume clutter model treated each fish school and individual marine mammal as discrete acoustic targets. Motion of the dominant marine organisms (in addition to source and receiver motion) was taken into account to observe clutter map variability during a series of pings. The volume clutter distribution from this physics‐based approach was compared against bottom clutter and against measured clutter‐rich reverberation from the GeoClutter experiment. [The Office of Naval Research (ONR) is sponsoring ...

Physics‐based clutter via semi‐deterministic simulation with bathymetric fractal realizations

A semi‐deterministic normal‐mode reverberation model has been developed that accounts for bistatic bottom micro‐slopes generated via bathymetric fractal realizations to simulate clutter scattering from seafloor interface features. This clutter simulator also accounts for uncertainties of the subbottom geoacoustics and on range‐dependent interface scattering that varies with the seafloor sediment type. This physics‐based approach is compared against measured clutter‐rich bottom reverberation from the TMAST02 experiment. [This work is sponsored by ONR.]

On the use of acoustic ambient noise to characterize backscattering strength

Strong correlation between acoustic ambient noise and backscattering strength was observed during analysis of data from the acoustic surface reverberation experment (ASREX) [Williams et al., J. Acoust. Soc. Am. 97, 3404 (1995)]. Traditional empirical formulas for acoustic backscattering strength, such as those of Chapman and Harris [R. P. Chapman and J. H. Harris, J. Acoust. Soc. Am. 34, 1592–1597 (1962)] and Ogden and Erskine [P. M. Ogden and F. T. Erskine, J. Acoust. Soc. Am. 95, 746–761 (1994)], utilize wind speed, frequency, and grazing angle as their input variables. An alternative approach which utilizes ambient noise in place of wind speed is presented here. Unlike wind speed, ambient noise measurements are easily made from subsurface platforms and this technique may therefore be advantageous for certain applications (i.e., submarines and covert operations). [Work supported by ONR.]

Multivariate Fourier and wavelet analysis of acoustic backscattering and environmental data from the acoustic surface reverberation experiment

Data from the acoustic surface reverberation experiment (ASREX) [Williams et al., J. Acoust. Soc. Am. 97, 3404 (1995)], is analyzed using multivariate Fourier and wavelet techniques to investigate relationships between environmental conditions and observed acoustic surface backscatter at 200, 400, and 800‐Hz frequencies. Environmental variables observed included wind speed and direction, wave height, air and sea temperatures, current speed and direction, ambient noise, and various measurements of bubble activity characterized by depth‐integrated air volume and e‐folding depth. The relative contribution of the environmental parameters to observed backscattering strengths at the various frequencies will be discussed. The analysis techniques will be described, compared, and contrasted. [Work supported by ONR.]

On the correlation of ambient noise levels and other parameters with backscattering strengths observed during the acoustic surface reverberation experiment (ASREX)

Preliminary results from a surface reverberation experiment performed in the North Atlantic during the winter of 1993–94 indicate that ambient noise levels may prove to be a convenient and accurate indicator of surface backscattering strengths. Backscattered energy from coherent sources ranging in frequency from 100 to 800 Hz were observed over a three month period. Both the backscattered returns and ambient noise were recorded by a 64‐element vertical array while simultaneous measurements were made of a variety of environmental parameters, including: wind speed and direction, wave spectra, air and sea temperature and current fields, bubble plumes, etc., by investigators from SIO, WHOI, and IOS/BC, Canada. The nature of the data set lends itself to time series analysis techniques. Multivariate analysis of backscattering strengths as a function of ambient noise and various other environmental parameters will be presented and discussed. [Work supported by ONR.]