richard keiffer - Academia.edu (original) (raw)
Papers by richard keiffer
Near-field acoustic scattering from rough surfaces
The Journal of the Acoustical Society of America, 1990
Modeling the azimuthal dependence of sea surface backscatter during CST7
Journal of The Acoustical Society of America, 1994
Mathematical and Computer Modelling, 1988
Wave Motion, 2011
A one-dimensional weakly-nonlinear model equation based on a Lagrangian-averaged Euler-α model of... more A one-dimensional weakly-nonlinear model equation based on a Lagrangian-averaged Euler-α model of compressible flow in lossless fluids is presented. Traveling wave solutions (TWS)s, in the form of a topological soliton (or kink), admitted by this fourth-order partial differential equation are derived and analyzed. An implicit finite-difference scheme with internal iterations is constructed in order to study soliton collisions. It is shown that, for certain parameters, the TWSs interact as solitons, i.e., they retain their "identity" after a collision. Kink-like solutions with an oscillatory tail are found to emerge in a signaling-type initial-boundary-value problem for the linearized equation of motion. Additionally, connections are drawn to related weakly-nonlinear acoustic models and the Korteweg-de Vries equation from shallow-water wave theory.
On the validity of the wedge assemblage method for pressure-release sinusoids
Journal of The Acoustical Society of America, 1993
Exact scattering theory applied to more realistic 2-D sea surfaces
Journal of The Acoustical Society of America, 1991
Near-field acoustic scattering from simulated two-dimensional, wind-driven sea surfaces
Journal of The Acoustical Society of America, 1992
Journal of The Acoustical Society of America, 1992
N PAGE Form Aptroved 3N AGI OBM No. 0704-0188' ubirc raiiiw por eiprse. indchiing the time for re... more N PAGE Form Aptroved 3N AGI OBM No. 0704-0188' ubirc raiiiw por eiprse. indchiing the time for reviewing instructions. searching existing 9-1i sources, gathering and ailiig AU-aaA25/ 804' ,a IJ l 1 1 , ntd corrrrrnlsregardlngohlsbjrden or any other aspecto thlscollection of Inlformnln .. including suggestions I' I Irl icII Il II operations and Repors, 1215 Jeerson Davis Highway, Suite 1204. Arlington. VA 2224302. and to 'he Office of Ington, DC 20503.
IEEE Journal of Oceanic Engineering, 1998
When modeling sound propagation through the uppermost layers of the ocean, the presence of bubble... more When modeling sound propagation through the uppermost layers of the ocean, the presence of bubble clouds cannot be ignored. Their existence can convert a range-independent sound propagation problem into a range-dependent one. Measurements show that strong changes in sound speed and attenuation are produced by the presence of swarms of microbubbles which can be depicted as patchy clouds superimposed on a very weak background layer. While models suitable for use in acoustic calculations are available for the homogeneous bubble layer (which results from long time averages of the total bubble population), no similar parameterizations are available for the more realistic inhomogeneous bubble layer. Based on available information and within the framework of a classification scheme for bubble plumes proposed by Monahan, a model for the range and depth dependence of the bubbly environment is developed to fill this void. This model, which generates a possible realization of the bubbly environment, is then used to calculate the frequencydependent change in the sound speed and attenuation induced by the presence of the bubbles plumes. Time evolution is not addressed in this work.
Wedge assemblage modeling of SAX99 backscatter strengths
Journal of The Acoustical Society of America, 2001
In SAX99 (Sediment Acoustics Experiment 1999), a number of high quality acoustic and environmenta... more In SAX99 (Sediment Acoustics Experiment 1999), a number of high quality acoustic and environmental measurements were made in support of an ONR Department Research Initiative on high-frequency sound interaction in ocean sediments [M. D. Richardson et al. and E. I. Thorsos et al., IEEE J. Ocean Eng. 26, 4-53 (2001)]. Supporting environmental measurements include digital stereo photography of the interface roughness which were converted [A. P. Lyons et al., IEEE J. Ocean Eng. 27, 515-524 (2002)] to digital elevation maps. Using these micro-bathymetry data sets, the measured backscatter strengths from SAX99 are modeled via an extension of the wedge assemblage model [R. S. Keiffer and J. C. Novarini, J. Acoust. Soc. Am. 107, 27-39 (2000)] that allows for density-contrast between the water and sediment but assumes no change in the sound speed. In particular, SAX99 data in which changes in the acoustic backscatter can be directly related to the time evolution of the fine-scale interface morphology will be modeled and compared. [Work supported by ONR, Program Element No. 61153N-32 and by the DoD High Performance Computing Shared Resource Center (Stennis). This document has been reviewed and approved for public release.]
The impact of the background bubble layer on reverberation-derived scattering strengths in the low to moderate frequency range
Journal of The Acoustical Society of America, 1995
Applied Acoustics, 1999
Davis and Scharstein have recently derived a complete solution for the impulse response of a dens... more Davis and Scharstein have recently derived a complete solution for the impulse response of a density contrast wedge (Davis AMJ, Scharstein RW. J. Acoust. Soc. Am. 1997, 101, 2821± 1835). However, the general solution given as a sum of residues, reduces to a closed form only for a discrete set of wedge angles. To circumvent this limitation and make the solution available for arbitrary angles, the explicit form for the residues is provided and the numerical implementation of the solution is discussed. The solution is then applied to the analysis of the diracted ®eld and physical aspects are analyzed. #
The impulse response of an aperture: Numerical calculations within the framework of the wedge assemblage method
Journal of The Acoustical Society of America, 1994
ABSTRACT
Forward scattering at 400 Hz from rough seas in the presence of a range-independent underwater bubble layer
Journal of The Acoustical Society of America, 1996
Journal of The Acoustical Society of America, 2000
A time domain method for calculating the acoustic impulse response of impenetrable, rough, two-di... more A time domain method for calculating the acoustic impulse response of impenetrable, rough, two-dimensional ͑2D͒ surfaces is presented. The method is based on an extension of the wedge assemblage ͑WA͒ method to 2D surfaces and objects. Like the WA method for one-dimensional ͑1D͒ surfaces, the approach for 2D surfaces uses Biot's and Tolstoy's exact solution for the impulse response of an infinite impenetrable wedge ͓J. Acoust. Soc. Am. 29, 381-391 ͑1957͔͒ as its fundamental building block. The validity of the WA method for backscattering from 2D sea surfaces is assessed through comparisons with calculations based on Milder's operator expansion ͑OE͒ method ͓J. Acoust. Soc. Am. 89, 529-541 ͑1991͔͒. Average intensities for backscattering from 2D fully developed seas ͑20 m/s wind speed͒ were computed by the WA and OE methods using 50 surface realizations and compared at 11 frequencies between 100 and 200 Hz. A single, moderately low grazing angle of incidence ͑20°͒ and several scattered grazing angles ͑90°, 45°, 20°, and 10°͒ were considered. Excellent overall agreement between the two models was obtained. The utility of the WA method as a tool to describe the physics of the scattering process is also discussed.
Applied Acoustics, 2003
A numerical model to calculate the impulse response of a two-dimensional, impenetrable, rough sur... more A numerical model to calculate the impulse response of a two-dimensional, impenetrable, rough surface directly in the time domain has been recently introduced [J. Acoust. Soc. Am. (2000) 107, 27]. This model is based on wedge diffraction theory and assumes that the halfspace containing the source and receiver is homogeneous. In this work, the model is extended to handle media where the index of refraction varies with the distance to the surface by merging the scattering model with a ray-based propagation model. The resulting hybrid model is tested against a Finite-Difference Time-Domain (FDTD) method for the problem of backscattering from a corrugated surface in the presence of a refractive layer. This new model can be applied, for example, to calculate acoustic reverberation from the sea surface in cases where the water mass is inhomogeneous and in the presence of a subsurface bubble layer at low frequencies where dispersion is negligible. It can also be used for atmospheric propagation problems where there is a sound speed gradient overlying rough terrain. Published by Elsevier Science Ltd.
Modeling the propagation from a horizontally directed high-frequency source in shallow water in the presence of bubble clouds and sea surface roughness
Journal of The Acoustical Society of America, 1998
ABSTRACT Among the many factors affecting the propagation of sound in shallow water, surface-gene... more ABSTRACT Among the many factors affecting the propagation of sound in shallow water, surface-generated microbubbles have remained virtually unexplored. The collection of microbubbles, bubbles which usually do not result in a uniform layer, presents a complex structure that varies not only in depth but also in range, and can be characterized as a collage of bubble clouds. A numerical procedure is developed in which the bubble clouds are modeled following a classification scheme proposed by Monahan [Natural Physical Sources of Underwater Sound, edited by B. V. R. Kerman (Kluwer Academic, Dordrecht, 1993), pp. 503-517]. An effective complex index of refraction of the bubble mixture is calculated for each point of the resulting range-dependent environment. The combined effect that the sea surface roughness and the bubbly environment have on forward propagation is then modeled through a high fidelity model [Norton et al., J. Acoust. Sec. Am. 97, 2173-2180 (1995)] which combines a finite element Parabolic Equation model with conformal mapping to handle surface roughness. The case of a horizontally directed source, operating at 20 kHz for a single realization of a shallow water environment is analyzed in detail. The presence of the bubble clouds severely affects the amplitude near the surface, but their influence rapidly diminishes in depth. For the case considered, the surface roughness, which has little effect on the transmission loss of the propagating field, is, however, responsible for interference effects of the forward field observed throughout the water column, while ignoring the rough surface and bubble clouds leads to a 12-15 dB error in the acoustic field near the surface, at a range of 150 m. To assume a uniformly stratified (range independent) bubble layer results in large errors near the surface at the location of the high void fraction packets, but is an acceptable approximation away from these features. (C) 1998 Acoustical Society of America.
FDTD modeling of low-frequency sea surface reverberation in the presence of a near-surface bubble layer
Journal of The Acoustical Society of America, 2000
ABSTRACT There exists a well‐documented discrepancy between the low to moderate frequency (150 Hz... more ABSTRACT There exists a well‐documented discrepancy between the low to moderate frequency (150 Hz<f<1.5 kHz), low‐grazing angle (<30 degrees) reverberation‐derived backscattering strengths collected at sea and the predictions provided by rough surface scattering models. One of the central features of the data/model comparisons in this regime is the strong wind speed dependence exhibited in the data and the very weak wind speed dependence predicted by surface scattering models. In a previous study [Keiffer et al., J. Acoust. Soc. Am. 97, 227–234 (1995)], a heuristic model was used to explore the hypothesis that bubble‐induced refraction may modify the insonification of the air/sea interface and significantly enhance the surface reverberation in this frequency range. In the present work, a finite difference time domain (FDTD) solution to the linear acoustic wave equation is exercised in numerical experiments designed to unambiguously demonstrate the significant impact that near‐surface, bubble‐induced refraction can have on the reverberation time series and, therefore, on the surface scattering strengths.
An evaluation of the Kirchhoff approximation in predicting the axial impulse response of hard and soft disks
Journal of The Acoustical Society of America, 1993
ABSTRACT
Finite-difference time-domain modeling of low to moderate frequency sea-surface reverberation in the presence of a near-surface bubble layer
Journal of The Acoustical Society of America, 2001
ABSTRACT A finite-difference time-domain (FDTD) solution to the two-dimensional linear acoustic w... more ABSTRACT A finite-difference time-domain (FDTD) solution to the two-dimensional linear acoustic wave equation is utilized in numerical experiments to test the hypothesis that near-surface, bubble-induced refraction can have a significant impact on low to moderate frequency sea-surface reverberation. In order to isolate the effects of bubble-modified propagation on the scattering from the air/sea interface from other possible phenomena such as scattering from bubble clouds, the bubbly environment is assumed to be range independent. Results of the study show that both the strong wind-speed dependence and the enhanced scattering levels of the order found in the reverberation data are obtained when a wind-speed-dependent bubble layer is included in the modeling. (C) 2001 Acoustical Society of America.
Near-field acoustic scattering from rough surfaces
The Journal of the Acoustical Society of America, 1990
Modeling the azimuthal dependence of sea surface backscatter during CST7
Journal of The Acoustical Society of America, 1994
Mathematical and Computer Modelling, 1988
Wave Motion, 2011
A one-dimensional weakly-nonlinear model equation based on a Lagrangian-averaged Euler-α model of... more A one-dimensional weakly-nonlinear model equation based on a Lagrangian-averaged Euler-α model of compressible flow in lossless fluids is presented. Traveling wave solutions (TWS)s, in the form of a topological soliton (or kink), admitted by this fourth-order partial differential equation are derived and analyzed. An implicit finite-difference scheme with internal iterations is constructed in order to study soliton collisions. It is shown that, for certain parameters, the TWSs interact as solitons, i.e., they retain their "identity" after a collision. Kink-like solutions with an oscillatory tail are found to emerge in a signaling-type initial-boundary-value problem for the linearized equation of motion. Additionally, connections are drawn to related weakly-nonlinear acoustic models and the Korteweg-de Vries equation from shallow-water wave theory.
On the validity of the wedge assemblage method for pressure-release sinusoids
Journal of The Acoustical Society of America, 1993
Exact scattering theory applied to more realistic 2-D sea surfaces
Journal of The Acoustical Society of America, 1991
Near-field acoustic scattering from simulated two-dimensional, wind-driven sea surfaces
Journal of The Acoustical Society of America, 1992
Journal of The Acoustical Society of America, 1992
N PAGE Form Aptroved 3N AGI OBM No. 0704-0188' ubirc raiiiw por eiprse. indchiing the time for re... more N PAGE Form Aptroved 3N AGI OBM No. 0704-0188' ubirc raiiiw por eiprse. indchiing the time for reviewing instructions. searching existing 9-1i sources, gathering and ailiig AU-aaA25/ 804' ,a IJ l 1 1 , ntd corrrrrnlsregardlngohlsbjrden or any other aspecto thlscollection of Inlformnln .. including suggestions I' I Irl icII Il II operations and Repors, 1215 Jeerson Davis Highway, Suite 1204. Arlington. VA 2224302. and to 'he Office of Ington, DC 20503.
IEEE Journal of Oceanic Engineering, 1998
When modeling sound propagation through the uppermost layers of the ocean, the presence of bubble... more When modeling sound propagation through the uppermost layers of the ocean, the presence of bubble clouds cannot be ignored. Their existence can convert a range-independent sound propagation problem into a range-dependent one. Measurements show that strong changes in sound speed and attenuation are produced by the presence of swarms of microbubbles which can be depicted as patchy clouds superimposed on a very weak background layer. While models suitable for use in acoustic calculations are available for the homogeneous bubble layer (which results from long time averages of the total bubble population), no similar parameterizations are available for the more realistic inhomogeneous bubble layer. Based on available information and within the framework of a classification scheme for bubble plumes proposed by Monahan, a model for the range and depth dependence of the bubbly environment is developed to fill this void. This model, which generates a possible realization of the bubbly environment, is then used to calculate the frequencydependent change in the sound speed and attenuation induced by the presence of the bubbles plumes. Time evolution is not addressed in this work.
Wedge assemblage modeling of SAX99 backscatter strengths
Journal of The Acoustical Society of America, 2001
In SAX99 (Sediment Acoustics Experiment 1999), a number of high quality acoustic and environmenta... more In SAX99 (Sediment Acoustics Experiment 1999), a number of high quality acoustic and environmental measurements were made in support of an ONR Department Research Initiative on high-frequency sound interaction in ocean sediments [M. D. Richardson et al. and E. I. Thorsos et al., IEEE J. Ocean Eng. 26, 4-53 (2001)]. Supporting environmental measurements include digital stereo photography of the interface roughness which were converted [A. P. Lyons et al., IEEE J. Ocean Eng. 27, 515-524 (2002)] to digital elevation maps. Using these micro-bathymetry data sets, the measured backscatter strengths from SAX99 are modeled via an extension of the wedge assemblage model [R. S. Keiffer and J. C. Novarini, J. Acoust. Soc. Am. 107, 27-39 (2000)] that allows for density-contrast between the water and sediment but assumes no change in the sound speed. In particular, SAX99 data in which changes in the acoustic backscatter can be directly related to the time evolution of the fine-scale interface morphology will be modeled and compared. [Work supported by ONR, Program Element No. 61153N-32 and by the DoD High Performance Computing Shared Resource Center (Stennis). This document has been reviewed and approved for public release.]
The impact of the background bubble layer on reverberation-derived scattering strengths in the low to moderate frequency range
Journal of The Acoustical Society of America, 1995
Applied Acoustics, 1999
Davis and Scharstein have recently derived a complete solution for the impulse response of a dens... more Davis and Scharstein have recently derived a complete solution for the impulse response of a density contrast wedge (Davis AMJ, Scharstein RW. J. Acoust. Soc. Am. 1997, 101, 2821± 1835). However, the general solution given as a sum of residues, reduces to a closed form only for a discrete set of wedge angles. To circumvent this limitation and make the solution available for arbitrary angles, the explicit form for the residues is provided and the numerical implementation of the solution is discussed. The solution is then applied to the analysis of the diracted ®eld and physical aspects are analyzed. #
The impulse response of an aperture: Numerical calculations within the framework of the wedge assemblage method
Journal of The Acoustical Society of America, 1994
ABSTRACT
Forward scattering at 400 Hz from rough seas in the presence of a range-independent underwater bubble layer
Journal of The Acoustical Society of America, 1996
Journal of The Acoustical Society of America, 2000
A time domain method for calculating the acoustic impulse response of impenetrable, rough, two-di... more A time domain method for calculating the acoustic impulse response of impenetrable, rough, two-dimensional ͑2D͒ surfaces is presented. The method is based on an extension of the wedge assemblage ͑WA͒ method to 2D surfaces and objects. Like the WA method for one-dimensional ͑1D͒ surfaces, the approach for 2D surfaces uses Biot's and Tolstoy's exact solution for the impulse response of an infinite impenetrable wedge ͓J. Acoust. Soc. Am. 29, 381-391 ͑1957͔͒ as its fundamental building block. The validity of the WA method for backscattering from 2D sea surfaces is assessed through comparisons with calculations based on Milder's operator expansion ͑OE͒ method ͓J. Acoust. Soc. Am. 89, 529-541 ͑1991͔͒. Average intensities for backscattering from 2D fully developed seas ͑20 m/s wind speed͒ were computed by the WA and OE methods using 50 surface realizations and compared at 11 frequencies between 100 and 200 Hz. A single, moderately low grazing angle of incidence ͑20°͒ and several scattered grazing angles ͑90°, 45°, 20°, and 10°͒ were considered. Excellent overall agreement between the two models was obtained. The utility of the WA method as a tool to describe the physics of the scattering process is also discussed.
Applied Acoustics, 2003
A numerical model to calculate the impulse response of a two-dimensional, impenetrable, rough sur... more A numerical model to calculate the impulse response of a two-dimensional, impenetrable, rough surface directly in the time domain has been recently introduced [J. Acoust. Soc. Am. (2000) 107, 27]. This model is based on wedge diffraction theory and assumes that the halfspace containing the source and receiver is homogeneous. In this work, the model is extended to handle media where the index of refraction varies with the distance to the surface by merging the scattering model with a ray-based propagation model. The resulting hybrid model is tested against a Finite-Difference Time-Domain (FDTD) method for the problem of backscattering from a corrugated surface in the presence of a refractive layer. This new model can be applied, for example, to calculate acoustic reverberation from the sea surface in cases where the water mass is inhomogeneous and in the presence of a subsurface bubble layer at low frequencies where dispersion is negligible. It can also be used for atmospheric propagation problems where there is a sound speed gradient overlying rough terrain. Published by Elsevier Science Ltd.
Modeling the propagation from a horizontally directed high-frequency source in shallow water in the presence of bubble clouds and sea surface roughness
Journal of The Acoustical Society of America, 1998
ABSTRACT Among the many factors affecting the propagation of sound in shallow water, surface-gene... more ABSTRACT Among the many factors affecting the propagation of sound in shallow water, surface-generated microbubbles have remained virtually unexplored. The collection of microbubbles, bubbles which usually do not result in a uniform layer, presents a complex structure that varies not only in depth but also in range, and can be characterized as a collage of bubble clouds. A numerical procedure is developed in which the bubble clouds are modeled following a classification scheme proposed by Monahan [Natural Physical Sources of Underwater Sound, edited by B. V. R. Kerman (Kluwer Academic, Dordrecht, 1993), pp. 503-517]. An effective complex index of refraction of the bubble mixture is calculated for each point of the resulting range-dependent environment. The combined effect that the sea surface roughness and the bubbly environment have on forward propagation is then modeled through a high fidelity model [Norton et al., J. Acoust. Sec. Am. 97, 2173-2180 (1995)] which combines a finite element Parabolic Equation model with conformal mapping to handle surface roughness. The case of a horizontally directed source, operating at 20 kHz for a single realization of a shallow water environment is analyzed in detail. The presence of the bubble clouds severely affects the amplitude near the surface, but their influence rapidly diminishes in depth. For the case considered, the surface roughness, which has little effect on the transmission loss of the propagating field, is, however, responsible for interference effects of the forward field observed throughout the water column, while ignoring the rough surface and bubble clouds leads to a 12-15 dB error in the acoustic field near the surface, at a range of 150 m. To assume a uniformly stratified (range independent) bubble layer results in large errors near the surface at the location of the high void fraction packets, but is an acceptable approximation away from these features. (C) 1998 Acoustical Society of America.
FDTD modeling of low-frequency sea surface reverberation in the presence of a near-surface bubble layer
Journal of The Acoustical Society of America, 2000
ABSTRACT There exists a well‐documented discrepancy between the low to moderate frequency (150 Hz... more ABSTRACT There exists a well‐documented discrepancy between the low to moderate frequency (150 Hz<f<1.5 kHz), low‐grazing angle (<30 degrees) reverberation‐derived backscattering strengths collected at sea and the predictions provided by rough surface scattering models. One of the central features of the data/model comparisons in this regime is the strong wind speed dependence exhibited in the data and the very weak wind speed dependence predicted by surface scattering models. In a previous study [Keiffer et al., J. Acoust. Soc. Am. 97, 227–234 (1995)], a heuristic model was used to explore the hypothesis that bubble‐induced refraction may modify the insonification of the air/sea interface and significantly enhance the surface reverberation in this frequency range. In the present work, a finite difference time domain (FDTD) solution to the linear acoustic wave equation is exercised in numerical experiments designed to unambiguously demonstrate the significant impact that near‐surface, bubble‐induced refraction can have on the reverberation time series and, therefore, on the surface scattering strengths.
An evaluation of the Kirchhoff approximation in predicting the axial impulse response of hard and soft disks
Journal of The Acoustical Society of America, 1993
ABSTRACT
Finite-difference time-domain modeling of low to moderate frequency sea-surface reverberation in the presence of a near-surface bubble layer
Journal of The Acoustical Society of America, 2001
ABSTRACT A finite-difference time-domain (FDTD) solution to the two-dimensional linear acoustic w... more ABSTRACT A finite-difference time-domain (FDTD) solution to the two-dimensional linear acoustic wave equation is utilized in numerical experiments to test the hypothesis that near-surface, bubble-induced refraction can have a significant impact on low to moderate frequency sea-surface reverberation. In order to isolate the effects of bubble-modified propagation on the scattering from the air/sea interface from other possible phenomena such as scattering from bubble clouds, the bubbly environment is assumed to be range independent. Results of the study show that both the strong wind-speed dependence and the enhanced scattering levels of the order found in the reverberation data are obtained when a wind-speed-dependent bubble layer is included in the modeling. (C) 2001 Acoustical Society of America.