Michael G. Brown

Iowa State University

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Ilya A. Udovydchenkov

Mitre (United States)

F. J. Beron‐Vera

University of Miami

Irina I. Rypina

Woods Hole Oceanographic Institution

F. D. Tappert

University of Miami

Oleg A. Godin

Naval Postgraduate School

N. A. Zabotin

University of Colorado Boulder

Neil J. Williams

University of Miami

Liudmila Zabotina

University of Colorado Boulder

M. J. Olascoaga

University of Miami

Peter F. Worcester

Scripps Institution of Oceanography

Cooperative Institutions

University of Miami

Woods Hole Oceanographic Institution

National Oceanic and Atmospheric Administration

Scripps Institution of Oceanography

University of California, San Diego

Institute of Oceanology. PP Shirshov Russian Academy of Sciences

University of Colorado Boulder

Naval Postgraduate School

University of Washington

Lomonosov Moscow State University

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A transformation of the environment eliminates parabolic equation phase errors

The Journal of the Acoustical Society of America (2006)

Irina I. Rypina Ilya A. Udovydchenkov Michael G. Brown

It is shown that phase errors associated with the standard parabolic wave equation can be eliminated in a range-independent environment by appropriately transforming the environment. Solutions to the standard parabolic equation (PE) in the transformed environment are close to solutions to the Helmholtz equation in the original environment in the sense that ray and asymptotic mode contributions to the PE wave fields have no phase errors relative to their Helmholtz equation counterparts. Although phase errors are eliminated in the PE wave fields, complete asymptotic equivalence to the Helmholtz equation wave fields is not attained. Ray- and mode-based wave-field expansions are used to illustrate similarities and differences between PE and Helmholtz equation wave fields. Numerical simulations show excellent agreement between PE wave fields in the transformed environment and mode-based solutions to the Helmholtz equation in the true environment.

Helmholtz equation

Helmholtz free energy

Mode (computer interface)

Phase velocity

10.1121/1.2227373

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Citations (7)

Efficient numerical simulation of stochastic internal-wave-induced sound-speed perturbation fields

The Journal of the Acoustical Society of America (1998)

John A. Colosi Michael G. Brown

An efficient method is presented to numerically simulate stochastic internal-wave-induced sound-speed perturbation fields in deep ocean environments. The sound-speed perturbation field is represented as an internal-wave eigenfunction expansion in which WKB amplitude scaling and stretching of the depth coordinate are exploited. Individual realizations of the sound-speed perturbation field are constructed by evaluating a multidimensional fast Fourier transform of a complex-valued function whose modulus has a known simple form and whose phase is random. Approximations made are shown to be consistent with approximations built into the Garrett–Munk internal-wave spectrum, which is the starting point of this analysis. Both time-varying internal-wave fields in three space dimensions and frozen fields in a vertical plane are considered.

Eigenfunction

WKB approximation

Speed of Sound

10.1121/1.421381

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Citations (136)

Ocean acoustic remote sensing using ambient noise: results from the Florida Straits

Geophysical Journal International (2016)

Michael G. Brown Oleg A. Godin Xiaoqin Zang Justin S. Ball N. A. Zabotin

Noise interferometry is the process by which approximations to acoustic Green's functions, which describe sound propagation between two locations, are estimated by cross-correlating time series of ambient noise measured at those locations. Noise-interferometry-based approximations to Green's functions can be used as the basis for a variety of inversion algorithms, thereby providing a purely passive alternative to active-source ocean acoustic remote sensing. In this paper we give an overview of results from noise interferometry experiments conducted in the Florida Straits at 100 m depth in December 2012, and at 600 m depth in September/October 2013. Under good conditions for noise interferometry, estimates of cross-correlation functions are shown to allow one to perform advanced phase-coherent signal processing techniques to perform waveform inversions, estimate currents by exploiting non-reciprocity, perform time-reversal/back-propagation calculations and investigate modal dispersion using time-warping techniques. Conditions which are favourable for noise interferometry are identified and discussed.

Ambient noise level

Seismic interferometry

Cross-correlation

10.1093/gji/ggw170

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Citations (18)

Fluctuations and predictability of sound propagation at moderate frequencies in bottom-limited sound channels

The Journal of the Acoustical Society of America (1993)

Harry A. DeFerrari Michael G. Brown F. D. Tappert Charles L. Monjo Lan Nghiem-Phu

Sound propagation in the moderate frequency range (depth to wavelength ratio, h/λ, between 20 and 100) in shallow water environments with bottom roughness is extremely complicated, variable, and poorly understood. Multipaths are generally neither resolvable (assuming Q≥1) nor stable. Fluctuations are dominated by a combination of continual formation and destruction of multipaths as well as by perturbations to otherwise stable paths—as is the case in deep water. Propagation characteristics observed in the Straits of Florida for several ranges have consistent features. Sound fields are slowly varying and phase coherent. Large fluctuations with periodicities of a few minutes to a few hours can be qualitatively accounted for as a slowly modulated complicated caustic structure. The quantitative description of these phenomena is beyond the domain of validity of existing theories because variables do not separate and rays behave chaotically. Advances in both theory and experiments are required to improve understanding of these propagation issues.

Predictability

Caustic (mathematics)

Sound propagation

Speed of Sound

10.1121/1.406606

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Citations (0)

Zonal jets as transport barriers in planetary atmospheres

AGU Fall Meeting Abstracts (2007)

F. J. Beron‐Vera Michael G. Brown M. J. Olascoaga Irina I. Rypina Hüseyin Koçak

Abstract The connection between transport barriers and potential vorticity (PV) barriers in PV-conserving flows is investigated with a focus on zonal jets in planetary atmospheres. A perturbed PV staircase model is used to illustrate important concepts. This flow consists of a sequence of narrow eastward and broad westward zonal jets with a staircase PV structure; the PV steps are at the latitudes of the cores of the eastward jets. Numerically simulated solutions to the quasigeostrophic PV conservation equation in a perturbed PV staircase flow are presented. These simulations reveal that both eastward and westward zonal jets serve as robust meridional transport barriers. The surprise is that westward jets, across which the background PV gradient vanishes, serve as robust transport barriers. A theoretical explanation of the underlying barrier mechanism is provided. It is argued that transport barriers near the cores of westward zonal jets, across which the background PV gradient is small, are found in Jupi...

Zonal flow (plasma)

Source

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Erratum: ‘‘Causality, caustics, and the structure of transient wave fields’’ [J. Acoust. Soc. Am. 8 , 251–255 (1986)]

The Journal of the Acoustical Society of America (1986)

Michael G. Brown

First Page

Causality

Transient (computer programming)

10.1121/1.394493

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Citations (2)

The Maslov integral representation of slowly varying dispersive wavetrains in inhomogeneous moving media

Wave Motion (2000)

Michael G. Brown

Abstract The Maslov integral representation of slowly varying surface gravity wavetrains is developed, allowing for smooth but otherwise arbitrary variations of both bathymetry and horizontal currents. Although we focus on the surface gravity wave problem, the results presented can be applied — and are presented in a form which facilitates their application — to any type of scalar small amplitude dispersive wave motion in a slowly varying environment, with or without background flow. The Maslov integral provides an asymptotically valid solution, on the wavelength scale, to the initial value problem under conditions in which exact solutions are unavailable owing to nonseparability of the equations of motion. Caustics of arbitrary complexity are properly treated. Away from caustics, stationary phase evaluation of the Maslov integral reduces it to a superposition of locally plane, slowly varying dispersive wavetrains that conserve wave action. An important step in the development of the Maslov integral is the geometric construction of exact solutions to the time-dependent wave action conservation equation. It is shown that in an unbounded homogeneous environment the Maslov integral reduces as a special case to the usual Fourier integral solution to the initial value problem.

Representation

10.1016/s0165-2125(00)00046-9

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Citations (20)

Predictability Limitations of Long-Range Sound Propagation

Michael G. Brown

Abstract : Our long-term scientific goal is to understand the basic physics of low-frequency long-range sound propagation in the ocean, and the effects of environmental variability on signal stability and coherence. We seek to understand the fundamental limits to signal processing imposed by ocean variability to enable advanced signal processing techniques, including matched field processing and other adaptive array processing methods.

Predictability

SIGNAL (programming language)

Array Processing

10.21236/ada531650

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Citations (0)

Long‐range acoustic transmission in the northwest Atlantic

Journal of Geophysical Research Atmospheres (1980)

Michael G. Brown Walter Munk John L. Spiesberger Peter F. Worcester

An acoustic propagation experiment conducted west of Bermuda during the fall of 1978 demonstrated that individual acoustic arrivals at 900‐km range were stable and identifiable over a 48‐day period. The observed arrival pattern is interpreted by geometric ray theory and by comparison with a ‘synthetic sofargram’ (equivalent to a synthetic seismogram) constructed from WKBJ theory. Using geometric ray theory, we examine the expected generation and annihilation of ray paths as a result of the seasonal erosion of the autumnal thermocline. Many rays are converted from purely refracted to surface reflected/refracted rays. The ultimate goal of this work is to interpret variability in acoustic transmission in terms of ocean variability.

Transmission loss

Seismogram

10.1029/jc085ic05p02699

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Citations (10)

Linearized travel time, intensity, and waveform inversions in the ocean sound channel—A comparison

The Journal of the Acoustical Society of America (1984)

Michael G. Brown

Linearized inversion schemes involving waveform (total record), intensity (dephased total record), and travel times (intensity peaks) are used on synthetically generated arrival patterns to solve for the sound speed structure in the upper km of a range-independent ocean model. In all three cases inversions are carried out for a variety of source center frequencies and bandwidths. The results are compared; conditions under which each inversion method performs poorly and well are outlined. The nature of the linearization approximation is discussed for each of the three types of inversion scheme as well as the methods used to solve the general linearized system.

Linearization

Intensity

10.1121/1.390852

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Citations (16)