Correcting the Kirchhoff rough boundary. Interaction model for scattering (original) (raw)

Sound scattering from a randomly rough fluid-solid interface

The Journal of the Acoustical Society of America, 1987

DDD12. High-resolution acoustic bottom roughness measurement in support of bottom echo interaction modeling. W. P. Dammann and C. A. Lauter (Ocean Acoustics Division, NOAA/AOML, 4301 Rickenbacker Causeway, Miami, FL 33149) A high-resolution acoustic bottom profiler using an extremely narrow-beam, 3-MHz echo sounder was developed at the Ocean Acoustic Division of NOAA/AOML. The device was used to measure bottom roughness over a range of scales from less than 1 cm to several meters. Roughness measurements were made in the lower Chesapeake Bay area over mud, fine to medium grain sand, and course grain sand. The data produced were used to appraise the performance of an acoustic echo formation model that predicts the effects of marine bottom characteristics on a reflected acoustic pulse envelope. Major aspects of the design and use of the system, procedures for processing generated data, and examples of processed output are presented. 2:23 DDD13. Characterization of seafloor type and roughness from 12-kHz acoustic backscattering measurements.

Measurements and modelling of high-frequency acoustic scattering by a rough seafloor and sea surface

Acoustic scattering by a rough, possibly dynamic interface is experimentally studied by insonifying the seabed and the sea surface at high frequency at various incident angles. A directional source working at 300 kHz was placed at the top of a 3.5 m high tower deployed on the seabed. A vertical array of 3 omnidirectional hydrophones was suspended from a portable frame, which was deployed in bistatic configuration at a variable range between 30 and 70 m. A selection of the results is presented to evaluate the sea surface and seabed scattering amplitude in the nominal specular reflection direction. Scattering by the sea surface was measured during relatively long periods of time in order to correlate its value with the sea state. Model-data comparison was conducted between the scattering data and a time-domain, three-dimensional rough surface scattering model (BORIS-SSA). Model- based analysis allows for a better understanding of some aspects of high-frequency multipath reverberation ...

The Effects of Seafloor Roughness on Acoustic Scattering: Manipulative Experiments

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The Dependence of Long-Range Reverberation on Bottom Roughness

2004

At long-range, shallow-water reverberation can be driven by sub-critical-angle scattering, i.e. by rough interrace scattering. The Naval Research Laboratory has recently developed a small-slope model for elastic seafloors that provides physics-based estimates of the dependence of scattering on the incident and scattered angles, and physical descriptors of the environment. In this paper, this incoherent model is used as kernels in reverberation models, which in turn are used to assess the sensitivity at 3.5 kHz of long-range monostatic reverberation to the roughness of the water-sediment interface. It is shown that when sub-critical-angle scattering dominates, the acoustic field could be quite sensitive to the parameter values of the roughness, thus arguing for the need for regional in-situ methods for its estimation.

Effects of Changing Roughness on Acoustic Scattering: (1) Natural Changes

1999

High resolution (~0.1-1 cm) measurements of seafloor roughness with underwater stereo photogrammetry were performed during the shallow-water SAX99 acoustic experiment. Changes in morphology due to hydrodynamic and biological processes were observed, and documented by changes in the values of measured slope and spectral strength of the seafloor roughness power spectrum. Roughness spectral and geoacoustic parameters were used in the first-order

High-frequency bottom backscattering: Roughness versus sediment volume scattering

The Journal of the Acoustical Society of America, 1992

High-frequency bottom acoustic and geoacoustic data from three well-characterized sites of different bottom composition are compared with scattering models in order to clarify the roles played by interface roughness and sediment volume inhomogeneities. Model fits to backscattering data from two silty sites lead to the conclusion that scattering from volume inhomogeneities was primarily responsible for the observed backscattering. In contrast, measured bottom roughness was sufficient to explain the backscattering seen at a sandy site. Although the sandy site had directional ripples, the model and data agree in their lack of anisotropy. PACS numbers: 43.30. Hw, 43.30. Gv, 43.20.Fn I. EXPERIMENTAL METHODS Geoacoustic and backscattering data were obtained at three separate shallow-water sites. The "Quinault" site ( 17 km west of the coast of the State of Washington, 47ø34'N, 124ø35'W) has a fine-sand bottom with pronounced directional ripples. The Arafura Sea site (255 km north-northwest of Cape Arnhem, Australia, 10ø0 I'S, 137ø50'E) has a relatively smooth bottom composed of a silt-clay mixture with numerous buried shell fragments. The "San Francisco" site (180 km northwest of San Francisco, 38ø39'N, 123ø29'W) has a silty bottom of moderate roughness. The San Francisco data were gathered as part of the STRESS ( Sediment TRansport Events on Shelves and Slopes) experiment. The site considered in the present work is known as the STRESS "mid-shelf' site. A. Geoacoustic measurements The Quinault, Arafura Sea, and San Francisco experiment sites were surveyed by means of box coring and stereophotography and found to be relatively uniform within a 1km 2 area surrounding the acoustic measurement locations. Subsamples from box cores were collected as described by Briggs et al. (1985) with 6.1-cm-diam cylindrical cores for 962 J. Acoust. Soc. Am. 92 (2), Pt. 1, August 1992 0001-4966/92/080962-16500.80

Roughness Spectra and Acoustic Response from a Diver-Manipulated Sea Floor

OCEANS 2006, 2006

The public reporting burden for this collection of information is estimated to average 1 hour per response, Including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information It it does not display a currently valid OMB control number. 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF RESPONSIBLE PERSON a. REPORT b. ABSTRACT c. THIS PAGE ABSTRACT OF Kevin Briggs PAGES SAR P S19b. TELEPHONE NUMBER (Include area code) Unclassified Unclassified Unclassified 5 228-688-5518 Standard Form 298 (Rev. 8/98)

Recent developments in modelling acoustic reflection loss at the rough ocean surface

2011

The transmission of sonar signals in a surface ducted environment, or in a shallow ocean, is affected by reflection losses at the ocean surface, when wind action or swell causes the surface to be roughened. Under these circumstances, the amplitude of the specular reflection of sound at the ocean surface is reduced by a number of complex phenomena, including: the sea surface shape; acoustic shadowing of parts of the surface to sound incident at small angles; diffraction of sound into the shadow zones; and bubble formation from white-caps. Recent work has shown that the inclusion of these effects within a ray model of transmission is a formidable prospect, as ray theory cannot describe all the phenomena explicitly, and the inclusion of acoustic wave effects in combination with a ray model is required. This paper addresses several of the complexities, in the search for a comprehensive solution to this modelling issue. In particular, the appropriateness of the Small-Slope Approximation roughness model used by Williams et al. (JASA, 116, Oct. 2004) is investigated, using a Parabolic Equation (PE) model. Also, the refraction near the ocean surface caused by wind-induced bubbles (e.g. Ainslie, JASA, 118, Dec. 2005) is investigated using the PE model. Lastly, the surface loss values obtained for received coherent sound pressure are compared with those relevant to received root-mean-square sound pressure. The paper speculates on the prospects for the future development of a surface loss model that includes all relevant effects.

Effects of changing roughness on acoustic scattering: (2) anthropogenic changes

2000

Deliberate modification of bottom roughness, including smoothing to eliminate centimetre scale natural roughness and raking to induce quasiperiodic roughness, was investigated using diver observations, quantification of bottom roughness from stereo photography, and measurement of acoustic backscattering strength. At 40 kHz, raking perpendicular to the acoustic line-of-sight with a tine spacing equal to one-half wavelength increased scattering by 12-18 dB, which

Bottom interaction of low-frequency acoustic signals at small grazing angles in the deep ocean

The Journal of the Acoustical Society of America, 1981

The results of a deep-ocean bottom interaction experiment are presented in which the effects of both bottom refraction and subbottom reflection were observed. Data were obtained in the Hatteras Abyssal Plain using a deep towed 220-Hz pulsed cw source and two receivers anchored near the bottom. For ranges between 1 and 6 km, corresponding to bottom grazing angles less than 13 °, the quadrature components of the received signals were recorded digitally. The observed amplitude shows a strong spatial interference pattern which is composed of the direct and bottom interacting arrivals. It is shown that for small source–receiver separations, the bottom return is dominated by a strong subbottom reflection. With increasing separation, this arrival evolves into a refracted arrival due to the presence of a positive sound-speed gradient in the sediment overlying the subbottom. Because of the gradient, a caustic is formed, and corresponding high intensity regions are observed in the data at the...

A Novel approach to bottom scattering using a narrow acoustic beam

Hydroacoustics, 2004

The acoustic response of the ocean bottom to a probing pulse is a complex and complicated process. This process is influenced by with the form of an acoustic transmitting/receiving beam and by the physical processes involved in sound scattering from the surface and the volume of the ocean bottom. The complexities of these phenomena often obscure an intuitive understanding of the underlying principles of echo formation and its reception. In this paper, we propose a simplistic model for this complex process using filter theory. The bottom is represented as a surface reflector with an acoustic wave front sweeping over it with time-varying velocity. The impulse response of a smooth flat bottom is characteristic of a low pass-filter that will greatly attenuate the impinging high frequency pulse. On the other hand, bottom undulations will modulate the reflected signal such that it can be represented by the impulse response of a band-pass filter. The received echo can be represented as the response of such filter to a high frequency pulse. The characteristics and amplitude of the echo are dependent on frequency spectrum overlap between the transmitted pulse spectrum and the filter frequency response. In the paper, we discuss several cases of interest with the intent to provide a solid intuitive understanding of the echo formation from the system point of view.

Bottom Interaction in Ocean Acoustic Propagation

The long term objective here is to understand the dominant physical mechanisms responsible for propagation and scattering over distances from tens to thousands of kilometers in the deep ocean where the sound channel is not bottom limited. The specific goal is to study the role of bottom interaction and bathymetry on the stability, statistics, spatial distribution and predictability of broadband acoustic signals observed just above and on the deep seafloor (greater than the critical depth). What is the relationship between the seismic (ground motion) noise on the seafloor and the acoustic noise in the water column? What governs the trade-offs in contributions from local and distant storms and in contributions from local and distant shipping? How effective is seafloor bathymetry at stripping distant shipping noise from the ambient noise field? This project addresses "the effects of environmental variability induced by ocean internal waves, internal tides and mesoscale processes, and by bathymetric features including seamounts and ridges, on the stability, statistics, spatial distribution and predictability of broadband acoustic signals..." (quote from the Ocean Acoustics web page). Understanding long range acoustic propagation in the ocean is essential for a broad range of Navy applications such as the acoustic detection of ships and submarines at long ranges, avoiding detection of ships and submarines, long range command and communications to submerged assets, and improving understanding of the environment through which the Navy operates. The long-term objective here is to understand the dominant physical mechanisms responsible for propagation and scattering in the deep ocean where the sound channel is not bottom limited.

Initial Phase of a Study of Bottom Interaction of Low Frequency Underwater Sound

1976

In Block 20, If different from Report) 18, SUPPLEMENTARY NOTES 19. KEY WORDS (Continue on reverse aide if necessary and iden!lfy by block number) underwater sound propagation bottom interaction propagation models \0bottom loss models sensitivity 16VIABSTRACT (Continue on reverse aide if necesaoy end Identify by block number) Bottom interaction is recognized as an important and only partially understood component of low frequency underwater sound propagation. Several phases of this complex problem have been investigated during the first year of a planned multiple year study. This report describes several aspects of the study including sensitivity of propagation loss to bottom loss variations, sensitivity of bottom loss to variations in ocean bottom physical parameters, bottom roughness effects, and propagation over a sloping bottom. (U) JAN73 1473 EDITION OF I NOV65 IS OBSOLETE UNCLASSIFIED SECURITY CLASSIFICATION OF THIS PAGE