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Flat topped beams and their characteristics in turbulent media

OPTICS EXPRESS 2006年第10期14卷 4196-4207页

作者： Eyyuboglu, Halil Tanyer Arpali, Caglar Baykal, Yahya Kemal Cankaya Univ Elect & Commun Engn Dept TR-06530 Ankara Turkey

The source and receiver plane characteristics of flat topped ( FT) beam propagating in turbulent atmosphere are investigated. To this end, source size, beam power and M(2) factor of source plane FT beam are derived. For a turbulent propagation medium, via Huygens Fresnel diffraction integral, the receiver plane intensity is found. Power captured within an area on the receiver plane is calculated. Kurtosis parameter and beam size variation along the propagation axis are formulated. Graphical outputs are provided displaying the variations of the derived source and receiver plane parameters against the order of flatness and propagation length. Analogous to free space behavior, when propagating in turbulence, the FT beam first will form a circular ring in the center. As the propagation length increases, the circumference of this ring will become narrower, giving rise to a downward peak emerging from the center of the beam, eventually turning the intensity profile into a pure Gaussian shape. (c) 2006 Optical Society of America.

关键词： .become propagating variations kurtosis flatness propagation axis source plane propagation length turbulent atmosphere source and receiver FT beam beam size receiver plane parameters

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Performance bounds on synchronous laser line scan systems

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OPTICS EXPRESS 2005年第3期13卷 738-748页

作者： Jaffe, JS Univ Calif San Diego Scripps Inst Oceanog Marine Phys Lab La Jolla CA 92093 USA

In this article a performance bound is derived for feasible image resolution among a class of imaging systems that can be referred to as synchronous laser line scan systems. Most often, these systems use a narrow beam projected source ( typically a laser) in conjunction with a very small field of view receiver that is synchronously scanned. Here, a bound on the maximum system resolution is derived when both source and receiver are "delta function like". The bound demonstrates that the best achievable overall system point spread function is the square of the one way point spread function for the medium. (C) 2005 Optical Society of America.

关键词： .medium overall underwater Point Spread imaging systems derived Synchronous narrow very small image formation field of view turbid optical imaging laser line scan systems source and receiver

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The Fresnel volume and transmitted waves

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GEOPHYSICS 2004年第3期69卷 653-663页

作者： Spetzler, J Snieder, R Delft Univ Technol Dept Geotechnol NL-2600 GA Delft Netherlands Colorado Sch Mines Dept Geophys Golden CO 80401 USA Colorado Sch Mines Ctr Wave Phenomena Golden CO 80401 USA

In seismic imaging experiments, it is common to use a geometric ray theory that is an asymptotic solution of the wave equation in the high-frequency limit. Con- sequently, it is assumed that waves propagate along in- nitely narrow lines through space, called rays, that join the source and receiver. In reality, recorded waves have a nite-frequency content. The band limitation of waves implies that the propagation of waves is extended to a nite volume of space around the geometrical ray path. This volume is called the Fresnel volume. In this tutorial, we introduce the physics of the Fresnel volume and we present a solution of the wave equation that accounts for the band limitation of waves. The nite-frequency wave theory species sensitivity kernels that linearly re- late the traveltime and amplitude of band-limited trans- mitted and reected waves to slowness variations in the ***-frequencysensitiv- ity kernels are closely connected through the concept of constructive interference of waves. The nite-frequency wave theory leads to the counterintuitive result that a pointlike velocity perturbation placed on the geometric ray in three dimensions does not cause a perturbation of the phase of the waveeld. Also, it turns out that Fer- mat s theorem in the context of geometric ray theory is a special case of the nite-frequency wave theory in the limit of innite frequency. Last, we address the miscon- ception that the width of the Fresnel volume limits the resolution in imaging experiments.

关键词： .seismic imaging traveltime sensitivity called equation perturbation propagate geometric source and receiver ray theory slowness kernels ected Fresnel ray path transmitted TUTORIAL nite-frequency wave

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Ultrasound diffusion for crack depth determination in concrete

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JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA 2004年第2期115卷 523-529页

作者： Ramamoorthy, SK Kane, Y Turner, JA Univ Nebraska Dept Engn Mech Lincoln NE 68588 USA

The determination of the depth of surface-breaking cracks in concrete specimens using an ultrasound diffusion technique is discussed. Experiments were carried out on precracked concrete specimens of varying crack depths (0%-40% of the specimen thickness). Contact transducers were placed at the specimen surface with source and receiver separated by the crack. Tone-burst excitations over a frequency range of 400-600 kHz were used. At these frequencies, ultrasound is scattered considerably by the heterogeneities in the concrete. In the limit of many scattering events, the evolution of energy may be modeled as a diffusion process. The arrival of the peak diffuse energy at the receiver is delayed due to the presence of crack. This delay is the prime indicator used for determining crack depth. Numerical and analytical analyses were also used for comparison. These results are in basic agreement with the experiments. In addition, these analyses are used to study the limits of this technique. In particular, it is shown that this technique is applicable to cracks greater than the scattering mean-free path, which is estimated at about 1 cm for these specimens. Aspects of practical implementation are also discussed. (C) 2004 Acoustical Society of America.

关键词： .arrival large cracks source and receiver diffusion process surface-breaking crack depths Ultrasound diffusion diffuse energy

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Three-dimensional imaging of multicomponent ground-penetrating radar data

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GEOPHYSICS 2003年第4期68卷 1241-1254页

作者： van der Kruk, J Wapenaar, CPA Fokkema, JT van den Berg, PM Delft Univ Technol Ctr Tech Geosci Delft Netherlands Delft Univ Technol Dept Appl Earth Sci NL-2628 RX Delft Netherlands

Scalar imaging algorithms originally developed for the processing of remote sensing measurements (e.g., the synthetic-aperture radar method) or seismic reflection data (e.g., the Gazdag phase-shift method) are commonly used for the processing of ground-penetrating radar (GPR) data. Unfortunately;these algorithms do not account for the radiation characteristics of GPR source and receiver antennas or the vectorial nature of radar waves. We present a new multicomponent imaging algorithm designed specifically for vector electromagnetic-wave propagation. It accounts for all propagation effects, including the vectorial characteristics of the source and receiver antennas and the polarization of the electromagnetic wavefield. A constant-offset source-receiver antenna pair is assumed to overlie a dielectric medium. To assess the performance of the scalar and multicomponent imaging algorithms, we compute their spatial resolution function, which is defined as the image of a point scatterer at a fixed depth using a single frequency. Application of the new multicomponent imaging algorithm results in a circularly symmetric resolution function, demonstrating that the radiation characteristics of the source and receiver antennas do not influence the derived image. In contrast, the two tested scalar imaging algorithms return distinctly asymmetric resolution functions with incorrect phase characteristics, which could result in erroneous images of the subsurface when these algorithms are applied to GPR data. The multicomponent and two scalar imaging algorithms are tested on data acquired across numerous buried objects with various dielectric properties and different strike directions. Phase differences between the different images are similar to those observed in the synthetic examples. Of the tested algorithms, we conclude that the multicomponent approach produces the most reliable results.

关键词： .three-dimensional spatial resolution contrast account source-receiver imaging algorithms radiation characteristics ground-penetrating radar GPR point scatterer Imaging of Multicomponent source and receiver scalar vectorial multicomponent imaging

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Multiple-point equalization of room transfer functions by using common acoustical poles

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IEEE TRANSACTIONS ON SPEECH AND AUDIO PROCESSING 1997年第4期5卷 325-333页

作者： Haneda, Y Makino, S Kaneda, Y IEEE

A multiple-point equalization filter using the common acoustical poles of room transfer functions is proposed, The common acoustical poles correspond to the resonance frequencies, which are independent of source and receiver positions. They are estimated as common autoregressive (AR) coefficients from multiple room transfer functions. The equalization is achieved with a finite impulse response (FIR) filter, which has the inverse characteristics of the common acoustical pole function. Although the proposed filter cannot recover the frequency response dips of the multiple room transfer functions, it can suppress their common peaks due to resonance;it is also less sensitive to changes in receiver position, Evaluation of the proposed equalization filter using measured room transfer functions shows that it can reduce the deviations in the frequency characteristics of multiple room transfer functions better than a conventional multiple-point inverse filter, Experiments show that the proposed filter enables 1-5 dB additional amplifier gain in a public address system without acoustic feedback at multiple receiver positions, Furthermore, the proposed filter reduces the reflected sound in room impulse responses without the pre-echo that occurs with a multiple-point inverse filter. A multiple-point equalization filter using common acoustical poles can thus equalize multiple room transfer functions by suppressing their common peaks.

关键词： .peaks frequency response source and receiver AUDIO PROCESSING inverse filter Multiple-Point Acoustical Poles equalization filter resonance frequencies multiple room transfer receiver position

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