Acoustics of Layered Media I

Brekhovskikh, Leonid M.; Godin, Oleg A.

doi:10.1007/978-3-642-52369-4

Cited by 266 publications

(109 citation statements)

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“…The thickness of the ice layer might vary between 0 and 5 m (Laxon et al, 2003). The effective reflection coefficient for a seismic wave being reflected from a water surface covered by a thin ice layer is given by (Brekhovskikh and Godin, 1998) where r 1 and r 2 are the reflection coefficients for water-ice and iceair, respectively. The phase term is given as…”

Section: Appendix a Reflection Coefficient For Ice Layermentioning

confidence: 99%

Reducing high-frequency ghost cavitation signals from marine air-gun arrays

Landrø

Amundsen

2016

GEOPHYSICS

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Ghost cavitation, which is a term describing that cavitation bubbles are generated acoustically, has been hypothesized to occur when the ghost reflected signals from many individual air guns beneath the sea surface produce a pressure that is close to zero in the water above the source array. Ghost cavitation is typically observed some milliseconds after the ghost reflection, and it may last for 5-15 ms, depending on the configuration of the source array. The cavitation process subsequently generates a weak high-frequency signal. To investigate this potential signal model and mechanism, we have performed a dedicated source experiment. We found that the distance between the source strings in a source array is a major factor that influences the amount and strength of the high-frequency signal. By increasing the separation distance from 6.5 to 8 m, we have observed a significant decrease in the high-frequency signal. Further, the amount of ghost cavitation can be reduced by increasing the distance between the guns. Also single sub-arrays may create ghost cavitation sound, of course weaker in signal strength compared with full arrays, in agreement with the model. Conventional air-gun modeling can be used to predict where ghost cavitation can occur. Therefore, in principle, a workflow could be developed to quantify grossly if and how much high-frequency signals could be generated by this mechanism, given the source array configuration, and further change the configuration to reduce to a very minimum the high-frequency signals, if deemed necessary. For an airgun array consisting of two subarrays separated by 6 m and fired at 9 m depth, we found that the high-frequency signals emitted between 1 and 10 kHz were of similar strength to the noise from conventional cargo ships, depending on their size and the vessels' speed.

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Section: Appendix a Reflection Coefficient For Ice Layermentioning

confidence: 99%

Reducing high-frequency ghost cavitation signals from marine air-gun arrays

Landrø

Amundsen

2016

GEOPHYSICS

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“…The Lame's equation for the longitudinal (volumetric) disturbances in the non-homogeneous, isotropic and stationary medium given in (Brekhovskikh, Godin, 1990) can be rewritten in the space-Fourier frequency domain as follows…”

Section: Basic Equationsmentioning

confidence: 99%

Example of Structure Modeling and Analysis of Ultrasound Scattering for Trabecular Bone

Wójcik¹,

Litniewski²,

Nowicki³

2010

Archives of Acoustics

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A trabecular bone consists of trabeculae whose mechanical properties differ significantly from the surrounding marrow, therefore an ultrasonic wave propagating within the bone structure is strongly scattered. The aim of this paper was to evaluate the contribution of the first, second and higher order scattering (multiple scattering) into the total scattering of ultrasound in a trabecular bone. The scattering due to the interconnections between thick trabeculae, usually neglected in trabecular bone models, has been also studied. The basic element in our model of the trabecular bone was an elastic cylinder with a various finite-length and diameter as well as orientation. The applied model was taking into account variation of both, elements size and their spatial configuration. The field scattered on the bone model was evaluated by solving numerically the integral form of the generalized Sturm-Liouville equation describing a scalar wave in inhomogeneous and lossy media. For the scattered fields calculated numerically the effective cross-sections were determined. The influence of absorption on the scattering coefficients was demonstrated. The results allowed to conclude that within the frequency range from 0.5 to 1.5 MHz contribution of the second order scattering to the effective backscattering cross-section is at least 500 times lower than that due to the first order scattering. It was noticed that for a frequency higher than 1.5 MHz fast growth of the backscattering (reflection) coefficients, calculated for the second order scattering, occurs.

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“…The vector potential in the solid is thus ql = [0, 4> ,0]. Therefore, in each solid layer j ( the layers are from layer 2 to layer 6, the medium 1 and 7 are fluid water), the scalar and vector potentials of waves are [5]: For the case of a 5-layer structure of rubber immersed in water, the shear stress vanishes for the interfaces between water and the solid. In addition the shear displacement is, in general, discontinuous.…”

Section: Theorymentioning

confidence: 99%

“…also are as in [5]. For the interfaces between a solid and a fluid, the displacement and stress matrix equation is: (14).…”

Section: (14) Where [A] = [A(6) ][A(s) ][A(4) ][A(3) ][A(2)] For Evementioning

confidence: 99%

Transmission Coefficient for Multi-Layered Structures at Arbitrary Incident Angle

Zhang

Peterson

1999

Review of Progress in Quantitative Nondestructive Evaluation

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Acoustics of Layered Media I

Cited by 266 publications

References 0 publications

Reducing high-frequency ghost cavitation signals from marine air-gun arrays

Reducing high-frequency ghost cavitation signals from marine air-gun arrays

Example of Structure Modeling and Analysis of Ultrasound Scattering for Trabecular Bone

Transmission Coefficient for Multi-Layered Structures at Arbitrary Incident Angle

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