scholarly journals Ultrasonic laboratory tests of geophysical tomographic reconstruction

Geophysics ◽  
1988 ◽  
Vol 53 (7) ◽  
pp. 947-956 ◽  
Author(s):  
Tien‐when Lo ◽  
M. Nafi Toksöz ◽  
Shao‐hui Xu ◽  
Ru‐Shan Wu
Keyword(s):  
Born Approximation ◽  
Laboratory Tests ◽  
Tomographic Reconstruction ◽  
Acoustic Properties ◽  
Diffraction Tomography ◽  
Frequency Range ◽  
Scaled Model ◽  
Surface Reflection ◽  
Rytov Approximation ◽  
Reflection Tomography

In this study, we test geophysical ray tomography and geophysical diffraction tomography by scaled model ultrasonics experiments. First, we compare the performance of these two methods under limited view‐angle conditions. Second, we compare the adaptabilities of these two methods to objects of various sizes and acoustic properties. Finally, for diffraction tomography, we compare the Born and Rytov approximations based on the induced image distortion by using these two approximation methods. Our experimental results indicate the following: (1) When the scattered field can be obtained, geophysical diffraction tomography is in general superior to ray tomography because diffraction tomography is less sensitive to the limited view‐angle problem and can image small objects of size comparable to a wavelength. (2) The advantage of using ray tomography is that reconstruction can be done using the first arrivals only, the most easily measurable quantity; and there is no restriction on the properties of the object being imaged. (3) For geophysical diffraction tomography, the Rytov approximation is valid over a wider frequency range than the Born approximation in the cross‐borehole experiment. In the VSP and the surface reflection tomography experiments, no substantial difference between the Born and Rytov approximations is observed.

Diffraction tomography and multisource holography applied to seismic imaging

Geophysics ◽  
1987 ◽  
Vol 52 (1) ◽  
pp. 11-25 ◽  
Author(s):  
Ru‐Shan Wu ◽  
M. Nafi Toksöz
Keyword(s):  
Sampling Interval ◽  
Imaging Method ◽  
Diffraction Tomography ◽  
Reconstruction Process ◽  
Numerical Examples ◽  
Surface Reflection ◽  
Reflection Data ◽  
Rytov Approximation ◽  
Long Tails ◽  
Vsp Data

Seismic tomography is emerging as an imaging method for determining subsurface structure. When the view‐angle coverage is limited and the scale of the medium inhomogeneities is comparable with the wavelength, as is often true in geophysical applications, the performance of ordinary ray tomography becomes poor. Other tomographic methods are needed to improve the imaging process. Here we study diffraction tomography and multisource holography and evaluate their performances for surface reflection profiling (SRP), vertical seismic profiling (VSP), and cross‐hole measurements. Theoretical formulations are derived for two‐dimensional geometry in terms of line sources along a source line and line receivers along a receiver line. The theory for diffraction tomography is based on the Born or Rytov approximation. The performances of diffraction tomography and multisource holography are evaluated by examining the information coverage in the spatial frequency domain and by numerical examples. Multisource holography, which is similar to Kirchhoff‐type migration, often gives distorted images of the object. This distortion causes long tails of the image in the case of SRP and a strong noise belt in the case of VSP and is due to incomplete and nonuniform coverage of the object spectrum. The filtering operation of diffraction tomography helps in correcting the nonuniform coverage (including duplication) of the object spectrum in the reconstruction process and therefore reduces the distortions. On the other hand, multisource holography is better suited for imaging sharp boundaries with large acoustic impedance contrasts since diffraction tomography is restricted, as presently formulated, to weak inhomogeneities. In addition, multisource holography has the flexibility to be used with an arbitrary number of sources (including a single source). Its sampling interval is not restricted by the Nyquist frequency. Numerical examples show that combined data sets (such as surface reflection data combined with VSP data, or cross‐hole data combined with surface data, etc.) improve the image quality.

High Resolution Low Frequency Ultrasonic Tomography

1997 ◽  
Vol 19 (4) ◽  
pp. 278-293 ◽  
Author(s):  
P. Lasaygues ◽  
J.P. Lefebvre ◽  
S. Mensah
Keyword(s):  
Cross Section ◽  
Born Approximation ◽  
Tomographic Reconstruction ◽  
Scattering Problem ◽  
Acoustic Scattering ◽  
Resolving Power ◽  
Ultrasonic Tomography ◽  
Fourier Synthesis ◽  
Central Wavelength ◽  
Reflection Tomography

Ultrasonic reflection tomography results from a linearization of the inverse acoustic scattering problem, named the inverse Born approximation. The goal of ultrasonic reflection tomography is to obtain reflectivity images from backscattered measurements. This is a Fourier synthesis problem and the first step is to correctly cover the frequency space of the object. For this inverse problem, we use the classical algorithm of tomographic reconstruction by summation of filtered backprojections. In practice, only a limited number of views are available with our mechanical rig, typically 180, and the frequency bandwidth of the pulses is very limited, typically one octave. The resolving power of the system is then limited by the bandwidth of the pulse. Low and high frequencies can be restored by use of a deconvolution algorithm that enhances resolution. We used a deconvolution technique based on the Papoulis method. The advantage of this technique is conservation of the overall frequency information content of the signals. The enhancement procedure was tested by imaging a square aluminium rod with a cross-section less than the wavelength. In this application, the central frequency of the transducer was 250 kHz so that the central wavelength was 6 mm whereas the cross-section of the rod was 4 mm. Although the Born approximation was not theoretically valid in this case (high contrast), a good reconstruction was obtained.

scholarly journals A computationally efficient hybrid 2D–3D subwoofer model

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Ahmad H. Bokhari ◽  
Martin Berggren ◽  
Daniel Noreland ◽  
Eddie Wadbro
Keyword(s):  
Hybrid Model ◽  
3D Model ◽  
Element Model ◽  
Acoustic Properties ◽  
Computational Time ◽  
Average Response ◽  
Frequency Range ◽  
Computationally Efficient ◽  
Lumped Element ◽  
Lumped Element Model

AbstractA subwoofer generates the lowest frequency range in loudspeaker systems. Subwoofers are used in audio systems for live concerts, movie theatres, home theatres, gaming consoles, cars, etc. During the last decades, numerical simulations have emerged as a cost- and time-efficient complement to traditional experiments in the design process of different products. The aim of this study is to reduce the computational time of simulating the average response for a given subwoofer design. To this end, we propose a hybrid 2D–3D model that reduces the computational time significantly compared to a full 3D model. The hybrid model describes the interaction between different subwoofer components as interacting modules whose acoustic properties can partly be pre-computed. This allows us to efficiently compute the performance of different subwoofer design layouts. The results of the hybrid model are validated against both a lumped element model and a full 3D model over a frequency band of interest. The hybrid model is found to be both accurate and computationally efficient.

Non-destructive ultrasonic method of testing the strength of backfill concrete at deep Talnakh mines

2020 ◽  
pp. 28-33
Author(s):  
A. V. Trofimov ◽  
◽  
A. E. Rumyantsev ◽  
A. P. Gospodarikov ◽  
A. P. Kirkin ◽  
...  
Keyword(s):  
Laboratory Tests ◽  
Underground Mining ◽  
Ultrasonic Method ◽  
Acoustic Method ◽  
Propagation Rate ◽  
Acoustic Properties ◽  
Quality Analysis ◽  
Transverse Waves ◽  
Longitudinal And Transverse Waves ◽  
Mechanical Methods

In underground mining with backfilling of mined-out areas, it is necessary to analyze the filling mass quality. Since coring and testing would make a complex process, the method of express analysis comes in handy. As the mechanical methods of express quality analysis are not very accurate, the best option would be to use an acoustic method. But before the latter can be used, it is necessary to conduct a series of laboratory tests in order to under stand the empirical correlation between strength and acoustic pro perties. This paper describes the results of laboratory tests that were carried out for samples of backfill concrete after 7, 28, 45, 70, 90 days of curing to test their strength and acoustic properties. A correlation has been found between the uniaxial compressive strength of the fill and the ultrasonic wave propagation rates. Some field measurements have been taken to determine the propagation rates of longitudinal and transverse waves in the filling mass. Calibration curves have been built for both longitudinal and transverse waves. It was established that for the filling mass that hasn’t cured for 90 days it is the propagation rate of transverse waves that correlates most accurately with the strength; whereas for the filling mass that is older than 90 days it is the propagation rate of longitudinal waves.

High Frequency Ultrasound, a Tool for Elastic Properties Measurement of Thin Films Fabricated on Silicon

2011 ◽  
Vol 324 ◽  
pp. 277-281 ◽  
Author(s):  
Pierre Campistron ◽  
Julien Carlier ◽  
Nadine Saad ◽  
Jamin Gao ◽  
Malika Toubal ◽  
...  
Keyword(s):  
Thin Films ◽  
Elastic Properties ◽  
High Frequency ◽  
Shear Waves ◽  
Thin Layers ◽  
Acoustic Properties ◽  
Frequency Range ◽  
Frequency Method ◽  
Longitudinal Waves

The main goal of this work is to develop an ultrasonic high frequency method for characterization of thin layers. The development of high frequency acoustic transducers for longitudinal waves and shear waves on silicon has enabeled the characterization of thin films deposited on this substrate. Three types of transducers have been achieved : (i) single crystal LiNbOSubscript text3 Y+163° for shear waves generation, and (ii) Y+36° for longitudinal waves, bonded and thinned on silicon substrate to achieve ultrasonic transducers in the frequency range 300-600 MHz ; (iii) thin films ZnO transducers were realized due to sputtering technologies working in the frequency range 1 GHz- 2.5 GHz. Using an inversion method and a network analyser which provide the scattering S11 parameter of the transducer versus the frequency we deduce the elastic properties of films deposited on the wafer surface. Thanks to these transducers the acoustic properties of thin films such as SU-8 based nanocomposites (doped with TiO2 , SrTiO3 or W nanoparticles) will be presented. In order to achieve mechanical impedance matching between silicon and water we control the mass of the embedded particles which provide a way to adjust the elastic properties of the characterized material. In another application an Indium metallic layer have been characterized in the high frequency range. We also use this method to characterize dielectric permittivity of the ZnO transducers.

Wave‐equation tomography

Geophysics ◽  
1992 ◽  
Vol 57 (1) ◽  
pp. 15-26 ◽  
Author(s):  
Marta Jo Woodward
Keyword(s):  
Wave Equation ◽  
Synthetic Data ◽  
Frequency Dispersion ◽  
Diffraction Tomography ◽  
Back Projection ◽  
Data Set ◽  
Rytov Approximation ◽  
Transmission Geometry ◽  
Ray Trace ◽  
Finite Aperture

The relation between ray‐trace and diffraction tomography is usually obscured by formulation of the two methods in different domains: the former in space, the latter in wavenumber. Here diffraction tomography is reformulated in the space domain, under the title of wave‐equation tomography. With this transformation, wave‐equation tomography projects monochromatic, scattered wavefields back over source‐receiver wavepaths, just as ray‐trace tomography projects traveltime delays back over source‐receiver raypaths. Derived under the Born approximation, these wavepaths are wave‐theoretic back‐projection patterns for reflected energy; derived under the Rytov approximation, they are wave‐theoretic back‐projection patterns for transmitted energy. Differences between ray‐trace and wave‐equation tomography are examined through comparison of wavepaths and raypaths, followed by their application to a transmission‐geometry, synthetic data set. Rytov wave‐equation tomography proves superior to ray‐trace tomography in dealing with geometrical frequency dispersion and finite‐aperture data, but inferior in robustness. Where ray‐trace tomography assumes linear phase delay and inverts the arrival time of one well‐understood event, wave‐equation tomography accommodates scattering and inverts all of the signal and noise on an infinite trace simultaneously. Interpreted through the uncertainty relation, these differences lead to a redefinition of Rytov wavepaths as monochromatic raypaths, and of raypaths as infinite‐bandwidth wavepaths (Rytov wavepaths averaged over an infinite bandwidth). The infinite‐bandwidth and infinite‐time assumptions of ray‐trace and Rytov, wave‐equation tomography are reconciled through the introduction of bandlimited raypaths (Rytov wavepaths averaged over a finite bandwidth). A compromise between rays and waves, bandlimited raypaths are broad back‐projection patterns that account for the uncertainty inherent in picking traveltimes from bandlimited data.

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