Spectral analysis of time domain diffraction tomography

Radio Science ◽  
1997 ◽  
Vol 32 (2) ◽  
pp. 593-603 ◽  
Author(s):  
Timor Melamed ◽  
Ehud Heyman
Keyword(s):  
Spectral Analysis ◽  
Time Domain ◽  
Diffraction Tomography

System Identification of Jack-Up Platform by Spectral Analysis

2010 ◽  
Author(s):  
X. M. Wang ◽  
C. G. Koh ◽  
T. N. Thanh ◽  
J. Zhang
Keyword(s):  
Spectral Analysis ◽  
System Identification ◽  
Time Domain ◽  
Time History ◽  
Offshore Structures ◽  
Wave Load ◽  
History Of ◽  
Structural Responses ◽  
Jack Up ◽  
Numerical Strategy

For the purpose of structural health monitoring (SHM), it is beneficial to develop a robust and accurate numerical strategy so as to identify key parameters of offshore structures. In this regard, it is difficult to use time-domain methods as the time history of wave load is not available unless output-only methods can be developed. Alternatively, spectral analysis widely used in offshore engineering to predict structural responses due to random wave conditions can be used. Thus the power spectral density (PSD) of structural response may be more appropriate than time history of structural responses in defining the objective (fitness) function for system identification of offshore structures. By minimizing PSD differences between measurements and simulations, the proposed numerical strategy is completely carried out in frequency domain, which can avoid inherent problems rising from random phase angles and unknown initial conditions in time domain. A jack-up platform is studied in the numerical study. A search space reduction method (SSRM) incorporating the use of genetic algorithms (GA) as well as a substructure approach are adopted to improve the accuracy and efficiency of identification. As a result, the stiffness parameters of jack-up legs can be well identified even under fairly noisy conditions.

scholarly journals Recent Applications and Advances of Numerical Modeling and Wavefield Inversion in Nondestructive Testing

2005 ◽  
Vol 3 ◽  
pp. 167-174 ◽  
Author(s):  
R. Marklein ◽  
K. J. Langenberg ◽  
K. Mayer ◽  
J. Miao ◽  
A. Shlivinski ◽  
...  
Keyword(s):  
Nondestructive Testing ◽  
Time Domain ◽  
Linear Time ◽  
Synthetic Aperture ◽  
Diffraction Tomography ◽  
Nonlinear Inversion ◽  
Nonlinear Frequency ◽  
Source Inversion ◽  
Wavefield Inversion ◽  
Domain Inversion

Abstract. This paper presents recent advances and future challenges of the application of different linear and nonlinear inversion algorithms in acoustics, electromagnetics, and elastodynamics. The presented material can be understood as an extension of our previous work on this topic. The inversion methods considered in this presentation vary from linear schemes, like the Synthetic Aperture Radar (SAR) applied electromagnetics and the Synthetic Aperture Focussing Technique (SAFT) as its counterpart in ultrasonics, and the linearized Diffraction Tomography (DT), to nonlinear schemes, like the Contrast Source Inversion (CSI) combined with different regularization approaches. Inversion results of the above mentioned inversion schemes are presented and compared for instance for time-domain ultrasonic data from the Fraunhofer-Institute for Nondestructive Testing (IZFP, Saarbrücken, Germany). Convenient tools for nondestructive evaluation of solids can be electromagnetic and/or elastodynamic waves; since their governing equations, including acoustics, exhibit strong structural similarities, the same inversion concepts apply. In particular, the heuristic SAFT algorithm can be and has been utilized for all kinds of waves, once a scalar approximation can be justified. Relating SAFT to inverse scattering in terms of diffraction tomography, it turns out that linearization is the most stringent inherent approximation. A comparison of the inversion results using the linear time-domain inversion scheme SAFT and well tested nonlinear frequency-domain inversion schemes demonstrates the considerable potential to extend and improve the ultrasonic imaging technique SAFT while consulting the mathematics of wavefield inversion, yet, in particular if the underlying effort is considered, the relatively simple and effective SAFT algorithm works surprisingly well. Since SAFT is a widely accepted imaging tool in ultrasonic NDE it seems worthwhile to check its formal restrictions and assumptions whether they could be overcome and whether they would outperform the standard and original SAFT algorithm.

Spectral analysis of time domain induced polarization waveforms

2020 ◽  
Vol 177 ◽  
pp. 104037
Author(s):  
Sara Johansson ◽  
Per Hedblom ◽  
Torleif Dahlin
Keyword(s):  
Spectral Analysis ◽  
Time Domain ◽  
Induced Polarization
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