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Refraction traveltime tomography based on damped wave equation for irregular topographic model
Park, Yunhui,Pyun, Sukjoon Elsevier 2018 Journal of applied geophysics Vol.150 No.-
<P><B>Abstract</B></P> <P>Land seismic data generally have time-static issues due to irregular topography and weathered layers at shallow depths. Unless the time static is handled appropriately, interpretation of the subsurface structures can be easily distorted. Therefore, static corrections are commonly applied to land seismic data. The near-surface velocity, which is required for static corrections, can be inferred from first-arrival traveltime tomography, which must consider the irregular topography, as the land seismic data are generally obtained in irregular topography.</P> <P>This paper proposes a refraction traveltime tomography technique that is applicable to an irregular topographic model. This technique uses unstructured meshes to express an irregular topography, and traveltimes calculated from the frequency-domain damped wavefields using the finite element method. The diagonal elements of the approximate Hessian matrix were adopted for preconditioning, and the principle of reciprocity was introduced to efficiently calculate the Fréchet derivative. We also included regularization to resolve the ill-posed inverse problem, and used the nonlinear conjugate gradient method to solve the inverse problem.</P> <P>As the damped wavefields were used, there were no issues associated with artificial reflections caused by unstructured meshes. In addition, the shadow zone problem could be circumvented because this method is based on the exact wave equation, which does not require a high-frequency assumption. Furthermore, the proposed method was both robust to an initial velocity model and efficient compared to full wavefield inversions. Through synthetic and field data examples, our method was shown to successfully reconstruct shallow velocity structures. To verify our method, static corrections were roughly applied to the field data using the estimated near-surface velocity. By comparing common shot gathers and stack sections with and without static corrections, we confirmed that the proposed tomography algorithm can be used to correct the statics of land seismic data.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A refraction traveltime tomography is developed to invert near-surface velocity. </LI> <LI> The finite-element method using unstructured mesh is used to handle irregular topography. </LI> <LI> Traveltimes and Fréchet derivatives are calculated using frequency-domain damped wavefield. </LI> <LI> The proposed algorithm is proved to be applicable to real seismic data. </LI> </UL> </P>
배출가스의 Engine-Out과 Tail-Out의 Time delay에 관한 고찰
박윤희(Yunhui Park),정종현(Chonghyeon Cheong) 한국자동차공학회 2012 한국자동차공학회 학술대회 및 전시회 Vol.2012 No.11
Nowadays, exhaust emissions is a key issue which can be often seen in mass media due to environment problem in the world. It is already known that exhaust emissions and increased- CO2 are the reason to cause the global warming. For these reasons, the regulations for exhaust emissions from vehicles have become much more stringent in recent years. These more stringent regulations request vehicle manufacturers to develop new emission control equipment and more precised analysis technologies for exhaust emissions. There are some factors related to analyzing exhaust emissions such as engine component and EMS(Engine Management System) and Catalyst Convertor. These can be affected to develop for reducing exhaust emission. The measuring points are engine-out and tail-out which can be used to analyzing time-delay during FTP-75 mode. Above these bases, this paper will discuss the time delay effect according to engine-out, tail-out and catalyst convertor for improvements which will require further improvements in emissions. So, this test can be used to determine more detailed exhaust emissions analysis when measuring tail-out emissions.
GEOLOGICALLY CONSTRAINED FULL WAVEFORM INVERSION
Sukjoon PYUN,Yunhui PARK 한국산업응용수학회 2013 한국산업응용수학회 학술대회 논문집 Vol.8 No.1
It is apparent that full waveform inversion with regularization shows a better result than with no regularization. However, it may fail to delineate sharp geological structures such as faults and salt boundaries. Hence, the geological information should be considered to recover accurate subsurface structures. In this paper, an adaptive smoothing regularization is implemented based on local slope. The adaptive regularization incorporates geological information into inversion process. Local slopes are calculated using the Hilbert transform and stabilized through isotropic and edge-enhancing smoothing before and after calculating local slopes, respectively. The adaptive smoothing regularization is applied to the full waveform inversion for a synthetic subsurface model. The inversion result shows that the layer interfaces are faithfully preserved and sharpened.