• Journal of the Korean Geophysical Society
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• v.8 no.1
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• pp.45-48
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• 2005

In order to investigate the velocity structure of the southern part of the Korean peninsula, exploded seismic signals were recorded for 120 s along a 294-km WNW-ESE line and 150 s along a 335-km NNW-SSE line in 2002 and 2004, respectively. Velocity tomograms were derived from inverting first arrival times. One-dimensional velocity models derived by joint analyses of teleseismic receiver functions and surface wave dispersion at several stations near the profiles were uesd to build initial models. The raypaths indicate several midcrust interfaces including ones at approximate depths of 2.0 and 14.9 km with refraction velocities of approximately 6.0 and 7.1 km/s, respectively. The deepest significant interface varies in depth from 30.8 km to 36.1 km. The critically refracting velocity varies from 7.8 to 8.1 km/s along this interface which may correspond to the Moho discontinuity. The velocity tomograms show (1) existence of a low-velocity zone centered at 6-7 km depth under the Okchon fold belt, (2) extension of the Yeongdon fault down to greater than 10 km, and (3) existence of high-velocity materials under the Gyeongsan basin whose thickness is less than 4.2 km.

• Journal of the Korean earth science society
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• v.21 no.6
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• pp.675-682
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• 2000

25 seismic shot gathers were obtained to study the two dimensional subsurface shear wave velocities in a landfilled area near the Keum river estuary. Borehole(BH#1 and BH#2) tests at two sites were made in the same area. Standard Penetration Tests were also performed at the same time. The 2-D shear wave velocity structure resulted from the inversion of the seismic data shows that the subsurface of the studied area consists of the upper 1${\sim}$3 meter thick layer(200 m/sec${\sim}$700 m/sec), the middle 5${\sim}$8 m thick low velocity layer(100 m/sec${\sim}$400 m/sec), and the lower layer of 1000m/sec or higher shear wave velocities. The thickness of the low velocity layer decreases from the BH #1 site to the BH #2 site. The depth to the basement also decreases toward the BH #2 site. The examination of the S wave velocity structure, the description of the geologic contents, and the Standard Penetration Test values indicate that the middle layer of low shear wave velocity may be related to the clay content of the layer. On the other hand, the Standard Penetration test values increase with depth, showing no significant relationship with the geologic contents of the subsurface. This study shows that the inversion of surface waves can be effective in the study of the shear wave velocity, especially in the area where low velocity layers can be found. The method of inversion of surface waves also can be used as a viable technique to overcome the limit of the seismic refraction method.

• The Korean Journal of Petroleum Geology
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• v.1 no.1 s.1
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• pp.29-35
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• 1993

Seismic reflection data can be used to extract information about the velocity structure of the earth. This process is called a velocity inversion of the seismic data. However, it is difficult to recover a broad band reflection coefficient series because the frequency band of seismic trace is limited. The linear programming method has been examined to find the simplest velocity model that has frequency components consistent with the usable frequencies of the seismic trace and interval velocity data. The velocity structure of the earth is displayed in pseudovelocity section. After the linear program had been tested with a synthetic seismic trace, it was applied to the seismic reflection data of the Block Ⅱ in the Yellow Sea. By comparing the pseudovelocity section with sonic logs obtained from the well in the same area, it was possible to define the lithostratigraphy and the boundaries of Cretaceous volcanics and Cretaceous metavolcanics.

• Journal of the Korean Geophysical Society
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• v.5 no.3
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• pp.233-245
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• 2002

The spectral-analysis-of-surface-waves (SASW) method is a nondestructive testing method based upon generation and detection of elastic stress waves. SASW is widely used as one of the techniques to determine stiffness profile in engineering geophysics. The essential steps involved are construction of an experimental dispersion curve from data collected in situ, and inversion of the dispersion curve to determine the stiffness profile. The main object of this study is to derive an analytical Jacobian for the inversion. If we set the subsurface to N homogeneous layer, it could save 2N times Jacobian calculation compared to numerical jacobian calculation during inversion. To reconstruct a stiffness profile, constrained damped least square method was applied for the inversion. The algorithm was tested for the numerical data and for the real asphalt and tunnel data, which were able to verify the stiffness profile. The stiffness profile reconstructed by the algorithm showed the possibility to appraise the soundness of tunnel with applications SASW.

• 한국지구물리탐사학회:학술대회논문집
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• 2006.06a
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• pp.61-70
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• 2006

3-D S-wave velocity model in the southern Korean Peninsula is investigated by using the joint inversion of receiver functions and surface-wave dispersion. A peninsula average Rayleigh-wave phase velocity in the 10-150 seconds range and tomographic estimates of the Rayleigh and Love wave group velocities in the 0.5-20 seconds period range determined using a $12.5{\times}12.5\;km$ grid for the southern part of the peninsula are used for the inversion. Receiver functions were determined from broadband (STS-2), short-period (SS-1) and acceleration (Episensor) channels of 95 stations. The dense distribution of the stations in the Peninsula permits us to examine the 3-D crustal structure in detail. The inversion result shows the variation and characteristics of S-wave velocity in the crust and upper mantle of the southern Korean Peninsula very well.

• The Journal of the Acoustical Society of Korea
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• v.24 no.6
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• pp.309-316
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• 2005

This paper Presents the geoacoustic inversion with Parallel Multi-Population Genetic Algorithm (PMPGA). This method is the modified form of simple genetic algorithm (SGA), which is devised for complementing the defects of simple genetic algorithm. The light bulb source and vertical line array (VLA) receiver are used for geoacoustic inversion. The results of this study show the geoacoustic Parameters can be estimated by PMPGA and the proposed algorithm is 1.7 times as fast as serial one on an average.

• 한국지구물리탐사학회:학술대회논문집
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• 2006.06a
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• pp.131-135
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• 2006

A seismic waveform inversion for prestack seismic data based on the Gauss-Newton method is presented. The Gauss-Newton method for seismic waveform inversion was proposed in the 80s but has rarely been studied. Extensive computational and memory requirements have been principal difficulties. To overcome this, we used different sizes of grids in the inversion stage from those of grids in the wave propagation simulation, temporal windowing of the simulation and approximation of virtual sources for calculating partial derivatives, and implemented this algorithm on parallel supercomputers. We show that the Gauss-Newton method has high resolving power and convergence rate, and demonstrate potential applications to real seismic data.

• Geophysics and Geophysical Exploration
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• v.25 no.4
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• pp.203-213
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• 2022

To understand an underlying geological structure via seismic imaging, the velocity information of the subsurface medium is crucial. Although the full-waveform inversion (FWI) method is considered useful for estimating subsurface velocity models, 3D FWI needs a lot-of computing power and time. Herein, we compare the calculation efficiency and accuracy of frequency-domain 2D and 3D acoustic FWIs. Thereafter, we demonstrate that the artifacts from 2D approximation can be partially suppressed via frequency-domain 2D FWI by employing diffraction angle filtering (DAF). By applying DAF, which employs only big reflection angle components, the impact of noise and out-of-plane reflections can be reduced. Additionally, it is anticipated that the DAF can create long-wavelength velocity structures for 3D FWI and migration.

• Geophysics and Geophysical Exploration
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• v.12 no.1
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• pp.85-98
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• 2009

The tau-p inversion algorithm is widely employed to generate starting models with most computer programs, which implement refraction tomography. This algorithm emphasises the vertical resolution of many layers, and as a result, it frequently fails to detect even large lateral variations in seismic velocities, such as the decreases which are indicative of shear zones. This study demonstrates the failure of the tau-p inversion algorithm to detect or define a major shear zone which is 50m or 10 stations wide. Furthermore, the majority of refraction tomography programs parameterise the seismic velocities within each layer with vertical velocity gradients. By contrast, the Generalized Reciprocal Method (GRM) inversion algorithms emphasise the lateral resolution of individual layers. This study demonstrates the successful detection and definition of the 50m wide shear zone with the GRM inversion algorithms. The existence of the shear zone is confirmed by a 2D analysis of the head wave amplitudes and by numerous closely spaced orthogonal seismic profiles carried out as part of a later 3D refraction investigation. Furthermore, an analysis of the shot record amplitudes indicates that a reversal in the seismic velocities, rather than vertical velocity gradients, occurs in the weathered layers. The major conclusion reached in this study is that while all seismic refraction operations should aim to provide as accurate depth estimates as is practical, those which emphasise the lateral resolution of individual layers generate more useful results for geotechnical and environmental applications. The advantages of the improved lateral resolution are obtained with 2D traverses in which the structural features can be recognised from the magnitudes of the variations in the seismic velocities. Furthermore, the spatial patterns obtained with 3D investigations facilitate the recognition of structural features such as faults which do not display any intrinsic variation or 'signature' in seismic velocities.

• Geophysics and Geophysical Exploration
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• v.1 no.1
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• pp.25-30
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• 1998

Time-lapse crosswell seismic data, recorded before and after the cavity filling, showed that the filling increased the velocity at a known cavity zone in an old mine site in Inchon area. The seismic response depicted on the tomogram and in conjunction with the geologic data from drillings imply that the size of the cavity may be either small or filled by debris. In this study, I attempted to evaluate the filling effect by analyzing velocity measured from the time-lapse tomograms. The data acquired by a downhole airgun and 24-channel hydrophone system revealed that there exists measurable amounts of source statics. I presented a methodology to estimate the source statics. The procedure for this method is: 1) examine the source firing-time for each source, and remove the effect of irregular firing time, and 2) estimate the residual statics caused by inaccurate source positioning. This proposed multi-step inversion may reduce high frequency numerical noise and enhance the resolution at the zone of interest. The multi-step inversion with different starting models successfully shows the subtle velocity changes at the small cavity zone. The inversion procedure is: 1) conduct an inversion using regular sized cells, and generate an image of gross velocity structure by applying a 2-D median filter on the resulting tomogram, and 2) construct the starting velocity model by modifying the final velocity model from the first phase. The model was modified so that the zone of interest consists of small-sized grids. The final velocity model developed from the baseline survey was as a starting velocity model on the monitor inversion. Since we expected a velocity change only in the cavity zone, in the monitor inversion, we can significantly reduce the number of model parameters by fixing the model out-side the cavity zone equal to the baseline model.