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REFERENCE LINKING PLATFORM OF KOREA S&T JOURNALS
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Geophysics and Geophysical Exploration
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Korean Society of Earth and Exploration Geophysicists
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Volume 11, Issue 4 - Nov 2008
Volume 11, Issue 3 - Aug 2008
Volume 11, Issue 2 - May 2008
Volume 11, Issue 1 - Feb 2008
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Two-Dimensional Magnetotelluric Interpretation by Finite-Element Method
Kim, Hee-Joon ; Choi, Ji-Hyang ; Han, Nu-Ree ; Lee, Seong-Kon ; Song, Yoon-Ho ;
Geophysics and Geophysical Exploration, volume 11, issue 2, 2008, Pages 85~92
Magnetotelluric (MT) methods are widely applied as an effective exploration technique to geothermal surveys. Two-dimensional (2-D) analysis is frequently used to investigate a complicated subsurface structure in a geothermal region. A 2-D finite-element method (FEM) is usually applied to the MT analysis, but we must pay attention to the accuracy of so-called auxiliary fields. Rodi (1976) proposed an algorithm of improving the accuracy of auxiliary fields, and named it as the MOM method. Because it introduces zeros into the diagonal elements of coefficient matrix of the FEM total equation, a pivoting procedure applied to the symmetrical band matrix makes the numerical solution far less efficient. The MOM method was devised mainly for the inversion analysis, in which partial derivatives of both electric and magnetic fields with respect to model parameters are required. In the case of forward modeling, however, we do not have to resort to the MOM method; there is no need of modifying the coefficient matrix, and the auxiliary fields can be elicited from the regular FEM solution. The computational efficiency of the MOM method, however, can be greatly improved through a sophisticated rearrangement of the total equation.
Elastic Wave Modeling Including Surface Topography Using a Weighted-Averaging Finite Element Method in Frequency Domain
Choi, Ji-Hyang ; Nam, Myung-Jin ; Min, Dong-Joo ; Shin, Chang-Soo ; Suh, Jung-Hee ;
Geophysics and Geophysical Exploration, volume 11, issue 2, 2008, Pages 93~98
Abstract: Surface topography has a significant influence on seismic wave propagation in a reflection seismic exploration. Effects of surface topography on two-dimensional elastic wave propagation are investigated through modeling using a weighted-averaging (WA) finite-element method (FEM), which is computationally more efficient than conventional FEM. Effects of air layer on wave propagation are also investigated using flat surface models with and without air. To validate our scheme in modeling including topography, we compare WA FEM results for irregular topographic models against those derived from conventional FEM using one set of rectangular elements. For the irregular surface topography models, elastic wave propagation is simulated to show that breaks in slope act as a new source for diffracted waves, and that Rayleigh waves are more seriously distorted by surface topography than P-waves.
Geophysical Study on the Geoelectrical Structure of the Hwasan Caldera in the Euisung Sub-basin Using Magnetotelluric Survey
Yang, Jun-Mo ; Kwon, Byung-Doo ; Cho, In-Ky ; Lee, Heui-Soon ; Park, Gye-Soon ; Um, Joo-Young ;
Geophysics and Geophysical Exploration, volume 11, issue 2, 2008, Pages 99~108
To extend our detailed knowledge for the Hwasan caldera, we carried out magnetotelluric (MT) survey, which is pretty sensitive to electrical property variation in both horizontal and vertical direction of subsurface, across the Hwasan caldera with the direction of EW. The 2-D inversion results of observed MT data lead to following conclusions. Firstly, the depth of the basin basement inferred by the MT inversion results matches well with that suggested by previous potential studies, but the basement resistivity seems fairly low when compared to that of general case. This feature might be related with the large-scaled, highly conductive layer beneath the Euisung Sub-basin suggested by the previous MT study. Secondly, the high resistivity zones reaching to 4000
are imaged around two external ring fault boundaries. These zones are thought of as the response of the rhyolitic dykes intruding along the ring fault, and in the previous gravity data correspond to relatively high density anomalies. Thirdly, low resistivity zone reaching to 200
is detected around a depth of 1km beneath the central part of the caldera, which has not been yet reported in korean geophysical literatures. If we take account of the evolution model of the Hwasan caldera, this zone is regarded as the past sedimentary layer that subsided during the period of forming external ring fault system. In addition, the relatively low density anomaly observed in the central part of the caldera may be attributed to this sedimentary layer.
A Study on the Shallow Marine Site Survey using Seismic Reflection and Refraction Method
Shin, Sung-Ryul ; Kim, Chan-Su ; Jo, Churl-Hyun ;
Geophysics and Geophysical Exploration, volume 11, issue 2, 2008, Pages 109~115
It is very important to estimate the physical properties of survey area and delineate the geological basement in marine site survey for the design of offshore structures. For the purpose of providing high quality data by means of engineering site survey, it is necessary to apply several survey techniques and carry out the integrated interpretation to each other. In this study, we applied single channel seismic reflection method and OBC (Ocean Bottom Cable) type seismic refraction method at shallow marine. We used a dual boomer-single channel streamer as a source-receiver in seismic reflection survey and airgun source-the developed OBC type streamer in seismic refraction survey. We made 24 channels OBC type streamer which has 4m channel interval and each channel is composed of single hydrophone and preamplifier. We tested the field applicability of the proposed method and applied the typical seismic data processing methods to the obtained reflection data in order to enhance the data quality and image resolution. In order to estimate the geological velocity distribution from refraction data, seismic refraction tomography technique was applied. Therefore, we could successfully perform time-depth conversion using the velocity information as an integrated interpretation. The proposed method could provide reliable geologic information such as sediment layer thickness and 3D basement depth map.
Seismic Amplitude and Frequency Characteristics of Gas hydrate Bearing Geologic Model
Shin, Sung-Ryul ; Lee, Sang-Cheol ; Park, Keun-Pil ; Lee, Ho-Young ; Yoo, Dong-Geun ; Kim, Young-Jun ;
Geophysics and Geophysical Exploration, volume 11, issue 2, 2008, Pages 116~126
In gas hydrate survey, seismic amplitude and frequency characteristics play a very important role in determining whether gas hydrate exists. According to the variation of source frequency and scatterer size, we study seismic amplitude characteristics using elastic modeling applied at staggered grids. Generally speaking, scattering occurs in proportion to the square of source frequency and the scatterer volume, which has an effect on seismic amplitude. The higher source frequency is, the more scattering occurs in gas hydrate bearing zone. Therefore, BSR is hardly observed in high frequencies. On the other side, amplitude blanking zone and BSR is clearly observed in lower frequencies although the resolution is poor as a whole. Seismic reflections traveling through free-gas layer below gas hydrate bearing zone decay so severely a high frequency component that a low frequency term is dominant. Amplitude anomaly of BSR result from high acoustic impedance contrast due to free-gas, which is a very crucial factor to estimate gas hydrate bearing zone. Seismic frequency analysis is carried out using wavelet transform method that frequency component could be decomposed with time variation. In application of wavelet transform to the seismic physical experiments data, we can observe that reflections traveling through air layer, which corresponds to the free-gas layer, decay a high frequency component.
2-D/3-D Seismic Data Acquisition and Quality Control for Gas Hydrate Exploration in the Ulleung Basin
Koo, Nam-Hyung ; Kim, Won-Sik ; Kim, Byoung-Yeop ; Cheong, Snons ; Kim, Young-Jun ; Yoo, Dong-Geun ; Lee, Ho-Young ; Park, Keun-Pil ;
Geophysics and Geophysical Exploration, volume 11, issue 2, 2008, Pages 127~136
To identify the potential area of gas hydrate in the Ulleung Basin, 2-D and 3-D seismic surveys using R/V Tamhae II were conducted in 2005 and 2006. Seismic survey equipment consisted of navigation system, recording system, streamer cable and air-gun source. For reliable velocity analysis in a deep sea area where water depths are mostly greater than 1,000 m and the target depth is up to about 500 msec interval below the seafloor, 3-km-long streamer and 1,035
tuned air-gun array were used. During the survey, a suite of quality control operations including source signature analysis, 2-D brute stack, RMS noise analysis and FK analysis were performed. The source signature was calculated to verify its conformity to quality specification and the gun dropout test was carried out to examine signature changes due to a single air gun's failure. From the online quality analysis, we could conclude that the overall data quality was very good even though some seismic data were affected by swell noise, parity error, spike noise and current rip noise. Especially, by checking the result of data quality enhancement using FK filtering and missing trace restoration technique for the 3-D seismic data inevitably contaminated with current rip noises, the acquired data were accepted and the field survey could be conducted continuously. Even in survey areas where the acquired data would be unsuitable for quality specification, the marine seismic survey efficiency could be improved by showing the possibility of noise suppression through onboard data processing.
Fault rupture directivity of Odaesan Earthquake (M=4.8, '07. 1. 20)
Yun, Kwan-Hee ;
Geophysics and Geophysical Exploration, volume 11, issue 2, 2008, Pages 137~147
Fault rupture directivity of the Odaesan earthquake, which was inferred to be the main cause of the high PGAvalue (> 0.1 g) unusually observed at the near-source region, was analyzed by using the data from the nearby (R < 100 km) dense seismic stations. The Boatwright's method (2007) was adopted for this purpose in which the azimuth and takeoff angle of the unilateral rupture directivity function could be estimated based on the relative peak ground-motions of seismic stations resulting from the nature of the rupture directivity. In this study, the approximate values of the relative peak ground-motions was derived from the difference between the log residuals of the point-source spectral model (Boore, 2003) for the main and secondary events based on the Random Vibration Theory. In this derivation, the spectral difference for a frequency range between the source corner frequencies of main and secondary events was considered to reflect only the effect of the fault directivity. The inversion result of the model parameters for the fault directivity function showed that the fault-plane of NWW-SEE direction dipping steeply to the North with high rupture velocity near upward in SE direction is responsible for the observed high level of ground-motion at the near-source region.
Seismic Evidence and Characteristics of Gas Hydrate in the Ulleung Basin
Kim, Han-Joon ; Jou, Hyeong-Tae ; Koo, Nam-Hyeong ; Yoo, Dong-G. ; Suk, Bong-Chool ; Yoo, Hai-Soo ; Lee, Ho-Young ; Park, Keun-Pil ;
Geophysics and Geophysical Exploration, volume 11, issue 2, 2008, Pages 148~152
Multichannel seismic profiles reveal a strong bottom simulating reflector (BSR) occurring below the seafloor in the plain of the Ulleung Basin, East Sea (Japan Sea). The essential characteristics of the BSR include its cross-cutting relationship to strata, strong amplitude, and reverse polarity with respect to the seafloor reflection, representing the base of the gas hydrate stability zone (BHSZ). The BSR reflection coefficient ranging from -0.23 to -0.26 is 1.5
1.7 times that of the seafloor reflection and interval velocities decrease to less than 700 m/s below the BSR. These features indicate the existence of free gas beneath the GHSZ. Heat flow, estimated from the BSR depth as
, is in good agreement with measured values. Therefore, the BSR can be efficiently used to estimate regional distribution of heat flow in the Ulleung Basin.
Boundary conditions for Time-Domain Finite-Difference Elastic Wave Modeling in Anisotropic Media
Lee, Ho-Yong ; Min, Dong-Joo ; Kwoon, Byung-Doo ; Lim, Seung-Chul ; Yoo, Hai-Soo ;
Geophysics and Geophysical Exploration, volume 11, issue 2, 2008, Pages 153~160
Seismic modeling is used to simulate wave propagation in the earth. Although the earth's subsurface is usually semi-infinite, we cannot handle the semi-infinite model in seismic modeling because of limited computational resources. For this reason, we usually assume a finite-sized model in seismic modeling. In that case, we need to eliminate the edge reflections arising from the artificial boundaries introducing a proper boundary condition. In this study, we changed three kinds of boundary conditions (sponge boundary condition, Clayton and Engquist's absorbing boundary condition, and Higdon's transparent boundary condition) so that they can be applied in elastic wave modeling for anisotropic media. We then apply them to several models whose Poisson's ratios are different. Clayton and Engquist's absorbing boundary condition is unstable in both isotropic and anisotropic media, when Poisson's ratio is large. This indicates that the absorbing boundary condition can be applied in anisotropic media restrictively. Although the sponge boundary condition yields good results for both isotropic and anisotropic media, it requires too much computational memory and time. On the other hand, Higdon's transparent boundary condition is not only inexpensive, but also reduce reflections over a wide range of incident angles. We think that Higdon's transparent boundary condition can be a method of choice for anisotropic media, where Poisson's ratio is large.