• Journal of the Korean Geophysical Society
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• v.2 no.3
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• pp.217-224
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• 1999

Although faults can be found by geological surveys, the surface traces of faults are not easily discovered by traditional geological surveys due to alluvia. In and around faults and fracture zones, the electrical resistivity appears to be lower than that of the surroundings due to the content of groundwater and clay minerals. Therefore, electrical resistivity surveys are effective to search buried faults and fracture zones. The dipole-dipole array electrical resistivity surveys, which could show the two dimensional subsurface electrical resistivity structure, were carried out in two areas, Yongdang-ri, Woongsang-eup, Yangsan-si, Kyungsangnam-do and Malbang-ri, Woedong-eup, Kyungju-si, Kyungsangpook-do. The one was next to the Dongrae Fault and the other near the Ulsan Fault was close to the region in which debatable quaternary fault traces had been found recently. From each measured data set, the electrical resistivity cross-section was obtained using the inversion program the reliability of which was analyzed using analytic solutions. A low resistivity zone was found in the inverted cross-section from the Yongdang-ri area survey data, and two low resistivity zones were found in that from the Malbang-ri area survey data. They were almost vertical and were 15∼20 m wide. Accounting the shape and the very low resistivity values of those zones (<100 Ωm)in the inverted section, they were interpreted as fracture zones although they should be proven by trenching. The reliability of the interpretation might be improved by adding some more parallel resistivity survey lines and interpreting the results in 3 and/or adding other geophysical survey.

• Journal of the Korean Geophysical Society
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• v.8 no.1
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• pp.15-21
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• 2005

The apparent source spectrum of the Fukuoka earthquake is estimated at the seismic basement by removing from the observed spectra at Korean seismic stations the path and site responses that were previously revealed through inversion process applied to large spectral D/B accumulated until 2004. The approximate source spectrum is also estimated by using data recorded near the epicenter from various Japanese seismic networks and compared with the Korean source spectrum. The comparison result shows that there is good agreement among source spectra estimated based on the data from seismic networks of Korea at large distances (190 km

• 한국지구물리탐사학회:학술대회논문집
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• 2001.09a
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• pp.122-140
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• 2001

We have applied the geophysical survey, mainly electric and electromagnetic (EM) methods, to a test site contaminated by hydrocarbon waste disposal and local spill. The multi-frequency, moving source & receiver EM survey along with ground penetrating radar (GPR) showed a fairly good performance in detection of buried metal pipes and objects. Magnetic survey measuring vertical and horizontal gradients were so sensitive to the small metallic objects spread over the surface that it's hard to discriminate the buried pipe. We chose electrical resistivity, EM and GPR survey to examine the soil contamination. Depth slices of resistivity distribution as the results of the inversion of resistivity and EM data coincided each other and closely matched the contaminated area determined by chemical analysis of the soil samples. GPR images did not show the reflection events related with contamination plume since there are no distinct spill in this site. We inferred the contamination using the penetration depth of the GPR energy, which could be used as auxiliary information to the resistivity and EM results. We summarized the applicability of each survey methods based on this results and proposed a desirable survey scheme for the determination of hydrocarbon contaminated site.

• Geophysics and Geophysical Exploration
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• v.6 no.3
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• pp.143-152
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• 2003

Among the various geophysical approaches to identify the leakage of leachate with conductivity variation, conventional electrical resistivity survey has been mainly used at waste landfill. We adopted small-loop electromagnetic (EM) survey using multi-frequencies in parallel with electrical resistivity survey to delineate the leakage of leachate through the shallow soil layer at a waste landfill in Jeju Island, and also with self-potential monitoring to detect the streaming potential produced by the movement of leachate. There were no evidences of leakage from waste landfill according to the results of the electrical resistivity survey and SP monitoring, and it was also true from the results of water quality analysis at stream around waste landfill periodically. On the other hand, the results of one-dimensional inversion of spatially-filtered small-loop EM survey data showed the anomalous zone of low resistivity with depth both around and inner waste landfill.

• Geophysics and Geophysical Exploration
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• v.10 no.4
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• pp.241-251
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• 2007

The observed spectra from Odaesan earthquake were fitted to a point-source spectral model to evaluate the source spectrum and spatial features of the modelling error. The source spectrum was calculated by removing from the observed spectra the path and site dependent responses (Yun, 2007) that were previously revealed through an inversion process applied to a large accumulated spectral dataset. The stress drop parameter of one-corner Brune's ${\omega}^2$ source model fitted to the estimated source spectrum was well predicted by the scaling relation between magnitude and stress drop developed by Yun et al. (2006). In particular, the estimated spectrum was quite comparable to the two-corner source model that was empirically developed for recent moderate earthquakes occurring around the Korean Peninsula, which indicates that Odaesan earthquake is one of typical moderate earthquakes representative of Korean Peninsula. Other features of the observed spectra from Odaesan earthquake were also evaluated based on the commonly treated random error between the observed data and the estimated point-source spectral model. Radiation pattern of the error according to azimuth angle was found to be similar to the theoretical estimate. It was also observed that the spatial distribution of the errors was correlated with the geological map and the $Q_0$ map which are indicatives of seismic boundaries.

• Geophysics and Geophysical Exploration
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• v.11 no.2
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• pp.137-147
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• 2008

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.

• Geophysics and Geophysical Exploration
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• v.10 no.4
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• pp.252-258
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• 2007

We have studied the feasibility of geostatistics approach to enhancing analysis of sparsely obtained seismic data by combining with satellite gravity data. The shallow depth and numerous fishing nets in The Yellow Sea, west of Korea, makes it difficult to do seismic surveys in this area. Therefore, we have attempted to use geostatistics to integrate the seismic data along with gravity data. To evaluate the feasibility of this approach, we have extracted only a few seismic profile data from previous surveys in the Yellow Sea and performed integrated analysis combining with the results from gravity data under the assumption that seismic velocity and density have a high physical correlation. First, we analyzed the correlation between extracted seismic profiles and depths obtained from gravity inversion. Next, we transferred the gravity depth to travel time using non-linear indicator transform and analyze residual values by kriging with varying local means. Finally, the reconstructed time structure map was compared with the original seismic section given in the previous study. Our geostatistical approach demonstrates relatively satisfactory results and especially, in the boundary area where seismic lines are sparse, gives us more in-depth information than previously available.

• Geophysics and Geophysical Exploration
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• v.11 no.2
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• pp.85-92
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• 2008

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.

• Journal of the Korean Geophysical Society
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• v.3 no.4
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• pp.235-244
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• 2000

We studied basic ideas of a network based Gravity/Magnetic data processing server/client system which provides functions of data processing, forward modeling, inversion and data process on Data Base. This Java technology was used to provide facilities, socket communication and JDBC(Java Database Connectivity) technology to produce an effective and practical client application. The server computers are linked by network to process the MPI parallelized computing. This can provide useful devices of the geophysical process and modeling that usually require massive computing performance and time. Since this system can be accessed by lots of users, it can provides the consistent and confident results through the verified processing programs. This system also makes it possible to get results and outputs through internet when their local machines are connected to the network. It can help many users who want to omit the jobs of system administration and to process data during their field works.

• Geophysics and Geophysical Exploration
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• v.4 no.3
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• pp.70-79
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• 2001

The integral equation method is a powerful tool for numerical electromagnetic modeling. But the difficulty of this technique is the size of the linear equations, which demands excessive memory and calculation time to invert. This limitation of the integral equation method becomes critical in inverse problem. The conventional Born approximation, where the electric field in the anomalous body is approximated by the background field, is very rapid and easy to compute. However, the technique is inaccurate when the conductivity contrast between the body and the background medium is large. Quasi-linear, quasi-analytical and extended Born approximations are novel approaches to 3-D EM modeling based on the linearization of the integral equations for scattered EM field. These approximation methods are much less time consuming than full integral equation method and more accurate than conventional Born approximation. They we, however, still approximate methods for 3-D EM modeling. Iterative series methods such as modified Born, quasi-linear and quasi-analytical can be used to increase the accuracy of various approximation methods. Comparisons of numerical performance against a full integral equation and various approximation codes show that the iterative series methods are very accurate and almost always converge. Furthermore, they are very fast and easy to implement on a computer. In this study, extended Born series method is developed and it shows more accurate result than that of other series methods. Therefore, Iterative series methods, including extended Born series, open principally new possibilities for fast and accurate 3-D EM modeling and inversion.