• Geophysics and Geophysical Exploration
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• v.1 no.1
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• pp.31-42
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• 1998

Recently crosshole seismic tomography has come to be widely used especially for the civil engineering, because it can provide more detail information than any other surface method, although the resolution of tomogram will be inevitably deteriorated to some extent due to the limited wavefield aperture on the nonuniqueness of traveltime inversion. In addition, our field sites often consist of a high-velocity bed rock overlain by low-velocity rock, sometimes with a contrast of more than 45 percent, and furthermore the bed rock is folded. The first arriving waves can be then the refracted ones that travel along the bed rock surface for some source/receiver distances. Thus, the desirable first arrivals can be easily misread that cause severe distortion of the resulting tomogram, if it is concerned with (straight ray) traveltime inversion procedure. In this case, comparision with synthetic data (forward modeling) is a valuable tool in the interpretation process. Besides, abundant information is contained in the crosshole data. For instance, examination of tube waves can be devoted to detecting discontinuities within the borehole such as breakouts, faults, fractures or shear zones as well as the end of the borehole. Specific frequency characteristics of marine silty mud will help discriminate from other soft rocks. The aim of this paper is to present several strategies to analyze and interpret the crosshole data in order to improve the ability at first to determine the spatial dimensions of interwell anomalies and furthermore to understand the underground structures. To this end, our field data are demonstrated. Possibility of misreading the first arrivals was illustrated. Tube waves were investigated in conjunction with the televiewer images. Use of shot- and receiver gathers was examined to benefit the detectabilities of discontinuities within the borehole.

• Tunnel and Underground Space
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• v.17 no.4
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• pp.255-265
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• 2007

As the mechanism and effect range of subsidence are altered according to the various conditions (the ground condition, the earth pressure, the geometric condition of underground cavity and the structure load), the analysis and prediction of subsidence in abandoned mining area are very difficult. Also, as the geological characteristics and the mining methods are differed in each mines, the application of the pre-existing reinforcements without improvement has a lot of difficulties and limits. In this study, the various underground investigation such as long-depth core drilling, seismic tomography and BIPS (borehole image processing system) were performed, the distribution of underground cavity and coal seam and rock relaxation condition were analyzed. And we predicted the type of subsidence and estimated the subsidence by theories of mining subsidence. With these results, we analyzed the mechanism of subsidence occurrence in the research object area. Finally, we improved existing methods which were applied to the abandoned mining area and also we established the rational reinforcement for the ground and structure foundation against each subsidence cause.

• Journal of the Korean Geophysical Society
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• v.5 no.3
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• pp.175-198
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• 2002

The techniques using geophysical methods were adopted to obtain quantitative criteria for assessment of grouting effectiveness. Various surface and borehole geophysical surveys including seismic, GPR(ground penetrating radar), resistivity and electromagnetic methods were conducted in fractured rock pilot site before and after grouting execution. However, it is not enough that geophysical data provide criteria for field engineers to confirm the grouting effectiveness in that site even though there is somewhat difference before and after grouting. This study will be continued for the detailed criteria and assessment of grouting effectiveness in other sites.

• 한국지구물리탐사학회:학술대회논문집
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• 2007.12a
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• pp.1-14
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• 2007

This paper briefly reviews the history of the Korean geophysics and analyze the current status of geophysical researches. And the future prospects of geophysics are discussed based on social demands for the science and technology in Korea. About thirty universities offer geophysics courses in their academic curricula. Although the number of Ph.D. graduates in geophysics had been small until the year of 1990, but is rapidly increasing. In recent years about $7{\sim}8$ Ph.D's are produced every year. The major geophysical methods used in Ph.D. theses are seismic, electrical and electromagnetic methods, and earthquake waves and research themes are computational geophysics, which involve data processing, modelling, inversion and tomography, geological structures, and paleomagnetic studies in the order of numbers. The Solid Earth Geophysics is generally distinguished in two categories such as "Global Geophysics" and "Exploration Geophysics". However, they are intimately connected, and overlap in many sectors, especially in large scale research projects. The global geophysics has a more academic and general scientific meaning, and several research groups in Korean universities are carrying out the earthquake seismology and paleomagnetic studies. On the other hand the exploration geophysics focuses on practical application of geophysical concepts, and the public research institutes conduct large projects for exploration of energy and mineral resources and to cope with environmental and natural disaster problems. The geophysical studies for local geology and regional crustal structure utilize various survey methods and usually cover both academic and exploration purposes. The computational geophysics constitutes the indispensable theoretical backgrounds for all geophysical sectors. Many young Korean geophysicists, who have strong background in mathematics and physics, devote to the computational geophysics and several groups have made the internationally highest level achievements. But, Korean geophysicists have to expand their research interests to include more global-scale, high-tech researches and collaborative works with various other science groups.

• Geophysics and Geophysical Exploration
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• v.5 no.1
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• pp.33-45
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• 2002

It was necessary to devise new techniques to overcome and enhance the resolution limits of traveltime tomography. Waveform inversion has been one of the methods for giving very high resolution result. High resolution image could be acquired because waveform inversion used not only phase but amplitude. But waveform inversion was much time consuming Job because forward and backward modeling was needed at each iteration step. Velocity-stress method was used for effective modeling. Resolution limits of imaging methods such as travel time inversion, acoustic and elastic waveform inversion were investigated with numerical models. it was investigated that Resolution limit of waveform inversion was similar tn resolution limit of migration derived by Schuster. Horizontal resolution limit could be improved with increased coverage by adding VSP data in cross hole that had insufficient coverage. Also, waveform inversion was applied to realistic models to evaluate applicability and using initial guess of travel time tomograms to reduce non-linearity of waveform inversion showed that the better reconstructed image could be acquired.

• Tunnel and Underground Space
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• v.12 no.2
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• pp.120-129
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• 2002

Natural cavitied were found at shallow depth during construction of a huge bridge in Cambro-Ordovician Limestone Basin in the central part or Korea. The distribution patterns of cavities in this area were investigated carefully with a supplementary field job such as a structural geological survey, a geophysical survey, and a rock mechanical test in laboratory or field. A structural geological mapping produced a detail geological map focusing the route of the Proposed highway. It suggested that there were three faults in this wet and these faults had an influence on the mechanism of natural cavities. Among many kinds of geophysical surveys, an electrical resistivity prospecting was applied first on the specific area that was selected by results from the geological survey. Many evidences far cavities were disclosed from this geophysical data. Therefore, a seismic tomography was tested on the target wet which was focused by results from the electrical resistivity Prospecting and was believed to have several large cavities. A distinct element numerical simulation using the UDEC was followed on the target area after completing all of field surveys. Data from field tests were directly dumped or extrapolated to numerical simulations as input data. It was verified from numerical analysis that several natural cavities underneath the foundation of the bridge should be reinforced Based on the project result, finally, most of fecundations far the bridge were re-examined and the cement grouting reinforcement was constructed on several foundations among them.

• Economic and Environmental Geology
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• v.39 no.2 s.177
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• pp.111-128
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• 2006

Three dimensional crustal structure and source features of earthquake hypocenters on the Korean peninsula were investigated using P and S-wave travel time tomography. The main goal of this research was to find Vp/Vs anomalies at earthquake hypocenters as well as those of crustal structure of basins and deep tectonic settings. This allowed fer the extrapolation of more detailed seismotectonic force from the Korean peninsula. The earthquake hypocenters were found to have high Vp/Vs ratio discrepancies (VRD) at the vertical sections. High V/p/Vs ratios were also found in the sedimentary basins and beneath the Chugaryong Rift Zone (CRZ), which was due to mantle plume that subsequently solidified with many fractures and faults which were saturated with connate water. The hypocenters of most earthquakes were found in the upper crust for Youngwol (YE), Kyongju (KE), Hongsung (HE), Kaesong (KSE), Daekwan (DKE), and Daehung (DHE) earthquakes, but near the subcrust or the Moho Discontinuity for Mt. Songni (SE), Sariwon (SRE) and Mt. Jiri (JE) earthquakes. Especially, we found hot springs of the Daekwan, Daehung and Unsan regions coincide with high VRD. Also, this cannot rule out the possibility that there are some partial meltings in the subcrust of this region. High VRD might indicate that many faults and fractures with connate water were dehydrated when earthquakes took place, reducing shear modulus in the hypocenter areas. This is can be explained by due to the fact that a point source which is represented by the moment tensor that may involve changes in volume, shear fracture, and rigidity. High Vp/Vs ratio discrepancies (VRD) were also found beneath Mt. Backdu beneath 40 km, indicating that magma chamber existed beneath Mt. Backdu is reducing shear modulus of S-wave velocity.

• 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.

• Journal of the Korean earth science society
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• v.21 no.5
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• pp.635-646
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• 2000

Geological achievements of the 20th century revolutionized our views about geological understanding and concept. A good example is the concept of continental drift suggested early in the 20th century and later explained in terms of seafloor spreading and plate tectonics. Our understanding of the compositions of materials forming earth has also improved during the20th century. Radio and stable isotopes together with biostratigraphy and sequence stratigraphy allow us to interpret the evolution of sedimentary basins in terms of plate movement and sedimentation processes. The Deep Sea Drilling Project initiated in 1960s and continued as the Ocean Drilling Project in 1980s is one of the most successful international research observations, and new developments in computational techniques have provided a wholly new view about the interior of the earth. Most of the geological features and phenomena observed in deep sea and around continental margins are now explained in terms of global tectonic processes such as superplumes flowing up from the interior of our planet and interacting with such as Rodinia Pannotia and Nena back in the Precambrian time. The space explorations which began in the late 1950s opened up a new path to astrogeology, astrobiology, and astropaleontology. The impact theory rooted in the discovery of iridium and associated phenomena in 1980s revived Cuvier's catastrophism as a possible explanation for the extinctions of biotas found in the geological record of this planet. Due to the geological achievements made in the 20th century, we now have a better understanding of geologic times and processes that were too long to be grasped by human records.