• Economic and Environmental Geology
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• v.35 no.5
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• pp.445-454
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• 2002

3-D P-wave velocity model in the southern Korean Peninsula is investigated by using the earthquake tomography method. This velocity model would be used to locate the exact hypocenter position, and also useful for our understanding of the crustal structure. The simultaneous inversion is used to get the minimum 1-D model and hypo-center relocation, which are used as an initial 3-D velocity model. The velocities in the minimum 1-D model are 6.04 km/s, 6.45 km/s, and 7.78 km/s between the depth of 0-19 km, 19-32 km, and 32-55 km respectively. In the 3-D P-wave velocity model, Layer 1 (0~3 km) has high velocities in Kyongsang basin, Yonglam massif, and Okchon folded belt, and low velocities in Kyonggi massif. In layer 2 (3~19 km) high velocities are predominent around Kyonsang basin and Yongnam massif except Yonil basin, but low velocities exist around Kyonggi massif and Okchon folded belt. In Laye. 3 (19~32 km) high velocities prevail throughout the southern part of Korean Peninsula, but low velocity does throughout the middle except SNU, YIN station in Konggi massif. In Layer 4 (32 km), the maximum velocity is showed in the middle and southwestern part, while the minimum velocity in the southeastern and coastal area. The depth of the velocity boundary corresponds to the crustal structure of the southern Korean Peninsula which is calculated by gravity data.

• 한국지구물리탐사학회:학술대회논문집
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• 2000.09a
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• pp.132-146
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• 2000

Seismic refraction survey was performed for 10 lines along NE-SW and NW-SE directions above Nampoong gallery at Makyo-ri, Dogye, Samcheok, Kangwon-do. 48 geophones were laid in line with the interval of 1m, and a 5Kg hammer was used as a source at 5 points for each line. Data processing was done using reciprocal time method, GRM, and traveltime tomography which utilizes wavefront expansion method for forward process and STRT for inversion. The result shows that the first layer has its lower boundary between 3.49m and 8.88m. The P-wave velocity of the first and the second layer were estimated as 270${\~}$360m/s and 1550${\~}$1940m/s respectively. When the boundary of the first and second layer is smooth enough and the velocity difference is large enough, GRM has little advantage over reciprocal time method. The result of reciprocal method and traveltime tomography shows consistency. The northeast part of the boundary has syncline structure, which is similar to the topography above. This implies that the collapse of the cavities of Nampoong gallery result in the subsidence of the ground surface. The subsidence is in progress across the Youngdong railroad, therefore a proper reinforcement work is required.

• Geophysics and Geophysical Exploration
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• v.2 no.4
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• pp.167-173
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• 1999

Since the resolution of the 2-D hole-to-hole seismic traveltime tomography is affected by the limited ray transmission angle, various methods were used to improve the resolution. Linear traveltime interpolation(LTI) ray tracing method was chosen for forward-modeling method. Inversion results using the LTI method were compared with those using the other ray tracing methods. As an inversion algorithm, SIRT method was used. In the iterative non-linear inversion method, the cost of ray tracing is quite expensive. To reduce the cost, each raypath was stored and the inversion was performed from this information. Using the proposed method, fast convergence was achieved. Inversion results are likely to be affected by the initial velocity guess, especially when the ray transmission angle was limited. To provide a good initial guess for the inversion, generalized regression neural network(GRNN) method was used. When the transmitted raypath angle is not limited or the geological model is very complex, the inversion results are not affected by initial velocity model very much. Since the raypath angles, however, are limited in most geophysical tomographic problems, the enhancement of resolution in tomography can be achieved by providing a proper initial velocity model by another inversion algorithm such as GRNN.

• The Journal of Engineering Geology
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• v.11 no.2
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• pp.163-174
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• 2001

For quantitative evaluation of geotechnical engineering properties such as rippability and diggability, clear interpretation on the subsUJiace velocity structures should be preceded by figuring out top soil, weathered and soft rock layers, shape of basement, fracture zones, geologic boundary and etC. from the seismic refraction data. It is very important to set up suitable field parameters, which are the configuration of profile and its length, spacings of geophones and sources and topographic conditions, for increasing field data Quality. Geophone spacing of 3 to 5m is reconunended in the land slope area of house land development site. In refraction tomography technique, the number of source points should be more than a Cluarter of available channel number of instrument and the subsurface structure interpretation can be decreased the artifact of inversion by topographic effect. Compared with core logging data, it is shown that the velocity range of the soil is less than 700m/s, weathered rock 700~1,200m/s, soft rock 1,200~1,800m/s on the velocity tomogram section. And the upper limit of P-wave velocity for rippability is estimated 1,200 to 1,800m/s in land slope area of gneiss.

• 한국지구물리탐사학회:학술대회논문집
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• 2008.10a
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• pp.141-144
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• 2008

Geological storage of carbon dioxide has been studying in advanced countries to reduce greenhouse gases and a pilot site for geological storage is also in operation in the deep saline aquifer. Seismic wave and electrical resistivity tomography methods are applicable to monitoring techniques and they are used to evaluate the distribution range and behavior of the carbon dioxide injected in the porous sandstone formations. This paper describes the construction of an experimental apparatus which consists of a high pressure vessel and a measurement system for geological storage of carbon dioxide. The experiment apparatus will be used to measure seismic velocities and resistivities during the injection of carbon dioxide at the supercritical phase in the porous sandstones.

• 한국지구물리탐사학회:학술대회논문집
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• 2007.06a
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• pp.317-322
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• 2007

Seismic exploration is divided by reflection and refraction method greatly, and reflection method can analyze complicated underground structure in the basis high resolution image, and refraction method can grasp the velocity structure of underground accurately. This thesis confirmed application of mixed exploration techniques using advantages of reflection and refraction. Reflection data processing applied conventional technique, and inversion of refraction data applied travel time tomographic technique that using SIRT method. Also, could establish initial information in model variable and improved the result of inversion by restricting model parameter value and dimension of area. Confirmed efficient fact in sequence and velocity structure grasping by utilizing accurate initial velocity model made out on the basis of marine reflection data, and mixed exploration technique using reflection and refraction have propriety that can trust in field application.

• Geophysics and Geophysical Exploration
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• v.6 no.1
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• pp.40-52
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• 2003

Despite the various opening models of the southwestern part of the East Sea (Japan Sea) between the Korean Peninsula and the Japan Arc, the continental margin of the Korean Peninsula remains unknown in crustal structure. As a result, continental rifting and subsequent seafloor spreading processes to explain the opening of the East Sea have not been adequately addressed. We investigated crustal and sedimentary velocity structures across the Korean margin into the adjacent Ulleung Basin from multichannel seismic reflection and ocean bottom seismometer data. The Ulleung Basin shows crustal velocity structure typical of oceanic although its crustal thickness of about 10 km is greater than normal. The continental margin documents rapid transition from continental to oceanic crust, exhibiting a remarkable decrease in crustal thickness accompanied by shallowing of Moho over a distance of about 50 km. The crustal model of the margin is characterized by a high-velocity (up to 7.4 km/s) lower crustal (HVLC) layer that is thicker than 10 km under the slope base and pinches out seawards. The HVLC layer is interpreted as magmatic underplating emplaced during continental rifting In response to high upper mantle temperature. The acoustic basement of the slope base shows an igneous stratigraphy developed by massive volcanic eruption. These features suggest that the evolution of the Korean margin can be explained by the processes occurring at volcanic rifted margins. Global earthquake tomography supports our interpretation by defining the abnormally hot upper mantle across the Korean margin and in the Ulleung Basin.

• Geophysics and Geophysical Exploration
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• v.18 no.3
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• pp.143-153
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• 2015

Polymer flooding for enhancing hydrocarbon production injects into a reservoir polymer solution that is viscous. It is very important to monitor the behavior pattern of the polymer solution in order to evaluate the effectiveness of polymer flooding. To monitor the distribution of polymer solution and thus fluid substitution within the reservoir, we first construct seismic and resistivity rock physics models (RPMs), which are functions of reservoir parameters such as rocks and type of fluid, fluid saturation. For the seismic and resistivity RPMs, responses of seismic and electromagnetic (EM) tomography are numerically simulated as polymer injection, using two dimensional (2D) staggered-grid finite difference elastic modeling and 2.5D finite element EM modeling algorithms, respectively. In constructing RPM for EM tomography, three different reservoir rocks are considered: clean-sand, dispersed shale-sand, and sand-shale lamination rocks. The polymer solution is assumed to have 2 wt% of polymer as normally generated, while water is freshwater or saltwater. Further, neutron logging is also considered to check its sensitivity to polymer flooding. The techniques discussed in the paper are important in monitoring not only hydrocarbon but also geothermal reservoirs.

• Economic and Environmental Geology
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• v.31 no.2
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• pp.127-139
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• 1998

A new technique of simultaneous inversion for 3-D seismic velocity structure by using direct, reflected, and refracted waves is applied to the southeast part of the Korean Peninsula including Pohang Basin, Kyongsang Basin and Ryongnam Massif. Pg, Sg, PmP, SmS, Pn, and Sn arrival times of 44 events with 554 seismic rays are inverted for locations and crustal structure. $6{\times}6$ with $0.5^{\circ}$ and 8 layers (4 km each layer) model was inverted. 3-D seismic crustal velocity tomography including eight sections from surface to Moho, ten profiles along latitude and longitude are analyzed. The results are as follows: 1) the average velocity and thickness of sediment are 5.04 km/s and 3-4 km, and the velocity of basement is 6.11 km/s. The shape of velocity in shallower layer is agreement with Bouguer gravity anomaly (Cho et al., 1997). 2) the velocities fluctuate strongly in the upper crust. The velocity distribution of the lower crust under Conrad appears basically horizontal. 3) the average depth of Moho is 30.4 km, and velocity is 8.01 km/s. 4) from the velocity and depth of the sediment, the thickness, velocity and form of the upper crust, and the depth and form of Moho, we can find the obvious differences among Ryongnam Massif, Kyongsang Basin and Pohang Basin. 5) the deep faults (a Ulsan series faults) near Kyongju and Pohang areas can be found to be normal and/or thrust faults with detachment extended to the bottom of the upper crust.

• Economic and Environmental Geology
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• v.31 no.3
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• pp.235-247
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• 1998

A new technique of simultaneons inversion for 3-D seismic velocity structure by using direct, reflected, and refracted waves is applied to the center of the Korean Peninsula including Pyongnam Basin, Kyonggi Massif, Okchon Fold Zone, Taebaeksan Fold Zone, Ryongnam Massif and Kyongsang Basin. Pg, Sg, PmP, SmS, Pn, and Sn arrival times of 32 events with 404 seismic rays are inverted for locations and crustal structure. 5 ($1^{\circ}$ along the latitude)${\times}6$ ($0.5^{\circ}$ along the longitude) ${\times}8$ block (4 km each layer) model was inverted. 3-D seismic crustal velocity tomography including eight sections from the surface to the Moho, eight profiles along latitude and longitude and the Moho depth distribution was determined. The results are as follows: (1) the average velocity and thickness of sediment are 5.15 km/sec and 3-4 km, and the velocity of basement is 6.12 km/sec. (2) the velocities fluctuate strongly in the upper crust, and the velocity distribution of the lower crust under Conrad appears basically horizontal. (3) the average depth of Moho is 29.8 km and velocity is 7.97 km/sec. (4) from the sedimentary depth and velocity, basement thickness and velocity, form of the upper crust, the Moho depth and form of the remarkable crustal velocity differences among Pyongnam Basin, Kyonggi Massif, Okchon Zone, Ryongnam Massif and Kyongsang Basin can be found. (5) The different crustal features of ocean and continent crust are obvious. (6) Some deep index of the Chugaryong Rift Zone can be located from the cross section profiles. (7) We note that there are big anisotropy bodies near north of Seoul and Hongsung in the upper crust, implying that they may be related to the Chugaryong Rift Zone and deep fault systems.