• Geophysics and Geophysical Exploration
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• v.16 no.4
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• pp.240-249
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• 2013

The seismic data quality of marine geological and engineering survey deteriorates because of the sea swell. We often conduct a marine survey when the swell height is about 1 ~ 2 m. The swell effect correction is required to enhance the horizontal continuity of seismic data and satisfy the resolution less than 1 m. We applied the swell correction to the 8 channel high-resolution airgun seismic data and 3.5 kHz subbottom profiler (SBP) data. The correct sea bottom detection is important for the swell correction. To detect the sea bottom, we used maximum amplitude of seismic signal around the expected sea bottom, and picked the first increasing point larger than threshold value related with the maximum amplitude. To find sea bottom easily in the case of the low quality data, we transformed the input data to envelope data or the cross-correlated data using the sea bottom wavelet. We averaged the picked sea bottom depths and calculated the correction values. The maximum correction of the airgun data was about 0.8 m and the maximum correction of two kinds of 3.5 kHz SBP data was 0.5 m and 2.0 m respectively. We enhanced the continuity of the subsurface layer and produced the high quality seismic section using the proper methods of swell correction.

• Journal of the Korean Geophysical Society
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• v.5 no.3
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• pp.219-231
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• 2002

Single beam echo sounding was used to delineate bathymetry sea bottom in the area of hydrography and marine navigation. This research was aimed at measuring the thickness of floating silty clay sediment with dual frequencies echo sounding system. There occur discrepancies in penetrating depth through sea beds between high frequency(200 KHz) and low(33 KHz) frequency. RI density logging was employed to characterize the floating silty clay sediment of Guangyang bay, which was chosen to investigate the proposed site for reclamation field. The volume of floating silty clay sediment was used to design by estimating size of reclamation site. The estimation strategies developed in this study will be readily applicable to measure the Pattern of sedimentation via regular hydrographic survey in the future.

• Geophysics and Geophysical Exploration
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• v.19 no.3
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• pp.164-173
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• 2016

Cable-free seismic technology is to acquire seismic data with independent receivers which are not connected by cables. This is an effective method for survey designs with less topographical conditions. With technology advancement for cable-free receivers, reliable data quality, easy deployment, and picking up the receivers, the cable-free technology has begun to apply to land seismic acquisition. In this study we introduced a cable-free seismic system and its equipment. We tried to build up the cable-free seismic technology through the field application. In the seismic tomography field applications, the seismic signals of the cable-free receiver and cabled receiver with the same distance from the source show the same phase in early stage. The difference of the first arrival times between two signals is less than 0.4 ms, which could be accepted. In the field application for seismic reflection exploration, we acquired shot gathers with different source depth and dynamite charge. The shot gathers from cable-free and cabled system are similar to each other. With an efficient method for receiver deployment and survey design, the application of the cable-free technology will increase.

• Geophysics and Geophysical Exploration
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• v.15 no.2
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• pp.75-84
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• 2012

The sea layer in marine Controlled-Source Electromagnetic (mCSEM) survey changes the conventional definition of apparent resistivity which is used in the land CSEM survey. Thus, the development of a new algorithm, which computes apparent resistivity for mCSEM survey, can be an initiative of mCSEM data interpretation. First, we compared and analyzed electromagnetic responses of the 1D stratified gas hydrate model and the half-space model below the sea layer. Amplitude and phase components showed proper results for computing apparent resistivity than real and imaginary components. Next, the amplitude component is more sensitive to the subsurface resistivity than the phase component in far offset range and vice versa. We suggested the induction number as a selection criteria of amplitude or phase component to calculate apparent resistivity. Based on our study, we have developed a numerical algorithm, which computes appropriate apparent resistivity corresponding to measured mCSEM data using grid search method. In addition, we verified the validity of the developed algorithm by applying it to the stratified gas hydrate models with various model parameters. Finally, by constructing apparent resistivity pseudo-section from the mCSEM responses with 2D numerical models simulating gas hydrate deposits in the Ulleung Basin, we confirmed that the apparent resistivity can provide the information on the geometric distribution of the gas hydrate deposit.

• 한국지구물리탐사학회:학술대회논문집
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• 2004.08a
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• pp.104-117
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• 2004

Dmitri Donskoi, which went down during the Russo-Japanese War occurred 100 years ago, was found by using geophysical exploration techniques at the 400 m water depth of submarine valley off Jeodong of Ulleung Island. In the submarine area with the rugged seabed topography and volcanic seamounts, in particular, the reliable seabed images were acquired by using the mid-to-shallow Multibeam exploration technique The strength of corrosion (causticity) of the sunken Donskoi, measured by the electrochemical method, decreased to 2/5 compared with the original strength.

• Geophysics and Geophysical Exploration
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• v.13 no.1
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• pp.88-95
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• 2010

An airborne gravity survey using a helicopter was carried out in October 2008, offshore along the northern Noto Peninsula, to understand the shallow and regional underground structure. Eleven flight lines, including three tie lines, were arranged at 2 km spacing within 20 km of the coast. The total length of the flight lines was ~700 km. The Bouguer anomalies computed from the airborne gravimetry are consistent with those computed from land and shipborne gravimetry, which gradually decrease in the offshore direction. So, the accuracy of the airborne system is considered to be adequate. A local gravity low in Wajima Bay, which was already known from seafloor gravimetry, was also observed. This suggests that the airborne system has a structural resolution of ~2 km. Reduction of gravity data to a common datum was conducted by compiling the three kinds of gravity data, from airborne, shipborne, and land surveys. In the present study, we have used a solid angle numerical integration method and an iteration method. We finally calculated the gravity anomalies at 300 m above sea level. We needed to add corrections of 2.5 mGals in order to compile the airborne and shipborne gravity data smoothly, so the accuracy of the Bouguer anomaly map is considered to be nearly 2 mGal on the whole, and 5 mGals at worst in limited or local areas.

• Journal of the Korean Geophysical Society
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• v.5 no.4
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• pp.305-319
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• 2002

The three-dimensional MT(Magneto-Telluric) modeling is performed to examine the validity of difference arrow of GDS(Geomagnetic Depth Sounding) survey, In this paper, we investigate the validity of the difference arrow on three configurations of conductors; which is located 1) at surface, 2) at the deep part and 3) vertically extended f개m surface to the deep part, respectively, For conductors located at surface, the validity of difference arrows is certified in our numerical model when long periods over 40 minutes are used or the distance between sea and conductor is over 150 km. However, for conductors located at the deep part, the validity of difference arrow is dependent on the size of conductors. Further, if the size of conductor is adequately larger than that of our model, we recognize the possibility that the mutual coupling of them influences up to longer periods, Moreover, in case of conductors which is vertically extended from surface to the deer part, the mutual coupling of them is reinforced for all periods, especially for longer periods, so that the validity of difference arrow is considerably in doubt. Therefore, to remove the known conductor effect such as the sea effect from the observed induction arrow, the mutual coupling between them must be examined. The difference arrow that certifies the validity in this way can only provide the Subsurface information based on physical supports.

• Geophysics and Geophysical Exploration
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• v.11 no.2
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• pp.116-126
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• 2008

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.

• Geophysics and Geophysical Exploration
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• v.12 no.2
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• pp.173-182
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• 2009

In April 2008, KIGAM carried out an ocean-bottom seismometer (OBS) survey in the central Ulleung Basin where strong bottom simulating reflectors (BSRs) were revealed from previous surveys and some gas-hydrate samples were retrieved by direct sampling. The purpose of this survey is to estimate the velocity structure near the BSR in the gas hydrate prospect area using wide-angle seismic data recorded on the ocean-bottom seismometers. Along with the OBS survey, a 2-D seismic survey was performed whereby stratigraphic and preliminary velocity information was obtained. Two methods were applied to wide-angle data for estimating P wave velocity; one is velocity analysis in the $\tau$-p domain and the other is seismic traveltime inversion. A 1-D interval velocity profile was obtained by the first method, which was refined to layered velocity structure by the latter method. A layer stripping method was adopted for modeling and inversion. All velocity profiles at each OBS site clearly show velocity reversal at BSR depths due to the presence of gas hydrates. In addition, we could confirm high velocity in the column/chimney structure.

• Geophysics and Geophysical Exploration
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• v.22 no.3
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• pp.116-131
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• 2019

In order to obtain high-quality high-resolution marine seismic data, the survey needs to be carried out at very low-sea condition. However, the survey is often performed with a slight wave, which degrades the quality of data. In this case, it is possible to improve the quality of seismic data by detecting the exact location of the sea bottom signal and eliminating the influence of waves or swells automatically during data processing. However, if noise is included or the sea bottom signal is weakened due to sea waves, sea bottom detection errors are likely to occur. In this study, we applied a method reducing such errors by estimating the sea bottom location, setting a narrow detection range and detecting the sea bottom location within this range. The expected location of the sea bottom was calculated using previously detected sea bottom locations for each channel of multi-channel data. The expected location calculated in each channel is also compared and verified with expected locations of other channels in a shot gather. As a result of applying this method to the noisy 8-channel high-resolution air-gun seismic data acquired off Yeosu, the errors in selecting the strong noise before sea bottom or the strong subsurface reflected signal after the sea bottom signal are remarkably reduced and it is possible to produce the high-quality seismic section with the correction of ~ 2.5 m swell effect.