• Geophysics and Geophysical Exploration
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• v.11 no.1
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• pp.34-40
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• 2008

Underwater seismic refraction with advanced interpretation approaches makes important contributions to shallow marine exploration and geotechnical investigations in Australia's coastal areas. A series of case studies are presented to demonstrate the recent applications of continuous and static USR methods to river crossing and port infrastructure projects at various sites around Australia. In Sydney, static underwater seismic refraction (USR) with bottom-placed receivers and borehole seismic imaging assisted the development of improved geotechnical models that reduced construction risk for a tunnel crossing of the Lane Cove River. In Melbourne, combining conventional boomer reflection and continuous USR with near-bottom sources and receivers improved the definition of a buried, variably weathered basalt flow and assisted dredging assessment for navigation channel upgrades at Geelong Ports. Sand quality assessment with continuous USR and widely spaced borehole information assisted commercial decisions on available sand resources for the reclamation phase of development at the Port of Brisbane. Buried reefs and indurated layers occur in Australian coastal sediments with the characteristics of laterally limited, high velocity, cap layers within lower velocity materials. If these features are not recognised then significant error in depth determination to deeper refractors can occur. Application of advanced refraction inversion using wavefront eikonal tomography to continuous USR data obtained along the route of a proposed offshore pipeline near Fremantle allowed these layers and the underlying bedrock refractor to be accurately imaged. Static USR and the same interpretation approach was used to image the drowned granitic regolith beneath sediments and indurated layers in the northern area of Western Australia at a proposed new berthing site where deep piling was required. This allowed preferred piling sites to be identified, reducing overall pile lengths. USR can be expected to find increased application to shallow marine exploration and geotechnical investigations in Australia's coastal areas as economic growth continues and improved interpretation methods are developed.

• Geophysics and Geophysical Exploration
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• v.11 no.1
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• pp.52-59
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• 2008

We have developed a new deep-towed marine DC resistivity survey system. It was designed to detect the top boundary of the methane hydrate zone, which is not imaged well by seismic reflection surveys. Our system, with a transmitter and a 160-m-long tail with eight source electrodes and a receiver dipole, is towed from a research vessel near the seafloor. Numerical calculations show that our marine DC resistivity survey system can effectively image the top surface of the methane hydrate layer. A survey was carried out off Joetsu, in the Japan Sea, where outcrops of methane hydrate are observed. We successfully obtained DC resistivity data along a profile ${\sim}3.5\;km$ long, and detected relatively high apparent resistivity values. Particularly in areas with methane hydrate exposure, anomalously high apparent resistivity was observed, and we interpret these high apparent resistivities to be due to the methane hydrate zone below the seafloor. Marine DC resistivity surveys will be a new tool to image sub-seafloor structures within methane hydrate zones.

• Geophysics and Geophysical Exploration
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• v.11 no.1
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• pp.1-14
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• 2008

We present a method for interpreting seismic records with arrivals and waveforms having characteristics which could be generated by extremely inhomogeneous velocity structures, such as non-typical oceanic crust, decollement at subduction zones, and seamounts in oceanic regions, by comparing them with synthetic waveforms. Recent extensive refraction and wide-angle reflection surveys in oceanic regions have provided us with a huge number of high-resolution and high-quality seismic records containing characteristic arrivals and waveforms, besides first arrivals and major reflected phases such as PmP. Some characteristic waveforms, with significant later reflected phases or anomalous amplitude decay with offset distance, are difficult to interpret using only a conventional interpretation method such as the traveltime tomographic inversion method. We find the best process for investigating such characteristic phases is to use an interactive interpretation method to compare observed data with synthetic waveforms, and calculate raypaths and traveltimes. This approach enables us to construct a reasonable structural model that includes all of the major characteristics of the observed waveforms. We present results here with some actual observed examples that might be of great help in the interpretation of such problematic phases. Our approach to the analysis of waveform characteristics is endorsed as an innovative method for constructing high-resolution and high-quality crustal structure models, not only in oceanic regions, but also in the continental regions.

• Geophysics and Geophysical Exploration
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• v.20 no.3
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• pp.146-162
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• 2017

Multibeam and seismic data in the northwestern part of the Ulleung Basin were analyzed to study stratigraphy and evolutionary history of submarine canyon. A detailed analysis reveals that the sedimentary sequences in this area consist of four stratigraphic units separated by erosional unconformities. On the continental slope, these units are dominated by well-stratified facies with some slope failures, whereas these units show well-stratified and chaotic facies toward the basin floor. Generally, the sediment thickness is relatively thin on the slope, whereas thick sediment accumulation occurs on the base of slope and basin floor. Based on seismic characteristics and distribution, the deposition of each units are well correlated with the evolutionary history of the submarine canyon. Unit 1 directly overlying the acoustic basement has thin sediment layer on the slope, whereas its thickness gradually increase toward the basin floor. Compared to other units, Unit 2 is relatively thick accumulations on the slope and contains some slope failures related to faults systems. The mass transport sediments due to slope failures, mainly deposited on the base of slope as a submarine fan. The width and depth of submarine canyon increase due to dominant of the erosional process rather than the sediment deposition. Unit 3 is thin accumulation on the slope around the submarine canyon. Toward the basin floor, its thickness gradually increases. Unit 4 is characterized by thin layers including slides and slumps on the slope, whereas it formed thick accumulations at the base of slope as a submarine fan. The increase in the width and depth of submarine canyon results from the dominant of the erosional process and slope failures around the submarine canyon. Consequently, the formation of sedimentary units combined with the development of submarine canyon in this area is largely controlled by the amounts of sediment supply originated from slope failures, regional tectonic effects and sea-level fluctuations.

• Economic and Environmental Geology
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• v.49 no.5
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• pp.381-388
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• 2016

The purpose of this study is to integrate and visualize using mapping techniques based on precise seabed geomorphology, seafloor backscattering images and high-resolution underwater images of the nearshore area around the Dokdo, in the East Sea. We have been obtained the precise topography map using multibeam echosounder system around the nearshore area(~50 m) of the southern part of the Seodo. Side scan sonar survey for analysis seafloor backscattering images was carried out in the same area of topography data. High-resolution underwater images(zone(a), zone(b), zone(c)) were taken in significant habitat scope of the nearshore area of the southern part of the Seodo. Using the results of bathymetry, seafloor backscattering images, high-resolution underwater images, we performed an integrated visualization about the nearshore area of the Dokdo. The integrated visualizing techniques are possible to make the seabed characteristic mapping results of the nearshore area of the Dokdo. The integrated visualization results present more complex and reliable information than separate geological products for seabed environmental mapping study and it is useful to understand the relation between seafloor characteristics and topographic environments of the study area. The integrated visualizing techniques and mapping analysis need to study sustainably and periodically, for effective monitoring of the nearshore ecosystem of the Dokdo.

• Geophysics and Geophysical Exploration
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• v.1 no.2
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• pp.101-109
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• 1998

Using airborne magnetic data, magnetic characteristics were studied at the Samryangjin caldera area developed in the volcanics of the Yuchon sub-basin, the south eastern part of the Gyeongsang basin. Residual magnetics, reduction to the pole, horizontal derivative, and vertical derivative maps are prepared. Using these maps, the magnetic lithofaces are zoned and the geological structures such as caldera and faults were qualitatively interpreted. In addition, the two quantitative interpretations were performed. Firstly, the forward modelling were done to the 14.5 line km crossing the caldera area to the northeast-southwest direction. Applying the 3-D Euler deconvolution method to the whole study area, the depth extent and the characteristics of the magnetic anomalous bodies were studied. According to the results, the magnetic lithofaces of the area are zoned by 4 units. In general, these are well matched with the geological distributions. But the biotite granites intruded in the northern boundary of the Samryangjin caldera show the high magnetic intensity, while the biotite granites of the other areas show the low magnetic intensity and the different magnetic lithofaces. Thus, we interpreted that the biotite granites are closely related with the volcanic activity of the Samryngjin caldera, and are intruded in the fracture zones developed along the caldera rim. The Samryangjin caldera and fault structures of the area can be easily recognized by the distinct magnetic structures from the various magnetic anomaly maps. Also the topographic characteristics well reflect these structures. The results of the forward modelling show that the magnetic basement depth of the Gyeongsang sedimentary basin is on the average about 6 km and in maximum 10 km. And the depth becomes shallower toward the caldera boundary due to the shallow intrusion of the volcanics. The results of the 3-D Euler method also show the caldera and fault structures. And the relatively shallow magnetic anomalous bodies which are related with the volcanics are generally developed to the east-west and northeast directions, while the deep magnetic anomalous bodies to the northwest direction.

• Journal of Korean Society of Environmental Engineers
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• v.28 no.5
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• pp.463-471
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• 2006

Electrical resistivity surveys were conducted at areas of abandoned landfills in Cheonan and Wonju. Geology and extent of leachate migration around the landfills were evaluated with collected resistivity data by 2-D and 3-D resistivity inverse modeling. The Cheonan landfill is located above the paddy fields and the resistivity survey lines were crossed to examine possible pollution at the paddy fields by leakage of the landfill leachate. In Wonju, the landfill and the downgradient paddy fields are divided by a concrete barrier wall. At the bottom of the landfill, there is a leachate settlement system, which has not been in operation. To evaluate leachate leakage into the paddy fields, a total of 4 survey lines were used. According to the resistivity survey results, the landfill leachate in Cheonan appeared to be restricted only within the interior of the landfill, not to migrate into the subsurface of the paddy fields. These results are well consistent with electrical conductivity values of groundwaters obtained from a periodic analysis of water qualities. In Wonju, however, it was inferred that the leachate emanating from the landfill migrated beneath the abandoned leachate settlement system and the leachate would reach the downgradient paddy fields. Low resistivity area was observed in the old reservoir area and it appeared to be derived from convergence of groundwater flows from the surrounding valley and the moist wet land. In addition, groundwater flow into the paddy fields occurs beneath the old reservoir embankment at depths of $7{\sim}8m$. This paper reports details of the resistivity surveys for the uncontrolled landfills.

• Geophysics and Geophysical Exploration
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• v.10 no.3
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• pp.191-202
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• 2007

To identify the detailed attenuation structure in the southern Korean Peninsula, a numerical test was conducted for the Q tomography inversion to be applied to the accumulated dataset until 2005. In particular, the stochastic pointsource ground-motion model (STGM model; Boore, 2003) was adopted for the 2D Q tomography inversion for direct application to simulating the strong ground-motion. Simultaneous inversion of the STGM model parameters with a regional single Q model was performed to evaluate the source and site effects which were necessary to generate an artificial dataset for the numerical test. The artificial dataset consists of simulated Fourier spectra that resemble the real data in the magnitude-distance-frequency-error distribution except replacement of the regional single Q model with a checkerboard type of high and low values of laterally varying Q models. The total number of Q blocks used for the checkerboard test was 75 (grid size of $35{\times}44km^2$ for Q blocks); Q functional form of $Q_0f^{\eta}$ ($Q_0$=100 or 500, 0.0 < ${\eta}$ < 1.0) was assigned to each Q block for the checkerboard test. The checkerboard test has been implemented in three steps. At the first step, the initial values of Q-values for 75 blocks were estimated. At the second step, the site amplification function was estimated by using the initial guess of A(f) which is the mean site amplification functions (Yun and Suh, 2007) for the site class. The last step is to invert the tomographic Q-values of 75 blocks based on the results of the first and second steps. As a result of the checkerboard test, it was demonstrated that Q-values could be robustly estimated by using the 2D Q tomography inversion method even in the presence of perturbed source and site effects from the true input model.

• Journal of the Korean Geophysical Society
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• v.2 no.1
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• pp.27-38
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• 1999

The errors between track segments or at the cross-over points of shipborne gravity were successfully reduced by applying a cross-over error adjustment technique using satellite gravity. The integration of shipborne and satellite altimeter-implied free-air gravity anomalies after the cross-over error adjustment resulted in a high resolution gravity map which contains both short and long wavelength components. The successful adjustment of the cross-over errors in the shipborne gravity using the satellite gravity suggests that the shipborne gravity can be combined with the satellite anomalies characterized by a stable and long wavelength component. The resulting free-air anomaly map is evenly harmonized with both short and long wavelength anomalies. Thus the corrected anomaly map can be better used for the geological interpretation. Free-air anomalies with more than 140 mGal in total variations generally correspond to the seafloor topographic changes in their regional patterns. A series of gravity highs are aligned from the Korea Plateau to the Oki Island, which are interpreted to be caused by seamounts or volcanic topographies. The gravity minima along the western and southern shelf edge are associated not only with the local basement morphology and thick sediment fill at the continental margin, but also possibly with the crustal edge effect known for passive continental margins. Series of NE-trending linear anomalies are possibly caused by a swarm of volcanic intrusions followed the initial opening of the Ulleung Basin. The linear high anomalies in the Ulleung Plateau are terminated by the straightly NNW-trending anomalies with a sharp gradient in its western boundary which indicates a fault-line scarp. The opposite side adjoined with the fault-line scarp shows no correlation with the fault-line scarp in geometry indicating that the block might be horizontally slided from the north. A gravity high in contrast to the deepening in seafloor toward the northeastern central Ulleung Basin is probably responsible for the thin crust and shallow seated mantle. The gravity minima along the western and southern shelf edge are associated not only with the local basement morphology and thick sediment fill at the continental margin, but also possibly with the crustal edge effect known for passive continental margins. Series of NE-trending linear anomalies are possibly caused by a swarm of volcanic intrusions followed the initial opening of the Ulleung Basin. The linear high anomalies in the Ulleung Plateau are terminated by the straightly NNW-trending anomalies with a sharp gradient in its western boundary which indicates a fault-line scarp. The opposite side adjoined with the fault-line scarp shows no correlation with the fault-line scarp in geometry indicating that the block might be horizontally slided from the north. A gravity high in contrast to the deepening in seafloor toward the northeastern central Ulleung Basin is probably suggestive of a thin crust and shallow seated mantle.

• Geophysics and Geophysical Exploration
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• v.15 no.1
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• pp.39-46
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• 2012

Recently, stratigraphic reservoirs are getting more attention than structural reservoirs which have mostly developed. However, recognizing stratigraphic thin gas reservoirs in a stacked section is usually difficult because of tuning effects. Moreover, if the reflections from the brine-saturated region of a thin layer have the same polarity with those from the gas-saturated region, we could not easily identify the gas reservoir with conventional data processing technique. In this study, we introduced a way to delineate the gas-saturated region in a thin layer reservoir using a spectral decomposition method. First of all, amplitude spectrum with the variation of the frequency and the incident angle was investigated for the medium which represents property of Class 3, Class 1 or Class 4 AVO response. The results show that the maximum difference in the amplitude spectra between brine and gas-saturated thin layers occurs around the peak frequency independent of the incident angle and the type of AVO responses. In addition, the amplitude spectra of the gas-saturated zone are greater than those of brine-saturated one in Class 3 and Class 4 at the peak frequency while those of phenomenon occur oppositely in Class 1. Based on the results, we applied spectral decomposition method to the stacked section in order to distinguish the gas-saturated zone from the brine-saturated zone in a thin layer reservoir. To verify our new method, we constructed a thin-layer velocity model which contains both gas and brine-saturated zones which have the same reflection polarities. As a result, in the spectral decomposed sections near the peak frequency obtained by Wigner-Ville Distribution (WVD), we could identify the difference between reflections from gas- and brinesaturated region in the thin layer reservoir, which was hardly distinguishable in the stacked section.