• Proceedings of the Korean Geotechical Society Conference
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• 2000.11a
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• pp.231-236
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• 2000

In order to predict depth of the pile forward modeling and inversion of magnetic logging data was conducted by using a finite line of dipoles model. The horizontal component as well as the vertical component of magnetic fields can be measured in the borehole, and the magnetic anomalies can be obtained by subtracting the Earth's magnetic field from the measurement. The magnetic anomalies of the pile are considered as vector sum of induced magnetization due to the Earth's magnetic field and remnant magnetization possessed by steel strings in the pile. The magnetic anomalies are used as input data for inversion from which the length, the magnetic moment per unit length, and the dip angle of the pile can be obtained. From the inversion of synthetic noisy data, and the data obtained from the field model test it is found that the driving depth of the pile can be determined as close to the order of measuring interval (5∼10㎝). It is also found that the resultant magnetic anomalies due to an individual steel string in the pile are almost same as those due to a group of steel strings located at the center of the pile. The magnetic logging method also can be used for locating reinforced bars, pipes, and steel casings.

• Anatomy and Cell Biology
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• v.56 no.4
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• pp.469-473
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• 2023

Anomalies of anterior cerebral artery (ACA) include aplasia, hypoplasia and variations in number. Magnetic resonance angiography (MRA) is a non-invasive diagnostic technique for assessment of anomalies of cerebral arteries. The aim of the study was to determine the role of MRA in detection of variants of ACA in adults. This study is an observational retrospective study. This study included forty-nine adult cases (28 males and 21 females), mean age 48±12.9 SD with anomalies of ACA in MRA. Magnetic resonance imaging of the brain and MRA were done to all patients. Cerebral MRA and magnetic resonance images were evaluated for frequency and distribution of variants of anterior cerebral arteries, associated aneurysms and infarctions. Odds ratios (ORs) and relative risk were calculated to determine risk of occurrence of cerebral infarctions in patients with anomalies of ACA. Hypoplasia of ACA was the commonest anomaly of ACA (51% of cases). Risk of occurrence of cerebral infarctions was higher in cases with azygos variant (OR, 3.3; P=0.35) than in those with hypoplastic ACA (OR, 2; P=0.58). MRA was highly reliable in identification of different variants of ACA and concomitant vascular changes.

• Journal of the Korean Geophysical Society
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• v.7 no.1
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• pp.1-10
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• 2004

Gravity, magnetic and high-resolution seismic surveys were carried out in the Powell Basin to examine tectonic structure and recent sedimentation on Dec. 2002. The trend of negative gravity anomalies along the spreading axis of the Powell Basin changes from northwest to east-west toward south. Both boundaries of the basin with the Antarctic Peninsula and the South Orkey micro-continent show negative magnetic anomalies, which indicates that the boundaries were continental rift areas in the initial stage of spreading. Magnitude of the magnetic anomalies corresponding to the axis of the basin is rather small compared to those of normal spreading axises in other regions. Such small anomalies would be caused by reduction of magnetic strength of oceanic crust below thick sediments due to thermal alternation. High-resolution (3.5 kHz) seismic profiles reveal that top of the South Scotia Ridge is a flat terrain coverd with thin coarse sediments by glacial erosion. Thick oceanic sediments are deposited in the central part of the basin. Little deformation in the oceanic sediments indicates that the Powell Basin has been in stable tectonic environment after spreading of the basin stopped.

• Journal of Korean Neurosurgical Society
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• v.37 no.4
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• pp.263-267
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• 2005

Objective: Middle cerebral artery(MCA) anomalies are found incidentally on conventional cerebral angiography and magnetic resonance angiography(MRA). Our goal is to examine the incidence and types of MCA anomalies. Methods: Cerebral angiography was performed in 448 patients and MRA in 743; the patients had or were suspected to have cerebrovascular disease. The images were retrospectively evaluated for arterial anatomic anomalies. We use Teal's classification for definition of accessory and duplicated MCAs. Results: On cerebral angiography, the following anomalies of the MCA were found in seven patients: fenestration (n = 2, incidence = 0.45%); duplication (n = 2, incidence = 0.45%); accessory MCA (n = 2, incidence = 0.45%); aplasia (n = 1, incidence = 0.22%). On MRA, eight patients had anomalous MCAs : fenestration (n = 1, incidence = 0.14%); duplication (n= 6, incidence = 0.81%); accessory (n = 1, incidence = 0.14%). Conclusion: Although the clinical significance is not great, we find a relatively high incidence of anomalous MCAs. Knowledge and recognition of these MCA anomalies are useful and important in the interpretation of cerebral images and during neurosurgical procedures.

• Journal of the Geological Society of Korea
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• v.54 no.6
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• pp.677-682
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• 2018

Magnetic anomalies are sensitive to magnetic properties present in deep Earth and near surface structures. Such geophysical characteristics often can be quantified by numerical analyses. In this study, we developed a finite element method (FEM) approach to compute magnetic anomalies using COMOL $Multiphysics^{(R)}$. This FEM approach was verified by comparing its numerical results with the previously known analytic solution for a uniformly magnetized sphere. Then, we used the method to compute magnetic reversal patterns near mid-ocean ridge with various faulting scenarios. This COMSOL-based approach can be incorporated into advanced multi-physical numerical models to understand the Earth.

• Geophysics and Geophysical Exploration
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• v.5 no.3
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• pp.157-168
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• 2002

Improved procedures were implemented in the production of the lithospheric magnetic anomaly map from Magsat satellite magnetometer data of East Asia between $90^{\circ}E-150^{\circ}E$ and $10^{\circ}S-50^{\circ}N$. Procedures included more effective selection of the do·it and dawn tracks, ring current correction, and separation of core field and external field effects. External field reductions included an ionospheric correction and pass-by-pass correlation analysis. Track-line noise effects were reduced by spectral reconstruction of the dusk and dawn data sets. The total field magnetic anomalies were differentially-reduced-to-the-pole to minimize distortion s between satellite magnetic anomalies and their geological sources caused by corefield variations over the study area. Aeromagnetic anomalies were correlated with Magsat magnetic anomalies at the satellite altitude to test the lithospheric veracity of anomalies in these two data sets. The aeromagnetic anomalies were low-pass filtered to eliminate high frequency components that may not be shown at the satellite altitude. Although the two maps have a low CC of 0.243, there are many features that are directly correlated (peak-to-peak and trough-to-trough). The low CC between the two maps was generated by the combination of directly- and inversely-correlative anomaly features between them. It is very difficult to discriminate directly, inversely, and nully correlative features in these two anomaly maps because features are complicatedly correlated due to the depth and superposition of the anomaly sources. In general, the lithospheric magnetic components were recovered successfully from satellite magnetometer observations and correlated well with aeromagnetic anomalies in the study area.

• Ocean and Polar Research
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• v.24 no.4
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• pp.491-500
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• 2002

Magnetic anomalies in the Dokdo and it's surroundings were investigated with respect to structure and origin of the Dokdo and surrounding seamounts. After normal and diurnal correction of measured magnetic data, crossover correction was applied to reduce errors between sets of magnetic anomalies. The errors from crossover operation result in decrease of about 51%, from 62.2 nT to 30.1 nT in standard deviation. Reduction-to-the-pole, second vertical derivative and analytic signal processing were applied to explore magnetic anomaly signatures in detail. Magnetic anomalies are most complicated in the 1st-Dok seamount, show SWW-NEE linear pattern in the 2nd-Dok seamount and lower to the 3rd-Dok seamount. Different magnetic anomaly patterns in three seamounts imply that three volcanic seamounts were formed at different times and are composed of rocks that were produced in different conditions. It seems that the 3rd-Dok seamount was first to form and followed by the 1st-Dok seamount. The complicated magnetic and second vertical derivative anomaly patterns in the 1st-Dok seamount may be due to subsidiary cones around crater or the presence of intruded magma bodies below sea surface and the Dokdo is probably a marginal subsidiary part of crater.

• Geophysics and Geophysical Exploration
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• v.12 no.1
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• pp.17-26
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• 2009

On Aogashima Island, a volcanic island located in the southernmost part of the Izu Seven Islands Chain, vector magnetic anomalies were obtained in a helicopter-borne magnetic survey. The purpose of this study was to understand the volcanic structure of Aogashima Island in order to mitigate future disasters. Commonly, to obtain the magnetic structure of a volcanic island, total intensity anomalies (TIA) have been used, even though they have intrinsic errors that have not been evaluated correctly. Because the total intensity magnetic anomaly (TIA) is not a physical value, it does not satisfy Maxwell's Equations, Laplace's Equation, etc., and so TIA is not suitable for any physical analyses. In addition, it has been conventionally assumed that TIA is the same as the projected total intensity anomaly vector (PTA) for analyses of TIA. However, the effect of the intrinsic error ($\varepsilon_T$ = TIA.PTA) on the analysis results has not been taken into account. To avoid such an effect, vector magnetic anomalies were measured so that a reliable analysis of Aogashima Island magnetization could be carried out. In this study, we evaluated the error in TIA and used vector anomalies to avoid this erroneous effect, in the process obtaining reliable analysis results for 3D, vector magnetization distributions. An area of less than 1 A/m magnetization was found in the south-west part of Aogashima Island at the depth of 1.2 km. Taking the location of fumarolic activity into consideration, the lower-magnetization area was expected to be the source of that fumarolic activity of Aogashima Island.

• Proceedings of the International Union of Geodesy And Geophysics Korea Journal of Geophysical Research Conference
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• 2003.05a
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• pp.20-20
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• 2003

In the summers of 1997 and 1998 and in February of 2000 we made 570 measurements of the ambient geomagnetic field 120 cm above the pavement surface of State Route 130, south of Pahoa, the island of Hawaii using a three-component fluxgate magnetometer. We measured at every 15.2 m (50 feet) interval covering a distance of 6, 310 m (20, 704 ft) where both historic and pre-historic highly magnetic basalt flows underlie. We also collected 197 core samples from eight road cuts, 489 specimens of which were subject to AF demagnetizations at 5 - 10 mT level up to a maximum field of 60 mT. We observed significant inclination anomalies ranging from a minimum of $31^{\circ}$ to a maximum $40^{\circ}$ where a uniform inclination value of $36.7^{\circ}$ (International Geomagnetic Reference Field, IGRF) was expected. Since the mean of the observed inclinations is approximately $35^{\circ}$ we assume that the study area is slightly affected by the magnetic terrain effect to a systematically shallower inclinations for being located in the regionally sloping surface of the southern side of the island (Baag, et al., 1995). We observed inclination anomalies showing wider (spacial) wavelength (160 - 600 m) and higher amplitudes in the historic lava flows area than in the northern pre-historic flows. Our observations imply that preexisting inclination anomalies such as those that we observed would have been interpreted as paleosecular variation (PSV). These inclination anomalies can best be attributed to concealed underground highly magnetic dikes, channel type lava flows, on-and-off hydrothermal activities through fissure-like openings, etc. Both the within- and between-site dispersions of natural remanent magnetization (NRM) are largest (up to ${\pm}7^{\circ}$) above the flows of 1955, while the area of pre-historic flows in the northern part of the study area exhibit the smallest dispersion. Nevertheless, mean inclinations of each historic flow of 1955 and 1790 are almost identical to that of the corresponding present field, whereas mean of NRM (after AF demagnetization) inclinations for each of the four pre-historic lava flow units is twelve to thirteen degrees lower than the present field inclination. We observed three cases of very large inclination variations from within a single flow, the best fitting curves of which are linear, second and third order polynomials each from within a single flow, whereas no present field variations are observed. This phenomena can be attributed to the notion that local magnetic anomalies on the surface of an active volcano are not permanent, but are transient. Therefore we believe that local magnetic anomalies of an active volcano may be constantly modified due to on going subsurface injections and circulations of hot material and also due to wide spacial and temporal distribution of highly magnetic basaltic flows that will constantly modify the topography which will in turn modify the local ambient geomagnetic field (Baag, et al., 1995). Our observations bring into question the general reliability of PSV data inferred from volcanic rocks, because on-going various geologic and geophysical activities associated with active volcano would continuously deflect and modify the ambient geomagnetic field.

• Korean Journal of Remote Sensing
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• v.21 no.1
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• pp.91-98
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• 2005

On board satellite magnetometer measures all possible magnetic components, such as the core and crustal components from the inner Earth, and magnetospheric, ionospheric and' its coupled components from the outer Earth. Due to its dipole and non-dipole features, separation of the respective component from the measurements is most difficult unless the comprehensive knowledge of each field characteristics and the consequent modeling methods are solidly constructed. Especially, regional long wavelength magnetic signals of the crust are strongly masked by the main field and dynamic external field and hence difficult to isolate in the satellite measurements. In particular, the un-modeled effects of the strong auroral external fields and the complicated behavior of the core field near the geomagnetic poles conspire to greatly reduce the crustal magnetic signal-to-noise ratio in the polar region relative to the rest of the Earth. We can, however, use spectral correlation theory to filter the static lithospheric and core field components from the dynamic external field effects that are closely related to the geomagnetic storms affecting ionospheric current disturbances. To help isolate regional lithospheric anomalies from core field components, the correlations between CHAMP magnetic anomalies and the pseudo-magnetic effects inferred from satellite gravity-derived crustal thickness variations can also be exploited, Isolation of long wavelengths resulted from the respective source is the key to understand and improve the models of the external magnetic components as well as of the lower crustal structures. We expect to model the external field variations that might also be affected by a sudden upheaval like tsunami by using our algorithm after isolating any internal field components.