• Journal of the Korean Magnetics Society
• /
• v.20 no.2
• /
• pp.52-60
• /
• 2010

The changes of magnetic flux density distribution and the degree of magnetic anomaly on the ground surface by underground fracture zones and empty spaces had been investigated through the variations of the measuring heights. The magnetic flux density distributions were monitored for the ground surfaces of fracture zones, empty spaces and tunnels by fluxgate-type magnetometer. The fracture zones showed the magnetic anomaly with (+) and (-) peak-pairs in the magnetic flux distribution measured at 0.15 m height from the ground surface, and this anomaly disappeared at the height of 1.15 m. The underground empty spaces and tunnels showed the decrease of magnetic flux densities, where the degree of this density decrease diminished with the increase of the underground depth. And, the existence and size of underground empty spaces, such as tunnels and sink holes, could be monitored by the phenomena of this decreasing flux density.

• Economic and Environmental Geology
• /
• v.28 no.5
• /
• pp.469-480
• /
• 1995

Gravity and magnetic surveys were carried out to investigate the three-dimensional configuration and characteristics of the landfills at Nanjido waste disposal site. For terrain correction and three-dimensional density inversion of gravity data an algorithm, which calculates the gravity effect of a three-dimensional body by using the solid angle method, is developed. This algorithm has been proved to give more accurate terrain correction values for the small survey area having varied topography like Nanjido site as compared with widely used methods such as Hammer's method and multiquadric equation method. Density inversion of gravity anomaly data gives very useful information about the lateral and vertical variation of the landfills, which can be used to discriminate the kinds of wastes. The average density of filled materials appears to be $1.7\;g/cm^3$ which is much higher than the value $(0.8\;g/cm^3)$ estimated by Seoul City. The lateral variation of density shows high correlation with the pattern of ongoing depression of the landfills. The northern region of the landfill no. 1, which shows low density and high depression, is closely associated with the industrial waste and sludge filled area. The magnetic anomaly data provide information about relative concentration of magnetic materials, which is also very useful to investigate characteristics of the fills. Several high positive anomaly regions on the reduced-to-pole magnetic anomaly map are appeared to be associated with the industrial waste fills, but certain industrial waste fills show low negative anomalies. This kind of magnetic information can be used in selecting drilling locations over landfills away from buried metal products during the stabilization process.

• Geophysics and Geophysical Exploration
• /
• v.8 no.1
• /
• pp.97-103
• /
• 2005

Recent development of marine magnetic gradient systems, using arrays of sensors, has made it possible to survey large contaminated areas very quickly. However, underwater Unexploded Ordnances (UXO) can be moved by water currents. Because of this mobility, the cleanup process in such situations becomes dynamic rather than static. This implies that detection should occur in near real-time for successful remediation. Therefore, there is a need for a fast interpretation method to rapidly detect signatures of underwater objects in marine magnetic data. In this paper, we present a fast method for location and characterization of underwater UXOs. The approach utilises gradient interpretation techniques (analytic signal and Euler methods) to locate the objects precisely. Then, using an iterative linear least-squares technique, we obtain the magnetization characteristics of the sources. The approach was applied to a theoretical marine magnetic anomaly, with random errors, over a known source. We demonstrate the practical utility of the method using marine magnetic gradient data from Japan.

• Economic and Environmental Geology
• /
• v.28 no.4
• /
• pp.395-404
• /
• 1995

The geologic structure of the Cheju volcanic island has been investigated by analyzing the gravity and magnetic data. Bouguer gravity map shows apparent circular low anomalies at the central volacanic edifice, and the maximum difference of the anomaly values on the island appears to be 30 mgal. The subsurface structure of the island is modeled by three-dimensional depth inversion of gravity data by assuming the model consists of a stacked grid of rectangular prisms of volcanic rocks bounded below by basement rocks. The gravity modeling reveals that the interface between upper volvanic rocks and underlying basement warps downward under Mt. Halla with the maximum depth of 5 km. Magnetic data involve aeromagnetic and surface magnetic survey data. Both magnetic anomaly maps show characteristic features which resemble the typical pattern of total magnetic anomalies caused by a magnetic body magnetized in the direction of the geomagnetic field in the middle latitude region, though details of two maps are somewhat different. The reduced-to-pole magnetic anomaly maps reveal that main magnetic sources in the island are rift zones and the Halla volcanic edifice. The apparent magnetic boundaries inferred by the method of Cordell and Grauch (1985) are relatively well matched with known geologic boundaries such as that of Pyosunri basalt and Sihungri basalt which form the latest erupted masses. Inversion of aeromagnetic data was conducted with two variables: depth and susceptibility. The inversion results show high susceptibility bodies in rift zones along the long axis of the island, and at the central volcano. Depths to the basement are 1.5~3 km under the major axis, 1~1.5 km under the lava plateau and culminates at about 5 km under Mt. Halla. The prominent anomalies showing N-S trending appear in the eastern part of both gravity and magnetic maps. It is speculated that this trend may be associated with an undefined fault developed across the rift zones.

• Economic and Environmental Geology
• /
• v.31 no.6
• /
• pp.535-545
• /
• 1998

Various geophysical methods have been applied to the survey of the lava tunnel of Manjanggul in Cheju Island to study the effectiveness of each method in investigating underground tunnels. The surveys employing gravity, magnetic, electrical, AMT and VLF methods were carried out along seven profiles across the Manjanggul; especially, all the five methods were used on one representative profile. Several aspects of different methods pertinent to their use in investigation of underground tunnels have been noted. The electrical method employing the dipole-dipole array appeared to be the most effective one among five methods. Therefore, we have tested the electrical method more carefully by using various electrode spacings, and obtained successful resistivity sections showing the existence of lava tunnels. The gravity method provided relatively successful responses associated with the tunnel although the gravity readings were contaminated by wind blowing during the survey. The gravity data were also useful for the quantitative modeling study. The magnetic data were also successful in delineating the tunnel qualitatively. The AMT data were not successful because the used frequency band was not appropriate in detecting very shallow target. The VLF data were severely influenced by the neighboring noise sources such as power lines and were not successful in detecting the tunnel responses. The comprehensive result of electrical, gravity and magnetic surveys suggests that undiscovered lava tunnels may exist adjacent to the Manjanggul.

• Geophysics and Geophysical Exploration
• /
• v.27 no.1
• /
• pp.57-68
• /
• 2024

Offshore wind power is increasingly regarded as a viable solution for reducing greenhous emissions due to the construction of wind farms and their superior power generation efficiency. Submarine power cables play a crucial role in transmitting the electricity generated offshore to land. To monitor cables and identify points of failure, analyzing the location or depth of burial of submarine cables is necessary. This study reviewed the technology and research for detecting submarine power cables, which were categorized into seismic/acoustic, electromagnetic, and magnetic exploration. Seismic/acoustic waves are primarily used for detecting submarine power cables by installing equipment on ships. Electromagnetic and magnetic exploration detects cables by installing equipment on unmanned underwater vehicles, including autonomous underwater vehicles (AUV) and remotely operated vihicles (ROV). This study serves as a foundational resource in the field of submarine power cable detection.

• The Bulletin of The Korean Astronomical Society
• /
• v.37 no.1
• /
• pp.78.1-78.1
• /
• 2012

We present near-IR imaging polarimetry of the 5${\times}$9 fields (-39'${\times}$69') centered at 30 Doradus in the Large Magellanic Cloud (LMC), using the InfraRed Survey Facility (IRSF). We obtained polarimetry data in J, H, and Ks bands using the JHKs-simultaneous imaging polarimeter SIRPOL in 2008 December and 2011 December. We measured Stokes parameters of point-like sources to derive the degree of polarization and the polarization position angle. Since our results are suffered from non-photometric weather, we compare the polarization results from 2008 and those from 2011, and examine the photometric reliabilities between the two runs. Our survey data will be compared with molecular and dust maps to reveal the large-scale magnetic field properties in the star-forming clouds.

• The Bulletin of The Korean Astronomical Society
• /
• v.41 no.2
• /
• pp.57.2-57.2
• /
• 2016

Polarimetry is an important tool for studying the physical processes in the interstellar medium, including star-forming regions. Polarimetry of young stellar objects and their circumstellar structures provides invaluable information about distributions of matter and configurations of magnetic fields in their environments. However, only a few near-infrared circular polarization (CP) observations were reported so far (before our survey). A systematic near-infrared CP survey has been firstly conducted in various star-forming regions, covering high-mass, intermediate-mass, and low-mass young stellar objects. All the observations were made using the SIRPOL imaging polarimeter on the Infrared Survey Facility (IRSF) 1.4 m telescope at the South African Astronomical Observatory (SAAO). In this presentation, we present the first CP survey results. The polarization patterns, extents, and maximum degrees of circular and linear polarizations are used to determine the prevalence and origin of CP in the star-forming regions. Our results are explained with a combination of circumstellar scattering and dichroic extinction mechanism generating the high degrees of CP in star-forming regions. The universality of the large and extended CPs in star-formaing regions can also be linked with the origin of homochirality of life.

• The Journal of Engineering Geology
• /
• v.27 no.3
• /
• pp.207-215
• /
• 2017

The northern extension of the Dongrae Fault is inferred to transect the Ulsan Fault in the Gueo-ri area, Oedong-eup, ~15 km SE of Gyeongju City, Gyeongbuk province, S Korea. We conducted geological and geophysical (magnetic, electrical resistivity, and frequency domain electromagnetic) surveys to identify the extent and orientation of the Dongrae Fault in this region. Through joint interpretation of the geological and geophysical data sets, we confirm the presence of the Dongrae Fault and determine its strike ($N14^{\circ}E$). The Dongrae Fault is thought to cross the Ulsan Fault near Ipsil Bridge in the Gwangeo-ri area. Geophysical surveying revealed a fault damage zone that widens to the south, with a typical width of >200 m. Geological field surveys did not delineate the geometry of the Dongrae Fault because alluvial deposits overlie the fault in this area.

• Journal of The Korean Astronomical Society
• /
• v.37 no.5
• /
• pp.427-431
• /
• 2004

We use simulations of large-scale structure formation to study the build-up of magnetic fields (MFs) in the intergalactic medium. Our basic assumption is that cosmological MFs grow in a magnetohy-drodynamical (MHD) amplification process driven by structure formation out of a magnetic seed field present at high redshift. This approach is motivated by previous simulations of the MFs in galaxy clusters which, under the same hypothesis that we adopt here, succeeded in reproducing Faraday rotation measurements (RMs) in clusters of galaxies. Our ACDM initial conditions for the dark matter density fluctuations have been statistically constrained by the observed large-scale density field within a sphere of 110 Mpc around the Milky Way, based on the IRAS 1.2-Jy all-sky redshift survey. As a result, the positions and masses of prominent galaxy clusters in our simulation coincide closely with their real counterparts in the Local Universe. We find excellent agreement between RMs of our simulated galaxy clusters and observational data. The improved numerical resolution of our simulations compared to previous work also allows us to study the MF in large-scale filaments, sheets and voids. By tracing the propagation of ultra high energy (UHE) protons in the simulated MF we construct full-sky maps of expected deflection angles of protons with arrival energies $E = 10^{20}\;eV$ and $4 {\times} 10^{19}\;eV$, respectively. Accounting only for the structures within 110 Mpc, we find that strong deflections are only produced if UHE protons cross galaxy clusters. The total area on the sky covered by these structures is however very small. Over still larger distances, multiple crossings of sheets and filaments may give rise to noticeable deflections over a significant fraction of the sky; the exact amount and angular distribution depends on the model adopted for the magnetic seed field. Based on our results we argue that over a large fraction of the sky the deflections are likely to remain smaller than the present experimental angular sensitivity. Therefore, we conclude that forthcoming air shower experiments should be able to locate sources of UHE protons and shed more light on the nature of cosmological MFs.