• Journal of the Korean earth science society
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• 제32권6호
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• pp.674-686
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• 2011

Choosing a seismic source and geophone type including a coupling method can be the most important factor in shallow seismic surveys. We studied the characteristics of seismic signals by analyzing 6 different seismic data sets that collected from several sources and geophone conditions. Geophones attached to weight plate (1.8 kg) can be easily and economically installed on the paved road where geophones with spikes would cause the coupling problem. In addition, experiments in this study revealed that a small handy hammer can be used as a seismic source by striking the paved road to generate the seismic signals within 200 ms two-way travel time. Attaching weight plates to geophones may change the geophone response curve which generally depends on the geophone mass, but the change seems not to give significant differences in the first arrival of refracted wave and in the pattern of reflection events. Consequently, using weight plates on paved roads can be an efficient and cost-saving method in the near-surface high-resolution seismic surveys.

• Geophysics and Geophysical Exploration
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• 제25권3호
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• pp.129-139
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• 2022

Analog-based streamers for ultra-high-resolution seismic surveys are capable of additional noise ingress in water, but the specifications cannot be expanded through interconnections. Foreign-produced digital streamers have been introduced and used primarily at domestic research institutes; however, the cost is high and smooth maintenance is challenging. This study investigates the localization of ultra-high-resolution digital streamers capable of high-resolution imaging of a geological structure. A digital streamer capable of 24-bit, 10 kHz digital sampling of up to 64 channel data was developed through research and development. Various quantitative specifications of the system were designed and developed close to the benchmark model, Geometrics' GeoEel streamer, and the number of modules that make up the system was drastically reduced, reducing development costs and making it easier to use. The field applicability of the developed streamer system was evaluated in an in situ experiment conducted in the waters around the Port of Yeong-il Bay in Pohang in April 2022.

• Journal of the Korean Geophysical Society
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• 제2권3호
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• pp.201-208
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• 1999

A high-resolution seismic profile acquired across the middle part of the Pungam Basin, one of the Cretaceous sedimentary basins in Korea, has been interpreted to delineate subsurface geological structures. Boundary faults, intrusive bodies, and unconformity surfaces are identified on the seismic section. Basin fills are divided into five depositional units (Units I, II, III, IV, and V in descending order). The normal faults were formed by transtentional movement along a sinistral strike-slip fault zone. Unconsolidated sediments, a weathered layer, and sedimentary layers overly the Precambrian gneiss. The granite body intruded at the southeastern part contacts the adjacent sedimentary rocks by a near-vertical fault. Granitic intrusions caused tectonic fractures and normal faults of various sizes. An andesitic intrusive body indicates post-depositional magmatic intrusions. Continuous strike-slip movements have deformed basin-filling sediments (Units I and II).

• Geophysics and Geophysical Exploration
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• 제4권4호
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• pp.133-144
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• 2001

An effective seismic reflection technique for mapping the cavities and bedrock surface in carbonate rocks is described. The high resolution seismic reflection images were successfully registered by using the hydrophones employed in the stream-water driven trench, and were effectively focused by applying optimal data processing sequences. The strategy included enhancement of the signal interfered with the large-amplitude scattering noise, through pre- and post stack processing such as time-variant filtering, bad-trace editing, residual statics, velocity analysis, and careful muting after NMO (normal moveout) correction. The major reflections including the bedrock surface were mapped with the desired resolution and were correlated to the seismic crosshole tomographic data. Shallow major reflectors could be identified and analyzed on the AGC (auto gain control)-applied field records. Three subhorizontal layers were identified with their distinct velocities; overburden (<3000 m/s), sediments (3000-4000 m/s), limestone bedrock (>4000 m/s). Taking into account of no diffraction effects in the field records, gravel-rich overburdens and sediments are considered to be well sorted. Based on the images mapped consistently on the whole survey line and seismic velocity increasing with depth, this area probably lacks in sizable cavities (if any, no air-filled cavities).

• Geophysics and Geophysical Exploration
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• 제9권4호
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• pp.255-260
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• 2006

Measurements in two adjacent (about 1.5 m separation) boreholes reveal that there is a significant degree of variations in the width and property (permeability) of shear zones in the granitic rock. A high frequency (>10 kHz) cross-well seismic tomography was conducted to characterize the features of permeability distribution at the shear zones in the inter-well region. At the shear zones, the correlation between the permeability at the well location and the velocity pattern shown in the cross-well velocity tomogram suggests that a high resolution velocity tomogram may provide useful information for the shear zone characteristics, such as permeability, fracture density, width, and length.

• Geophysics and Geophysical Exploration
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• 제22권3호
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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.

• The Journal of Engineering Geology
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• 제31권3호
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• pp.269-281
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• 2021

Land creeping is the imperceptibly slow, steady, downward movement o f slope-forming soil or rock. Because creep-related failures occur frequently on a large scale without notice, they can be hazardous to both property and human life. Korea Forest Service has operated the prevention and response system from land creeping which has been on the rise since 2018. We categorized and proposed three survey steps (e.g., preliminary, regional, detailed) for investigation of creeping susceptibility site with a focus on geophysical mapping of a selected test site, Yongheung-dong, Pohang, Korea. The combination of geophysical (dipole-dipole electrical resistivity tomography and reciprocal seismic refraction technique, well-logging), geotechnical studies (standard penetrating test, laboratory tests), field mapping (tension cracks, uplift, fault), and comprehensive interpretation of their results provided the reliable information of the subsurface structures including the failure surface. To further investigate the subsurface structure including the sliding zone, we performed high-resolution geophysical mapping in addition to the regional survey. High-resolution seismic velocity structures are employed for stability analysis because they provided more simplified layers of weathering rock, soft rock, and hard rock. Curved slip plane of the land creeping is effectively delineated with a shape of downslope sliding and upward pushing at the apex of high resistive bedrock in high-resolution electrical resistivity model with clay-mineral contents taken into account. Proposed survey steps and comprehensive interpretation schemes of the results from geological, geophysical, and geotechnical data should be effective for data sets collected in a similar environment to land-creeping susceptibility area.

• Proceedings of the Korean Geotechical Society Conference
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• 한국지반공학회 2004년도 춘계학술발표회
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• pp.799-806
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• 2004

Recently, as the forerunner in establishing the Northeast Asia's logistics base, a lot of marine engineering works such as new ports and container terminals, extension of old ports, new bridges, land reclamation etc. have been progressed. Parallel to it, there is also an increasing demand for improving the safety of construction. In this situation, high resolution seismic reflection profiling can be well used, attempting to classify rocks and sediments under water, if possible, to delineate the distribution of grain sizes in sediments not only for calculating the cost of removing sediments from harbour's channels, but also for estimating the bearing capacities for bridge or port construction. However, the results from the corresponding reflection survey that has been in operation in our country can not be effectively used for engineering purposes mostly due to the insufficient resolution. Thus. in this paper, two innovative strategies are introduced to enhance resolution. The one deals with a newly designed exploration barge that will help reduce several kinds of noises encountered electrically or operationally. The other is associated with an establishment of optimum measuring system comprising e.g. a specially devised hydrophone with a combination of 7 piezoelectric elements. Field experiments performed at Busan harbour are illustrated. The quality of acquired data was thereby fundamentally improved in comparison with that obtained at the same time in a conventional way.

• Journal of the Korean Geotechnical Society
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• 제22권4호
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• pp.41-49
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• 2006

This paper demonstrates the feasibility of using shallow S-wave, high-resolution seismic reflection surveys to characterize geological structure and stratigraphy of basement rocks for civil engineering purposes. S-wave seismic reflections from depths less than 20 m were recorded along the top of steep readout slopes. Seismic reflection data were recorded using a standard CDP acquisition method with a 24-channel seismograph and a sledge-hammer SH-wave source. The data were acquired using a split-spread source-receiver geometry with a 2 m shot-and-receiver interval, and then were processed to enhance S/N ratio of the data, to improve resolvable power of the seismic section, and to get velocity information of the basement rock. The final seismic reflection profiles using the CDP technique has imaged surfaces as shallow as less than 1m and resolved beds as thin as 1m. The migrated reflection sections possess sufficient quality to correlate the prominent reflection events to the bedding planes and faults identified on the readout outcrops. Similar S-wave reflection surveys could also be used to produce the necessary details of a geological structure of shallow bedrocks to pinpoint optimum locations for monitor wells of civil engineering purposes.

• 한국지구물리탐사학회:학술대회논문집
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• 한국지구물리탐사학회 2005년도 공동학술대회 논문집
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• pp.15-20
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• 2005

Defects of dam body which can be induced in seepage or leakage procedure can directly affect dam safety. Therefore, a proper inspection method should be carried out in the first place to find out their positions and sizes, After that, some reinforcement works such as grouting and the corresponding assessment could be taken in a proper way. The dam(center core type earth dam) issued in this study has been in need for intensive diagnosis and reinforcement work, because a lot of slumps similar to cracks, seepage and some boggy area have been observed on the downstream slope. High resolution seismic reflection method was performed on the crest profile twice before and after grouting work(Aug. 2001 and Nov. 2004) aimed at the dam inspection and the assessment of grouting efficiency as well. To enhance the data resolution, P-beam energy radiation technique which can reduce the surface waves and hence to reinforce the reflection events was used. Strong reflection events were recognized in the stack section before grouting work, It seems that the events would be caused by e.g. horizontal cracks with a considerable aperture, Meanwhile such strong reflection events were not observed in the section after grouting. That is, the grouting work was dear able to reinforce the defects of dam body. Hence, the section showed an well arranged picture of dam inner structure. In this sense, seismic reflection method will be a desirable technique for dam inspection and for monitoring dam inner structure as well.