• Geophysics and Geophysical Exploration
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• v.3 no.1
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• pp.19-24
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• 2000

Seismic reflection data was obtained by using 28 and 100 Hz geophones at the ground subsidence sites in an old coal mine area. Frequency spectrum of the geophone analyzed with offset revealed that 1) In the near offset ($1\~10m$), the signals in the 100 Hz geophone data contains higher frequency components (up to 300 Hz) than that of the 28 Hz (<200 Hz), 2) In the intermediate offset ($11\~39m$), although the 28 Hz geophone data showed very similar frequency characteristics as the near offset data, the 100Hz geophone data seemed to be contaminated by noise at high frequency zone (>200 Hz). In the far offset ($\geq40\;m$), the signals in both the 28 and 100 Hz geophone data are attenuated to noise level at high frequency Bone more than 150 Hz.

• 한국지구물리탐사학회:학술대회논문집
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• 2005.05a
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• pp.223-226
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• 2005

In order to acquire 3 components data which has the good signal to noise ratio with only one shot, 3-C geophones were used, As a result, the vertical component showed the distinct first arrival of P-wave, and the horizontal component was improved the signal to noise ratio of S-wave, while was attenuated P-wave. The 2-D Poisson's ratio section was computed from P- and S-wave cell velocities included velocity tomograms of the P- and S-waves. The Poisson's ratio values were computed in the range of $0.2{\~}0.3$. With one shot, we can obtain 2-D distribution of dynamic Poisson's ratio as well as velocity tomograms of P- and S-waves.

• 한국지구물리탐사학회:학술대회논문집
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• 2006.06a
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• pp.95-100
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• 2006

To reveal subsurface structures of the Ulsan fault, seismic data were recorded along a 750-m long line near Nongso-Eup in Ulsan. P and S waves were generated simultaneously by impacting a 5 kg sledgehammer on a tilted plate. The data were received by 16 10-Hz 3-component geophones at 3 m intervals. Refracted P waves were inverted using the tomography method. Dip moveout and migration were applied to reflection data processed following a general sequence. Four layers were identified based on P-wave velocities and P- and S-wave stacked image. From top to bottom, the P-wave velocity of each layer ranges in $300{\sim}1100\;m/s$, $1100{\sim}1700\;m/s$, $1700{\sim}2700\;m/s$, and greater than 2700 m/s. The corresponding thickness of the top three layers averages 3.9 m, 5.9 m, 4.4 m, respectively. The S-wave stack section is effective to define subsurface structures shallower than 10 m.

• Geophysics and Geophysical Exploration
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• v.2 no.4
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• pp.209-214
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• 1999

A field experiment generating shear waves by trench method was conducted at two places in Taejun area. We were able to separate the P- and S-waves by summing and subtracting the vertical and horizontal component of the data recorded at a three component downhole geophone in the borehole. The analysis of the records revealed that the shear waves were polarized to NS and EW directions. The faster shear waves were polarized to NS direction. The NS direction generally agrees with the dominant joints direction observed from the cores collected from the borehole.

• Journal of the Korean Geophysical Society
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• v.8 no.2
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• pp.81-87
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• 2005

Acquisition and processing of vector seismic waves were conducted through simultaneous generation of P, SH, and SV waves and receiving those waves using three-component geophones. Test data were received by 24 8-Hz geophones at an interval of 2 m along a 94-m profile. The data were recorded for 512 ms with sampling intervals of 0.2 ms. Raw data indicate that both reflected and refracted P waves are strongly recorded on the vertical component while SH waves are significant on the transverse horizontal component. On the inline horizontal component, both direct P and converted PS waves are recorded. First arrivals of P and SH waves were detected simultaneously on the vertical and transverse horizontal axes, respectively. The recorded vector data were separately inverted using traveltime tomography to yield P- and SH-wave sections. Using those two velocity sections, Poisson's ratios were able to be obtained effectively.

• Proceedings of the KSR Conference
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• 2005.11a
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• pp.412-417
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• 2005

To ensure the safety of the tunnelling without the loss of economy, the tunnel seismic profiling(TSP) method for the prediction ahead of tunnel face, begins to be used routinely in these days. TSP method does not interfere the tunnelling works while the horizontal drilling does, and its prediction length is longer than that of the drilling. Yet the most frequently adopted technique of TSP in Korea is the multi-shot and 2 receiver array using in-hole receivers, even though this array requires as many as 26 drill-holes for receiver installation and ballasting, which results in 3-6 hours of suspension in excavation work. In this paper, multi-receiver and lesser shot array using side-wall attached 3 component geophones is to be described to prove the efficiency in terms of the survey time as well as the reliability of the method by comparison of the predicted weak points and the face mapping results. The predictions mostly agreed with the real fractures or joint developed zones which have been confirmed during the excavation. It also has been found that TSP method can be effectively applied to perform draining ground water ahead of tunnel face by imaging the geologic discontinuities.

• Geophysics and Geophysical Exploration
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• v.12 no.3
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• pp.255-262
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• 2009

Conventionally, vertical-seismic-profiling (VSP) survey that provides high-resolution information has mainly performed to obtain the exact depth of the gas hydrate-bearing sediment, which is one of the key factors in the development of the gas hydrate. In this study, we extracted interval velocities and created corridor stacks from the first domestic zero-offset VSP data, which were acquired with three component receivers at UBGH09 borehole in Ulleung Basin where gas hydrate exists. Then we compared the corridor stacks with a CMP stacked section from surface seismic data. First of all, we converted the signals recorded with three component receivers to true vertical and horizontal components by phase rotation, and divided the data into direct waves and reflected waves by wavefield separation processing. The trend of the interval velocity extracted from the zero-offset VSP was similar to that of the sonic log obtained at the same borehole. Because the interval velocity of the gas hydrate-bearing sediment above the BSR was high, and it decreased suddenly through the BSR, we could infer that free gas is accumulated below the BSR. The results of comparing the corridor stacks to the CMP stacked section of the surface seismic data showed that most reflection events agreed well with those in the surface CMP stacked section and that the phase-rotated VSP data corresponded better with the surface seismic data than the VSP data without phase rotation. In addition, by comparing a corridor stack produced from the transverse component with the CMP stacked section of the surface seismic data, we could identify PS mode-converted reflections in the CMP stacked section.

• 한국지구물리탐사학회:학술대회논문집
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• 2006.06a
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• pp.125-130
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• 2006

Both surface seismic and far-offset VSP data were recorded alongtwo mutually perpendicular profiles in the Pungam basin. The first-arrival times were simultaneously inverted using the tomography method. For the surface data, seismic energy was generated by a 5-kg sledgehammer at 48 stations and detected by 21 surface geophones at 3 m intervals and one 3-component geophone in test borehole for the purpose of static corrections. For the VSP data, seismic waves generated by the sledgehammer on the ground were detected by a 3-component borehole geophone in a depth range of $9{\sim}99\;m$. Delay times of the hammer data were corrected using the seisgun data before the inversion to yield velocity tomograms. The tomograms indicates that the soil layer with velocities less than 750 m/s averages 1.8 m thick. The velocity varies from 5353 m/s at the depth range of $31{\sim}40\;m$ to 4262 m/s at the depth range of $65{\sim}73\;m$. Compared with core samples, the relatively large variation in velocity may due to lithology changes and fracture effects with depth.

• Economic and Environmental Geology
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• v.44 no.3
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• pp.229-238
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• 2011

S-wave, which provides lithology and pore fluid information, plays a key role in estimating gas-hydrate saturation. In general, P- and S-wave velocities increase in the presence of gas-hydrate and the P-wave velocity decreases in the presence of free gas under the gas-hydrate layer. Whereas there are very small changes, even slightly increases, in the S-wave velocity in the free gas layer because S-wave is not affected by the pore fluid when propagating in the free gas layer. To verify those velocity properties of the BSR (bottom-simulating reflector) depth in the gas-hydrate prospect area in the Ulleung Basin, P- and S-wave velocity profiles were derived from multi-component ocean-bottom seismic data which were acquired by Korea Institute of Geoscience and Mineral Resources (KIGAM) in May 2009. OBS (ocean-bottom seismometer) hydrophone component data were modeled and inverted first through the traveltime inversion method to derive P-wave velocity and depth model of survey area. 2-D multichannel stacked data were incorporated as an initial model. Two horizontal geophone component data, then, were polarization filtered and rotated to make radial component section. Traveltimes of main S-wave events were picked and used for forward modeling incorporating Poisson's ratio. This modeling provides S-wave profiles and Poisson's ratio profiles at every OBS site. The results shows that P-wave velocities in most OBS sites decrease beneath the BSR, whereas S-wave velocities slightly increase. Consequently, Poisson's ratio decreased strongly beneath the BSR indicating the presence of a free gas layer under the BSR.