• Geophysics and Geophysical Exploration
• /
• v.12 no.4
• /
• pp.367-369
• /
• 2009

• The Journal of Engineering Geology
• /
• v.19 no.1
• /
• pp.1-8
• /
• 2009

Geophysical surveys were conducted on an upper part of a natural slope located at Daejeon University. Electrical resistivity and seismic refraction measurements were carried out to obtain information on a weathered zone and internal structure at shallow depth, while AMT measurement a bed rock and geological structure at deep depth. With all the techniques applied, these results show a good correlation between electrical resistivity images and refraction velocity distributions for the characterization of a weathering and geological structure at depth. In particular, AMT survey seems to be the powerful tool for detecting a distribution of a bed rock with deep depth. The combined geophysical investigation produced a detailed image of a subsurface structure and improved well in the interpretation.

• Journal of the Korean Geotechnical Society
• /
• v.38 no.10
• /
• pp.49-60
• /
• 2022

Underground urbanization appears to be a promising solution in response to the shortage of construction sites in the above-ground space. In this context, an accurate evaluation of a construction site ensures the long-term performance of geosystems. This study characterizes potential sites for complex plants built in underground space using geophysical methods (i.e., seismic refraction exploration and electrical resistivity survey) and in situ tests (i.e., standard penetration tests (SPTs) and downhole tests). SPTs are conducted in nine boreholes BH-1-BH-9 to estimate the groundwater level and vertical distribution of geological structures. The seismic refraction method enables us to obtain the elastic wave velocity and thickness of each soil layer for each cross-sectional area. An electrical resistivity survey conducted using the dipole array method provides the electrical resistivity profiles of the cross-sectional area. Data obtained using geophysical techniques are used to assess the classification of the soil layer and bedrock, particularly the fracture zone. This study suggests that geotechnical information using in situ tests and geophysical methods are useful references to design an underground complex plant construction.

• Journal of the Korean Geophysical Society
• /
• v.9 no.4
• /
• pp.343-349
• /
• 2006

In order to study unconsolidated sediments upstream of the Soyang Dam, seismic refraction data were recorded along five profiles of 94 m length. Receiver interval and record length were 2 m and 204 ms, respectively. Recorded data were analyzed using the traveltime tomography and delay-time methods in order to reveal sediment distribution in the investigation site. The acoustic basements are buried at approximately 14 m deep and their thickness shallows to southeast approaching to a hill. On the basement, pre-existing sediments deposited before construction of the Soyang dam lies. This sedimentary layer is composed of paleo-soil and sandy sediments. Above this layer, recent sediments comprise the top layer which is believed to have been deposited since the formation of the dam. Average thickness of this uppermost layer is approximately 1.6 m, which is much thicker than in the downstream.

• Journal of the Korean Geophysical Society
• /
• v.2 no.3
• /
• pp.209-216
• /
• 1999

In order to get geologic information necessary for underground installation of water pipe, seismic refraction profiling was applied to the southwest side of the Seomjin River which flows between Namwon-gun, Cholabuk-do and Gokseong-gun, Cholanam-do. Before obtaining the in-line refraction data, walkaway data were recorded with 1 m geophone interval and -36∼+36 m offset range. From the walkaway data, it is interpreted that a dry soil layer with the average velocity of 585 m/s covers wet sediments with the average velocity of 1,326 m/s. The second layer overlies basements nearly horizontally with the average velocity of 4,218 m/s. Refraction profiling of 220 m long with the geophone interval of 2 m is interpreted with the Generalized Reciprocal Method (GRM). Three layers are identified with average velocities of 688 m/s, 1,473 m/s, and 3,776 m/s, respectively. The depth to the bedrock impossible for ripping ranges between two extremes, 1.51∼2.43 m and 2.25∼3.54 m, depending upon thickness of the hidden layer. A typical shortcoming of refraction method, the hidden layer problem, prevents accurate estimation in depth of the second layer.

• Economic and Environmental Geology
• /
• v.43 no.6
• /
• pp.615-623
• /
• 2010

Seismic refraction survey is a geophysical method that delineates subsurface velocity structure using direct wave and critically refracted wave. The generalized reciprocal method(GRM) is an inversion technique which uses travel-time data from several forward and reverse shots and which can provide the geometry of irregular inclined refractors and structures underlain by hidden layer such as low velocity zone and thin layer. In this study, a simple Excel-GRM routine was tested for fast mapping of the interface between weathering layer and bedrock during the survey, with employing a pair of forward and reverse shots. This routine was proved to control the maximum dip of approximately $30^{\circ}C$ and maximum velocity contrast of 0.6, based on the panel tests in terms of dipping angle and velocity contrast for the two-layer inclined models. In contrast with conventional operation of five to seven shots with sufficient offset distance and indoor data analysis thereafter, this routine was performed in the field shortly after data acquisition. Depth to the bedrock provided by Excel-GRM, during the field survey for Cheongju granite area, correlates well with the elevation of the surface of soft rock from the drill core and SPS logging data. This cost-effective routine developed for quickly delineating the bedrock surface in the field survey will be readily applicable to mapping of weathering zone in narrow zone with small variation of elevation of bedrock.

• Journal of the Korean Geophysical Society
• /
• v.4 no.2
• /
• pp.113-120
• /
• 2001

The discrete wavelet transform(DWT) has potential as a tool for supplying discriminatory attributes with which to distinguish seismic events. The wavelet transform has the great advantage over the Fourier transform in being able to localize changes. In this study, a discrete wavelet transform is applied to seismic traces for identifying seismic events and picking of arrival times for first breaks and S-wave arrivals. The precise determination of arrival times can greatly improve the quality of a number of geophysical studies, such as velocity analysis, refraction seismic survey, seismic tomography, down-hole and cross-hole survey, and sonic logging, etc. provide precise determination of seismic velocities. Tests for picking of P- and S- wave arrival times with the wavelet transform method is conducted with synthetic seismic traces which have or do not have noises. The results show that this picking algorithm can be successfully applied to noisy traces. The first arrival can be precisely determined with the field data, too.

• 한국지구물리탐사학회:학술대회논문집
• /
• 1999.08a
• /
• pp.48-64
• /
• 1999

For quantitative evaluation of geotechnical engineering properties such as rippability and diggability, clear interpretation on the subsurface velocity structures should be preceded by figuring out top soil, weathered and soft rock layers, shape of basement, fracture zones, geologic boundary and etc. from the seismic refraction data. It is very important to set up suitable field parameters, which are the configuration of profile and its length, spacings of geophones and sources and topographic conditions, for increasing field data quality Geophone spacing of 3 to 5m is recommended in the land slope area for house land development and 5 to 10m in the tunnel site. In refraction tomography technique, the number of source points should be more than a half of available channel number of instrument, which can make topographic effect ignorable. Compared with core logging data, it is shown that the velocity range of the soil is less than 700m/s, weathered rock 700${\~}$1,200m/s, soft rock 1,200${\~}$1,800m/s. And the upper limit of P-wave velocity for rippability is estimated 1,200 to 1,800m/s in land slope area of gneiss. In case of tunnel site, it is recommended in tunnel design and construction to consider that tunnel is in contact with soft rock layer where three lineaments intersecting each other are recognized from the results of the other survey.

• Economic and Environmental Geology
• /
• v.36 no.6
• /
• pp.501-510
• /
• 2003

Several geophysical surveys(electrical resistivity, electromagnetic, seismic refraction, CPR) were conducted to primarily investigate the gallery and the geomembrane at an abandoned mine(Imcheon mine). The subsurface structure mapped from seismic refraction survey mainly consists of three velocity layers(>1000 m/s, 1000∼2000 m/s,＜2000 m/s). Top of the bedrock, whose velocities exceed 2000 m/s, appears to be at depth of 7.5∼10m. Higher resistivities (of ten thousands-hundred of thousands ohm-m) are interpreted to be associated with a open(cavities) gallery. The events at depth of approximately 0.5∼0.7m in GPR sections are probably caused by high-density-poly-ethylene geomembrane. Taking into consideration of the differences in the spatial resolution between georadar and electrical surveys, the events of geomembrane correspond to the top of the high resistivities at depth of about 2m. The segments, characterized with the higher conductivities in the electromagnetic data and the lower resistivities in the electrical resistivity data, are probably associated with surface water or tear zone of geomembrane.

• Proceedings of the International Union of Geodesy And Geophysics Korea Journal of Geophysical Research Conference
• /
• 2003.05a
• /
• pp.15-15
• /
• 2003

The first crustal refraction survey in the Korean Peninsula was carried out over the survey line connecting Seosan-Yeongdong-Kyeongiu on Dec. 15, 2002. The total length of the survey line was about 300 Km and 198 portable seismometers were deployed with approximately 1.5-km interval. The survey line itself was geologically important since it was almost normal to the so-called Sino-Korean structural trend. Two shots, one at Seosan (west end point) and the other at Yeongdong (mid-point), were exploded. They were 100-m deep drill well explosions. The Seosan shot consisted of a ton emulsion type explosive, while Youngdong consisted of 500 kg one. Both shots generated signals with good S/N ratios to the farthest receivers. Seismic signals were recorded by 195 receivers out of 198 ones. Although the originally planned Kyeongju shot (east end point) could not be exploded due to public discontent, the experiment was evaluated very successful. First breaks in all recorded traces were picked up and two preliminary analyses were carried out. The one is conventional flat layer analysis and the other was refraction tomographic analysis. The one resulted in average 32-km thick two-layer crust and the underlying mantle with 8.05-km/s P-velocity. The top crust layer with 3.86 kw/s P-velocity was 2.5-km thick and the lower crust layer with 6.0l km/s P-velocity was 29.5-km thick. The other resulted in a velocity cross-section. The confidence level of the velocity cross-section could not be evaluated at this time because only two shot were exploded. Detailed analyses such as surface wave dispersion are on going. Continuing crustal scale refraction surveys are planned in Korea.