• 한국지구물리탐사학회:학술대회논문집
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• 2002.09a
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• pp.85-100
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• 2002

This paper deals with a development of combined seismic methods, based on the application of Televiewer and seismic tomography, for the quantitative assessment of reclaimed body or dam at seashores in our county. The underground structure of reclaimed dam is very complex, mainly due to the unexpected exchange of rock fragments with the marine silty mud in conjunction with S.C.P. (Sand Compaction Pile) foundation, so that for several reasons only the use of Televiewer and seismic tomography for general application might not lead to a desirable resolution. Kinds of upgraded measuring and evaluation techniques for that are needed. For examples, a novel strategy for capturing the returning impulses from the outer side of casing plastic pipe is desired to be developed. For the tomograhy, one should be being more focussed on the study of raw data based on the wave propagation theory. In this paper, it is shown that such multidisciplinary approaches can be, by attempting to compare and jointly interpret the results from two methods, much efficient for understanding the reclaimed dam structure.

• 한국지구물리탐사학회:학술대회논문집
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• 2007.06a
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• pp.48-53
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• 2007

A new way to integrate various geophysical information for evaluation of RQD was developed. In this study, we did not directly define the RQD value where borehole data are not sampled. Instead, we infer the probability of RQD values with prior probability from borehole direct data, and secondary supporting probability from resistivity and seismic tomography data. For the integration, we applied the geostatstical indicator kriging to get prior probability of RQD value, and indicator kriging with soft data to get the supporting probability from resistivity and seismic data. And we finally use the permanence ratio rule to integrate these information. The finally obtained result was also analyzed to fully utilize the probabilistic features. We show the probability of wrongly classifying the RQD evaluation and vice versa. This result may be used for decision making process based on the geophysical exploration.

• Journal of Industrial Technology
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• v.27 no.B
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• pp.115-119
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• 2007

Currently, geophysical method is applied for understanding the subsurface geologic structure economically and systematically, but there exists some limitations on recognizing complex subsurface structures precisely by a single geophysical method. In order to understand the complex subsurface structures, we applied various geophysical methods including seismic refraction survey, two-dimensional resistivity survey, seismic tomography survey, suspension-ps log, and understood distribution of low velocity, low resistivity range of resistivity survey and correlation of an intersecting point, velocity distribution of seismic tomography survey.

• Proceedings of the Korean Geotechical Society Conference
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• 2004.03b
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• pp.491-498
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• 2004

In recent years, as an effort to grasp the leading position in the field of maritime trading, new ports and container terminals arc now under construction. Old ports are extended. At the beginning, stones were thrown down to form stone embankments, that is stone-dams, in the outer and inner boundaries of the planned reclamation-land. S.C.P(Sand Compaction Pile) works are often needed to improve the stability of stone-dams, where marine sediments arc relatively thick. Here, interests are centered on the shape of stone body. In this, drilling work won't provide a sufficient resolution. In addition, the result corresponds to only one borehole point information. Thus, the aim of this paper is to introduce an affordable technology, that is, a combined geophysical method(seismic tomography + Televiewer) enables to get the whole information about stone-dam section. The measuring and evaluating procedure is described in detail with an emphasis on dealing with the use of seismic detonator, proper borehole deployment and integrated data analysis. Examples of field experiments at Busan new port are illustrated, which will prove the benefit of combined geophysical method.

• Geophysics and Geophysical Exploration
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• v.9 no.1
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• pp.30-36
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• 2006

Japan's first pilot-scale $CO_2$ sequestration experiment has been conducted in Nagaoka, where 10400 t of $CO_2$ have been injected in an onshore aquifer at a depth of about 1100 m. Among various measurements conducted at the site for monitoring the injected $CO_2$, we conducted time-lapse crosswell seismic tomography between two observation wells to determine the distribution of $CO_2$ in the aquifer by the change of P-wave velocities. This paper reports the results of the crosswell seismic tomography conducted at the site. The crosswell seismic tomography measurements were carried out three times; once before the injection as a baseline survey, and twice during the injection as monitoring surveys. The velocity tomograms resulting from the monitoring surveys were compared to the baseline survey tomogram, and velocity difference tomograms were generated. The velocity difference tomograms showed that velocity had decreased in a part of the aquifer around the injection well, where the injected $CO_2$ was supposed to be distributed. We also found that the area in which velocity had decreased was expanding in the formation up-dip direction, as increasing amounts of $CO_2$ were injected. The maximum velocity reductions observed were 3.0% after 3200 t of $CO_2$ had been injected, and 3.5% after injection of 6200 t of $CO_2$. Although seismic tomography could map the area of velocity decrease due to $CO_2$ injection, we observed some contradictions with the results of time-lapse sonic logging, and with the geological condition of the cap rock. To investigate these contradictions, we conducted numerical experiments simulating the test site. As a result, we found that part of the velocity distribution displayed in the tomograms was affected by artefacts or ghosts caused by the source-receiver geometry for the crosswell tomography in this particular site. The maximum velocity decrease obtained by tomography (3.5%) was much smaller than that observed by sonic logging (more than 20%). The numerical experiment results showed that only 5.5% velocity reduction might be observed, although the model was given a 20% velocity reduction zone. Judging from this result, the actual velocity reduction can be more than 3.5%, the value we obtained from the field data reconstruction. Further studies are needed to obtain more accurate velocity values that are comparable to those obtained by sonic logging.

• International Journal of Safety
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• v.7 no.2
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• pp.17-22
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• 2008

Recent developments in contactless, air-coupled sensing of seismic and ultrasonic waves in concrete structures are presented. Contactless sensing allows for rapid, efficient and consistent data collection over a large volume of material. Two inspection applications are discussed: air-coupled impact-echo scanning of concrete structures using seismically generated waves, and air-coupled imaging of internal damages in concrete using ultrasonic tomography. The first application aims to locate and characterize shallow delamination defects within concrete bridge decks. Impact-echo method is applied to scan defected concrete slabs using air coupled sensors. Next, efforts to apply air-coupled ultrasonic tomography to concrete damage imaging are discussed. Preliminary results are presented for air-coupled ultrasonic tomography applied to solid elements to locate internal defects. The results demonstrate that, with continued development, air-coupled ultrasonic tomography may provide improved evaluation of unseen material defects within structures.

• Geophysics and Geophysical Exploration
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• v.1 no.1
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• pp.43-48
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• 1998

Recently seismic tomography has been widely used to visualize subsurface structure for resource explorations and construction site evaluation. We studied a way to include fresnel zone concept in the conventional ray-based traveltime tomography. The algorithm developed uses the same order of computing time as the conventional traveltime to mography but incorporates the rigorous wavepath concept of wave-equation tomography. Some experiments to synthetic and real data show reasonable results compared to conventional ray-based traveltime tomography.

• Geophysics and Geophysical Exploration
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• v.19 no.3
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• pp.164-173
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• 2016

Cable-free seismic technology is to acquire seismic data with independent receivers which are not connected by cables. This is an effective method for survey designs with less topographical conditions. With technology advancement for cable-free receivers, reliable data quality, easy deployment, and picking up the receivers, the cable-free technology has begun to apply to land seismic acquisition. In this study we introduced a cable-free seismic system and its equipment. We tried to build up the cable-free seismic technology through the field application. In the seismic tomography field applications, the seismic signals of the cable-free receiver and cabled receiver with the same distance from the source show the same phase in early stage. The difference of the first arrival times between two signals is less than 0.4 ms, which could be accepted. In the field application for seismic reflection exploration, we acquired shot gathers with different source depth and dynamite charge. The shot gathers from cable-free and cabled system are similar to each other. With an efficient method for receiver deployment and survey design, the application of the cable-free technology will increase.

• Proceedings of the Korean Geotechical Society Conference
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• 2000.11a
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• pp.577-584
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• 2000

Seismic refraction survey was peformed for 10 lines along NE-SW and NW-SE directions above Nampoong gallery at Makyo-ri, Dogye, Samcheok, Kangwon-do. 48 geophones were laid in line with the interval of 1m, and a 5Kg hammer was used as a source at 5 points for each line. Data processing was done using reciprocal time method, GRM, and traveltime tomography which utilizes wavefront expansion method for forward process and SIRT for inversion. The result shows that the first layer has its lower boundary between 3.49m and 8.88m. The P-wave velocity of the first and the second layer were estimated as 270 360m/s and 1550 1940m/s respectively. When the boundary of the first and second layer is smooth enough and the velocity difference is large enough, GRM has little advantage over reciprocal time method. The result of reciprocal method and traveltime tomography shows consistency. The northeast part of the boundary has syncline structure, which is similar to the topography above. This implies that the collapse of the cavities of Nampoong gallery result in the subsidence of the ground surface. The subsidence is in progress across the Youngdong railroad, therefore a proper reinforcement work is required.

• Proceedings of the KSR Conference
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• 2005.05a
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• pp.610-614
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• 2005

Rock mass classification by using electrical resistivity tomography(ERT) method is widely performed for the determination of rock support type in tunnel design. In the region of high electrical noise level, however, the result of the ERT will have many erroneous features. In this study, the back ground electrical noise had been measured to find out the reason why the results of ERT in this area did not agree to the expected geology confirmed by boreholes. In order to overcome this limitation of ERT, a hybrid surface-borehole array seismic refraction tomography had been followed. Using this technique, we could get P-wave velocity section including the depth level of tunnel. The comparison of the P-wave velocity and RMR shows fairly good statistical relationship to make it possible to set up the rock mass classification for the entire tunnel line.