• Proceedings of the Korean Geotechical Society Conference
• /
• 2002.03a
• /
• pp.695-702
• /
• 2002

Groundwater flow is primarily influenced by the presence of fractures, functioning as conduits. To block the flow, grouting operation is commonly used. Thereby the fractures are then expected to be sealed, which will add to enhance the shear strength in rock. This far, regarding the assessment of grouting efficiency, however, there's been a considerable uncertainty That is, several geophysical methods of high resolution such as tomography, S-wave logging have produced a significant amount of measurable response caused by grouting, but they can inevitably be used only for the qualitative assessment. Thus, this paper deals with an accurate quantitative assessment about the grouting result. In this, a new strategy is introduced, based mainly on evaluating the opening of fractures. For fracture-opening investigation purposes, borehole Televiewer has already proven to be an excellent logging technique that produces both amplitude image and traveltime image. As well known, the traveltime image can be converted to a high precision 3D caliper log with max. 288 arms, which allows to observe the opening of fractures. To evaluate the fracture opening from the traveltime image, an algorithm of practical use was developed, in which image correction due to the borehole deviation, feature discrimination of wall roughness from fractures, automatic evaluation procedure etc. were considered. Field examples are shown to confirm the efficiency of the suggested method.

• 한국지구물리탐사학회:학술대회논문집
• /
• 2007.06a
• /
• pp.209-214
• /
• 2007

In Korean geology that crystalline rock is dominant, the properties of subsurface including the anisotropy are distributed complexly and changed abruptly. Because of such geological environments, cross-hole seismic traveltime tomography is widely used to obtain the high resolution image of the subsurface for the engineering purposes in the geotechnical sites. However, because the cross-hole tomography has a wide propagation angle coverage relatively, its data tend to include the seismic velocity anisotropy comparing with the surface seismic methods. It can cause the misinterpretation that the cross-hole seismic data including the anisotropic effects are analyzed and treated with the general processing techniques assuming the isotropy. Therefore, we need to consider the seismic anisotropy in cross-hole seismic traveltime tomography. The seismic anisotropic tomography algorithm, which is developed for evaluation of the velocity anisotropy, includes several inversion schemes in order to make the inversion process stable and robust. First of all, the set of the inversion parameters is limited to one slowness, two ratios of slowness and one direction of the anisotropy symmetric axis. The ranges of the inversion parameters are localized by the pseudo-beta transform to obtain the reasonable inversion results and the inversion constraints are controlled efficiently by ACB(Active Constraint Balancing) method. Especially, the inversion using the Fresnel volume is applied to the anisotropic tomography and it can make the anisotropic tomography more stable than ray tomography as it widens the propagation angle coverage.

• Journal of the Korean earth science society
• /
• v.22 no.1
• /
• pp.20-29
• /
• 2001

P wave traveltime delays has been measured and a preliminary tomouaphic inversion has been performed using the data collected from the network deployed by Korea National University of Education in the southcentral part Korea. The maximum variation in relative traveltime residuals is almost 0.7 seconds. A large azimuthal variation in traveltime residuals is observed at the stations in the northwestern part of the study area. This might indicate the existence of lateral velocity heterogeneity beneath the study area. Although, the inverted tomographic image seems to be correlated with the tectonic boundary between Kyunggi Massif and Okcheon Belt, the confidence level is presently low. We should place on the importance of this study for finding preferable inversion parameters and predicting probable result. Better tomographic image of the study area can be obtained in the near future when the data are accumulated.

• Geophysics and Geophysical Exploration
• /
• v.6 no.3
• /
• pp.138-142
• /
• 2003

Crosswell seismic data were acquired in three sections crossing a tunnel of 3 different types; one was empty, another was ailed by sand, and the other was filled by rock debris. Both the P- and S-wave first arrivals were picked and the traveltime tomography was conducted to generate the P- and S- wave velocity tomograms on the all three sections. Among six tomograms, only one tomogram shows a low velocity zone that can be interpreted as a tunnel image. The tomogram is the P wave velocity image of a section that crosses an empty tunnel. The result of numerical analysis for the spatial resolution of the traveltime tomography was consistent to this finding.

• 한국지구물리탐사학회:학술대회논문집
• /
• 1999.08a
• /
• pp.137-155
• /
• 1999

Recently, Televiewer(Borehole Acoustic Scanner(Televiewer)) has come to be widely used specially for the general engineering construction design. The Televiewer tool using a focussed acoustic beam is to detect the amplitude and traveltime of each reflected acoustic signal at the wall, resulting in the amplitude- and traveltime image respectively. Fractures can be well detected, because they easily scatter the acoustic energy due to the highly narrow beam. In addition, the drilling work will rough the borehole wall so that the acoustic energy can be scattered simply due to the roughness of the wall. Thus, the amplitude level can be directed associated with the elastic properties(impedance) and the hardness of the rock as well. Meanwhile, the traveltime image provides an information about the borehole shape and can be converted to a high precision 3D caliper log(max. 288 arms). In this paper, based on the high resolution of Televiewer images, general evaluation methods are illustrated to derive very reliable rock parameters.

• Proceedings of the Korean Geotechical Society Conference
• /
• 2004.03b
• /
• pp.815-822
• /
• 2004

A knowledge of in situ stress state is important to design various engineering structures such as dams, tunnels and so on. There are about three wellknown indicators that is, borehole will breakouts, hydraulic fracturing, ellipsoidal cross section of borehole that have been attributed to the state of stress in the vicinity of borehole. Fortunately, Televiewer traveltime image can be used as a caliper log with 144 or 288 arms, which allows to determine the borehole shape. Televiewer amplitude image will give detailed information about the distribution and character of breakouts and hydraulic fracturing as well. For investigation purposes, a series of boreholes(total 195 boreholes: 12.239m) that have been logged all over the country during past 10 years are analyzed. The primary objective of this paper are to examnine the ability of a Televiewer to determine the shape of borehole, to present data inferred by stress indicators, to indicate their possible relationship with the anisotropic horizontal stresses. It is shown that in most cases the fracture orientation statistically estimated from observed fractures denotes an excellent correlation with the orientations inferred by stress indicators. Many intervals of breakouts are terminated at the intersection of oblique fracture with the borehole. The results from Televiewer data are further compared with those of hydraulic fracturing techniques.

• Proceedings of the Korean Geotechical Society Conference
• /
• 2002.03a
• /
• pp.627-634
• /
• 2002

To facilitate a drilling plan for grouting in fractured rock, an algorithm of practical use associated with a new term “fracture density distribution”or“fracture tomogram”is developed. It is well known that Televiewer data(amplitude and traveltime image) provide detailed information about not only dip and dip direction of each fracture but also its aperture size estimated by an appropriate evaluation algorithm. A selected plane section of medium around a borehole or the cross section between two boreholes is discretized into a two dimensional grid of cells(rectangular elements). As each elongated(straight) fracture passes through the cells, the corresponding aperture size value is successively summed up in each cell, depending on the fracture length segment. In this, the fracture lines can be determined by intersecting of each fracture plane with the selected plane section. If the fracture line does not pass through a particular grid element, the segment length is set to zero. The final value(aperture size value of each cell) derived from all the detected fractures constitutes the fracture density distribution of the selected plane section, Field examples are illustrated, which will prove the benefit of the suggested algorithm for several kinds of grouting works.

• Geophysics and Geophysical Exploration
• /
• v.5 no.1
• /
• pp.33-45
• /
• 2002

It was necessary to devise new techniques to overcome and enhance the resolution limits of traveltime tomography. Waveform inversion has been one of the methods for giving very high resolution result. High resolution image could be acquired because waveform inversion used not only phase but amplitude. But waveform inversion was much time consuming Job because forward and backward modeling was needed at each iteration step. Velocity-stress method was used for effective modeling. Resolution limits of imaging methods such as travel time inversion, acoustic and elastic waveform inversion were investigated with numerical models. it was investigated that Resolution limit of waveform inversion was similar tn resolution limit of migration derived by Schuster. Horizontal resolution limit could be improved with increased coverage by adding VSP data in cross hole that had insufficient coverage. Also, waveform inversion was applied to realistic models to evaluate applicability and using initial guess of travel time tomograms to reduce non-linearity of waveform inversion showed that the better reconstructed image could be acquired.

• Geophysics and Geophysical Exploration
• /
• v.10 no.4
• /
• pp.314-321
• /
• 2007

Recently, multi-dimensional joint inversion of geophysical data based on fundamentally different physical properties is being actively studied. Joint inversion can provide a way to obtaining much more accurate image of the subsurface structure. Through the joint inversion, furthermore, it is possible to directly estimate non-geophysical material properties from geophysical measurements. In this study, we developed a new algorithm for jointly inverting dc resistivity and seismic traveltime data based on the multiple constraints: (1) structural similarity based on cross-gradient, (2) correlation between two different material properties, and (3) a priori information on the material property distribution. Through the numerical experiments of surface dc resistivity and seismic refraction surveys, the performance of the proposed algorithm was demonstrated and the effects of different regularizations were analyzed. In particular, we showed that the hidden layer problem in the seismic refraction method due to an inter-bedded low velocity layer can be solved by the joint inversion when appropriate constraints are applied.