• Geophysics and Geophysical Exploration
• /
• v.8 no.1
• /
• pp.59-66
• /
• 2005

The main purpose of this paper is to describe a highly efficient common mid-point (CMP) data acquisition method for ground-penetrating radar (GPR) surveying, which is intended to widen the application of GPR. The most important innovation to increase the efficiency of CMP data acquisition is continuous monitoring of the GPR antenna positions, using a real-time kinematic Global Positioning System (RTK-GPS). Survey time efficiency is improved because the automatic antenna locating system that we propose frees us from the most time-consuming process-deployment of the antenna at specified positions. Numerical experiments predicted that the data density and the CMP fold would be increased by the increased efficiency of data acquisition, which results in improved signal-to-noise ratios in the resulting data. A field experiment confirmed this hypothesis. The proposed method makes GPR surveys using CMP method more practical and popular. Furthermore, the method has the potential to supply detailed groundwater information. This is because we can convert the spatially dense dielectric constant distribution, obtained by using the CMP method we describe, into a dense physical value distribution that is closely related to such groundwater properties as water saturation.

• Geophysics and Geophysical Exploration
• /
• v.6 no.2
• /
• pp.64-70
• /
• 2003

A new approach of estimating the coefficient of permeability (COP) from resistivity has been developed, which can provide another good application tool of geophysical methods to geo-technical field. Borehole electrical logging and Lugeon test results in Yangsan area showed that resistivity is inversely proportional to the COP. For granite and andesite in Yansan area, the relation between the resistivity ($\rho$) and the COP (k) revealed that, $log(k){\approx}-0.85621\;log({\rho})+0.0031$. Derived relation is applied to AMT data acquired from a survey line along the tunnel. Two-dimensional resistivity distribution from AMT data was converted to two-dimensional COP section. The final COP section can be used as good input data for groundwater modeling.

• Geophysics and Geophysical Exploration
• /
• v.7 no.1
• /
• pp.62-69
• /
• 2004

The objective of this study is to estimate the distribution of a zone disturbed by excavation (EDZ) around tunnels that have been excavated at about 500 m depth in pre-Tertiary hard sedimentary rock. One of the most important tasks is to evaluate changes in the dynamic stability and permeability of the rock around the tunnels, by investigating the properties of the rock after the excavation. We performed resistivity and acoustic tomography using two boreholes, 5 m in length, drilled horizontally from the wall of a tunnel in pre-Tertiary hard conglomerate. By these methods, we detected a low-resistivity and low-velocity zone 1 m in thickness around the wall of the tunnel. The resulting profiles were verified by permeability and evaporation tests performed at the same boreholes. This anomalous zone matched a high-permeability zone caused by open fractures. Next, we performed resistivity monitoring along annular survey lines in a tunnel excavated in pre-Tertiary hard shale by a tunnel-boring machine (TBM). We detected anomalous zones in 2D resistivity profiles surrounding the tunnel. A low-resistivity zone 1 m in thickness was detected around the tunnel when one year had passed after the excavation. However, two years later, the resistivity around the tunnel had increased in a portion, about 30 cm in thickness, of this zone. To investigate this change, we studied the relationship between groundwater flow from the surroundings and evaporation from the wall around the tunnel. These features were verified by the relationship between the resistivity and porosity of rocks obtained by laboratory tests on core samples. Furthermore, the profiles matched well with highly permeable zones detected by permeability and evaporation tests at a horizontal borehole drilled near the survey line. We conclude that the anomalous zones in these profiles indicate the EDZ around the tunnel.

• Geophysics and Geophysical Exploration
• /
• v.7 no.1
• /
• pp.80-87
• /
• 2004

During excavation work for the construction of a highway bridge in a limestone area in Korea, several cavities were found, and construction work was stopped temporarily. Cavities under the bridge piers might seriously threaten the safety of the planned bridge, because they could lead to excessive subsidence and differential settlement of the pier foundations. In order to establish a method for reinforcement of the pier foundations, borehole radar reflection and tomography surveys were carried out, to locate cavities under the planned pier locations and to determine their sizes where they exist. Since travel time data from the crosshole radar survey showed anisotropy, we applied an anisotropic tomography inversion algorithm assuming heterogeneous elliptic anisotropy, in order to reconstruct three kinds of tomograms: tomograms of maximum and minimum velocities, and of the direction of the symmetry axis. The distribution of maximum velocity matched core logging results better than that of the minimum velocity. The degree of anisotropy, defined by the normalized difference between maximum and minimum velocities, was helpful in deciding whether an anomalous zone in a tomogram was a cavity or not. By careful examination of borehole radar reflection and tomography images, the spatial distributions of cavities were delineated, and most of them were interpreted as being filled with clay and/or water. All the interpretation results implied that two faults imaged clearly by a DC resistivity survey were among the most important factors controlling the groundwater movement in the survey area, and therefore were closely related to the development of cavities. The method of reinforcement of the pier foundations was based on the interpretation results, and the results were confirmed when construction work was resumed.

• Geophysics and Geophysical Exploration
• /
• v.12 no.1
• /
• pp.27-32
• /
• 2009

In this study, we examined the applicability of a 3D electrical resistivity technique for the probing of underground cavities at a field test site in a karst area in Yongweol-ri, Muan-gun, in the south-western part of the Korean peninsula. At the test site, where the ground has subsided in the past, underground cavities are commonly found in the limestone bedrock, which is overlain with alluvial deposits. The limestone cavities at the test site are mostly filled with groundwater and clay; hence, they show levels of electrical resistivity that are significantly lower than those of the surrounding host bedrock. The results of this study demonstrate that the zones of low resistivity correspond to the zones of the cavities identified in the boreholes at the site, and that our 3D electrical resistivity survey is a very effective tool for detecting and mapping underground cavities in a karst area.

• Geophysics and Geophysical Exploration
• /
• v.12 no.1
• /
• pp.8-16
• /
• 2009

The holistic inversion approach for frequency domain airborne electromagnetic data has previously been employed to simultaneously calibrate, process and invert raw frequency-domain data where prior information was available. Analternative formulation has been developed, which is suitable in the case where explicit prior information is not available. It incorporates: a multi-layer vertically-smooth conductivity model; a simplified bias parameterisation; horizontal smoothing with respect to elevation; and cluster computer parallelisation. Without using any prior data, an inversion of 8.0 million data for 3.4 million parameters yields results that are consistent with independently derived calibration parameters, downhole logs and groundwater elevation data. We conclude that the success of the holistic inversion method is not dependent on a sophisticated conceptual model or the direct inclusion of survey-area specific prior information. In addition, acquisition costs could potentially be reduced by employing the holistic approach which largely eliminates the need for high altitude zero-level measurements.

• Geophysics and Geophysical Exploration
• /
• v.9 no.3
• /
• pp.193-206
• /
• 2006

Among various borehole seismic testing techniques for determining body wave velocity, crosshole seismic method has been known as one of the most suitable technique for evaluating reliably geotechnical dynamic properties. In this study, to perform successfully the crosshole seismic test for rock as well as soil layers regardless of the groundwater level, multi-purposed spring-loaded source which impact horizontally a subsurface ground in vertical borehole was developed and applied at major facility sites in Korea. The geotechnical dynamic properties were evaluated by determining efficiently the body wave velocities such as shear wave velocity and compressional wave velocity from the horizontally impacted crosshole seismic tests at study sites, and were provided as the fundamental parameters for the seismic performance evaluation and seismic design of the target facilities.

• Geophysics and Geophysical Exploration
• /
• v.3 no.3
• /
• pp.88-93
• /
• 2000

The present study may be summarized as follows. By means of a pole-pole electrical survey, major geological features, i.e, fresh rocks, fractured & weak & saturated rocks, fresh water bearing aquifer, were successfully delineated in the surveyed area of a granite region in Korea. The subsequent Schlumberger sounding and drilling confirmed the existence of the acquifer at 60 m depth as expected. But one more minor acquifer which does not show up in the resistivity depth section was met at 100 m depth. A simple forward modelling leads the authors to believe that any other electrode configuration, e.g., Wenner, Schlumberger, dipole-dipole, pole-dipole, would not detect the deeper aquifer. Under these circumstances, further studies remain to be done in connection with the spatial resolution in the vertical direction.

• Geophysics and Geophysical Exploration
• /
• v.8 no.3
• /
• pp.207-217
• /
• 2005

We present a new approximate formulation of the integral equation (IE) method for models with variable background conductivity. This method overcomes the standard limitation of the conventional If method related to the use of a horizontally layered background only. The new approximate IE method still employs the Green's functions for a horizontally layered 1-D model. However, the new method allows us to use an inhomogeneous background with the IE method. The method was carefully tested for modeling the EM field for complex structures with a known variable background conductivity. It can find wide application in modeling EM data for multiple geological models with some common geoelectrical features, like a known inhomogeneous overburden, or salt dome structures.

• Journal of the Korean Geophysical Society
• /
• v.5 no.1
• /
• pp.29-39
• /
• 2002

Geophysical data sets from the Chojeong area in the Chungbok-Do are compositely studied in terms of multi-attribute interpretations for the subsurface mappings of shallow fracture zones, associated with groundwater reservoir. Utilizing a GIS software, the attribute data were implemented to a database; a lineament from the satellite image, electrical resistivities and its standard deviation, radioactivity, seismic velocity, and bedrock depth. In an attempt to interpret 1-D electrical sounding data in 3-D views, 1-D data are firstly performed horizontal and vertical inter- and extrapolation. Reconstruction of a resistivity volume is found to be an effective scheme for subsurface mapping of shallow fracture zones. Shallow fracture zones are located in the southeastern part of the study area, which are commonly correlated with the various exploration data.