Soon Jee Seol

Application of Geophysical Methods to the Safety Analysis of an Earth Dam

A series of events occurred at an earth-filled dam which raised a serious question about the safety of the dam. An integrated geophysical survey, including dc resistivity, controlled source magnetotelluric (CSMT), ground penetrating radar (GPR), and seismic refraction and tomography methods, has been conducted on the dam. The purpose of the survey was to provide basic data through imaging the internal structure of the embankment for a precise safety and stability examination of the dam. One of the essential reasons applying various kinds of geophysical methods is to view the physical properties of the internal structure of the dam and to compare the results each other. The second reason is to accomplish two survey objectives in the safety examination of the dam: imaging of the internal condition of the dam and investigation of the basement structures beneath the dam and its vicinity. All the applied geophysical investigation showed their own characteristic responses to anomalous zones. The zones delineate...

Elastic modelling in tilted transversely isotropic media with convolutional perfectly matched layer boundary conditions

To simulate wave propagation in a tilted transversely isotropic (TTI) medium with a tilting symmetry-axis of anisotropy, we develop a 2D elastic forward modelling algorithm. In this algorithm, we use the staggered-grid finite-difference method which has fourth-order accuracy in space and second-order accuracy in time. Since velocity-stress formulations are defined for staggered grids, we include auxiliary grid points in the z-direction to meet the free surface boundary conditions for shear stress. Through comparisons of displacements obtained from our algorithm, not only with analytical solutions but also with finite element solutions, we are able to validate that the free surface conditions operate appropriately and elastic waves propagate correctly. In order to handle the artificial boundary reflections efficiently, we also implement convolutional perfectly matched layer (CPML) absorbing boundaries in our algorithm. The CPML sufficiently attenuates energy at the grazing incidence by modifying the damping profile of the PML boundary. Numerical experiments indicate that the algorithm accurately expresses elastic wave propagation in the TTI medium. At the free surface, the numerical results show good agreement with analytical solutions not only for body waves but also for the Rayleigh wave which has strong amplitude along the surface. In addition, we demonstrate the efficiency of CPML for a homogeneous TI medium and a dipping layered model. Only using 10 grid points to the CPML regions, the artificial reflections are successfully suppressed and the energy of the boundary reflection back into the effective modelling area is significantly decayed. In this study, we developed a forward modelling algorithm with the staggered-grid finite-difference method. The elastic wave propagation in 2D tilted transversely isotropic media is well simulated using this algorithm. We also implement convolutional perfectly matched layer absorbing boundaries in our algorithm in order to handle the artificial boundary reflection efficiently.

Crosswell monitoring using virtual sources and horizontal wells

Time-lapse crosswell seismic provides an efficient way to monitor the migration of a C O2 plume or its leakage after C O2 injection into a geologic formation. Recently, crosswell seismic has become a powerful tool for monitoring underground variations, using the concept of a virtual source, with virtual sources positioned at the receivers installed in the well and thus the positions of sources and receivers can be invariant during monitoring. However, time-lapse crosswell seismic using vertical wells and virtual sources has difficulty in describing the front of a C O2 plume, which usually is parallel to the vertical wells, and in obtaining sufficient ray coverage for the first-arrival tomography. These problems arise because of the theoretical downward-illumination-directivity limitation of the virtual source. We have developed an effective monitoring method that uses virtual sources and two horizontal wells: one above and one below the C O2 sequestration reservoir. In our method, we redatum the traces th...

A Radar Survey at a Granite Quarry to Delineate Fractures and Estimate Fracture Density

Borehole radar, both reflection and tomography methods, and GPR surveys were conducted at a granite quarry to delineate rock inhomogeneities, including major fractures, and to estimate the fracture density. The borehole reflection survey used a directional antenna to get the spatial orientations of the reflectors. The center frequency was 20MHz for the borehole radar reflection and tomography surveys and 100MHz for GPR. Through the interpretation of borehole reflection data using dipole and directional antennas, as well as surface GPR images, we could determine the orientation of the major fractures in three dimensions. Portions of the tomography travel time curves exhibited velocity anisotropy, which is uncommon in granite. Comparing the tomography data with Televiewer images showed that the anisotropy effect in this area is closely related to the alignment of fine fissures. From the borehole radar, tomography, and GPR images as well as the distribution of anisotropy, we conclude that the area bounded by...

mCSEM inversion for CO2 sequestration monitoring at a deep brine aquifer in a shallow sea

Carbon dioxide injection monitoring in offshore environments is a promising future application of the marine controlled-source electromagnetic (mCSEM) method. To investigate whether the mCSEM method can be used to quantitatively monitor variations in the distribution of the injected CO2, we developed a mCSEM inversion scheme and conducted numerical analyses. Furthermore, to demonstrate the monitoring capability of the mCSEM method in challenging environments, we used a deep brine aquifer model in shallow sea as an injection target. The mCSEM responses of the injected CO2 in the deep brine aquifer were severely decayed and heavily masked by the air wave due to the proximity of the free space. Therefore, the accurate computation of small mCSEM responses due to the injected CO2 and the proper incorporation into the inversion process are critically important for the mCSEM method to be successful. Additionally, in monitoring situations, some useful a priori information is usually available (e.g. well logs and seismic sections), and the proper implementation of this to our inversion framework is crucial to ensure reliable estimation of the distribution of the injected CO2 plume. In this study, we developed an efficient 2.5D mCSEM inversion algorithm based on an accurate forward modelling algorithm and the judicious incorporation of a priori information into our inversion scheme. The inversion scheme was tested with simplified and realistic CO2 injection models and successfully recovered the resistivity distributions of the injected CO2, although it still required the presence of a considerable amount of the injected CO2. Based on these inversion experiments, we demonstrated that the mCSEM method is capable of quantitatively monitoring variations in the distribution of injected CO2 in offshore environments. We developed an efficient 2.5D mCSEM inversion algorithm based on an accurate forward modelling algorithm and the judicious incorporation of a priori information into our inversion scheme. We demonstrated the successful recovery of resistivity distributions of the injected CO2 from the deep brine aquifer model in the shallow sea.

Automatic velocity analysis using bootstrapped differential semblance and global search methods

The goal of automatic velocity analysis is to extract accurate velocity from voluminous seismic data with efficiency. In this study, we developed an efficient automatic velocity analysis algorithm by using bootstrapped differential semblance (BDS) and Monte Carlo inversion. To estimate more accurate results from automatic velocity analysis, the algorithm we have developed uses BDS, which provides a higher velocity resolution than conventional semblance, as a coherency estimator. In addition, our proposed automatic velocity analysis module is performed with a conditional initial velocity determination step that leads to enhanced efficiency in running time of the module. A new optional root mean square (RMS) velocity constraint, which prevents picking false peaks, is used. The developed automatic velocity analysis module was tested on a synthetic dataset and a marine field dataset from the East Sea, Korea. The stacked sections made using velocity results from our algorithm showed coherent events and improved the quality of the normal moveout-correction result. Moreover, since our algorithm finds interval velocity (vint) first with interval velocity constraints and then calculates a RMS velocity function from the interval velocity, we can estimate geologically reasonable interval velocities. Boundaries of interval velocities also match well with reflection events in the common midpoint stacked sections.

Joint Electromagnetic Inversion with Structure Constraints Using Full-waveform Inversion Result

Compared with the separated inversion of electromagnetic (EM) and seismic data, a joint inversion using both EM and seismic data reduces the uncertainty and gives the opportunity to use the advantage of each data. Seismic fullwaveform inversion allows velocity information with high resolution in complicated subsurface. However, it is an indirect survey which finds the structure containing oil and gas. On the other hand, marine controlled-source EM (mCSEM) inversion can directly indicate the oil and gas using different EM properties of hydrocarbon with marine sediments and cap rocks whereas it has poor resolution than seismic method. In this paper, we have developed a joint EM inversion algorithm using a cross-gradient technique. P-wave velocity structure obtained by full-waveform inversion using plane wave encoding is used as structure constraints to calculate the cross-gradient term in the joint inversion. When the jointinversion algorithm is applied to the synthetic data which are simulated for subsea reservoir exploration, images have been significantly improved over those obtained from separate EM inversion. The results indicate that the developed joint inversion scheme can be applied for detecting reservoir and calculating the accurate oil and gas reserves.

Imaging tilted transversely isotropic media with a generalised screen propagator

One-way wave equation migration is computationally efficient compared with reverse time migration, and it provides a better subsurface image than ray-based migration algorithms when imaging complex structures. Among many one-way wave-based migration algorithms, we adopted the generalised screen propagator (GSP) to build the migration algorithm. When the wavefield propagates through the large velocity variation in lateral or steeply dipping structures, GSP increases the accuracy of the wavefield in wide angle by adopting higher-order terms induced from expansion of the vertical slowness in Taylor series with each perturbation term. To apply the migration algorithm to a more realistic geological structure, we considered tilted transversely isotropic (TTI) media. The new GSP, which contains the tilting angle as a symmetric axis of the anisotropic media, was derived by modifying the GSP designed for vertical transversely isotropic (VTI) media. To verify the developed TTI-GSP, we analysed the accuracy of wave propagation, especially for the new perturbation parameters and the tilting angle; the results clearly showed that the perturbation term of the tilting angle in TTI media has considerable effects on proper propagation. In addition, through numerical tests, we demonstrated that the developed TTI-GS migration algorithm could successfully image a steeply dipping salt flank with high velocity variation around anisotropic layers. We expand the vertical transversely isotropic generalised screen propagator into a tilted transversely isotropic generalised screen propagator (TTI-GSP) by adopting tilted coordinates. The validity of the developed algorithm has been tested by increasing the accuracy of wide-angle propagation in TTI-GSP with high-order expansion of the velocity perturbation.

Computation of Apparent Resistivity from Marine Controlled-source Electromagnetic Data for Identifying the Geometric Distribution of Gas Hydrate

요약: 해양 전자탐사의 겉보기 비저항은 해수층으로 인해 지표탐사와 그 정의가 달라지게 되며, 이를 적절히 계산할 수있는 알고리듬의 개발은 해양 전자탐사의 출발점이 될 수 있다. 이를 위해, 1차원 층서 가스 하이드레이트 수치모형과 해수층과 그 하부의 반 무한매질로 이루어진 수치모형에서 계산한 전자기적 반응을 비교분석하였다. 겉보기 비저항을 계산하기 위해서는 실수와 허수 성분 보다는 진폭과 위상을 사용하는 것이 더 적합하였으며 해양 전자탐사 반응의 민감도를정량적으로 분석하여, 근거리 영역에서는 위상이 원거리 영역에서는 진폭 성분이 더 안정적인 결과를 주는 것을 알았다.또한 위상과 진폭의 선택기준으로써 유도상수의 값을 제안하였다. 이러한 분석을 토대로 격자 탐색법(grid search)을 사용하여 겉보기 비저항을 계산하는 수치알고리듬을 개발하였다. 개발된 알고리듬을 이용하여 1차원 층서 가스 하이드레이트수치모형의 다양한 변수를 변화시켜가며 겉보기 비저항을 계산해봄으로써 알고리듬의 타당성을 검증하였다. 마지막으로,계산한 겉보기 비저항 값을 이용한 가스 하이드레이트 부존양상 정보의 도출가능성을 살펴보았다. 동해 울릉분지의 가스하이드레이트 부존양상을 모사한 2차원 가스 하이드레이트 수치모형에서 계산된 자료의 겉치레 단면도는 가스 하이드레이트 부존양상 정보 추출이 가능함을 보여주었다. 주요어: 해양 전자탐사, 겉보기 비저항, 가스 하이드레이트, 부존양상Abstract: The sea layer in marine Controlled-Source Electromagnetic (mCSEM) survey changes the conventionaldefinition of apparent resistivity which is used in the land CSEM survey. Thus, the development of a new algorithm,which computes apparent resistivity for mCSEM survey, can be an initiative of mCSEM data interpretation. First, wecompared and analyzed electromagnetic responses of the 1D stratified gas hydrate model and the half-space model belowthe sea layer. Amplitude and phase components showed proper results for computing apparent resistivity than real andimaginary components. Next, the amplitude component is more sensitive to the subsurface resistivity than the phasecomponent in far offset range and vice versa. We suggested the induction number as a selection criteria of amplitudeor phase component to calculate apparent resistivity. Based on our study, we have developed a numerical algorithm, whichcomputes appropriate apparent resistivity corresponding to measured mCSEM data using grid search method. In addition,we verified the validity of the developed algorithm by applying it to the stratified gas hydrate models with various modelparameters. Finally, by constructing apparent resistivity pseudo-section from the mCSEM responses with 2D numericalmodels simulating gas hydrate deposits in the Ulleung Basin, we confirmed that the apparent resistivity can provide theinformation on the geometric distribution of the gas hydrate deposit.Keywords: mCSEM, apparent resistivity, gas hydrate, geometrical distribution

The potential of fracture imaging using high-frequency, single-hole electromagnetic data

This paper presents an inversion scheme for high‐frequency electromagnetic (EM) data from a single borehole for detection and characterization of fluid‐filled fractures. Water in the fracture zone may be characterized by its high electrical permittivity and, if saline, by high electrical conductivity. High electrical conductivity results in increased attenuation of EM fields, whereas high electrical permittivity reduces the phase velocity of propagating EM fields. Taking advantage of these effects, we use high‐frequency EM fields to detect and characterize fluid‐filled fractures. To demonstrate the feasibility of single‐hole EM imaging, we develop a three‐step inversion scheme to map a fluid‐filled fracture near the borehole and to evaluate its electrical conductivity and permittivity.We assume that a fluid‐filled fracture can be simulated by a conductive thin sheet. To test our inversion scheme, we generated synthetic data using the thin‐sheet integral equation method. A vertical magnetic dipole was used...