• Geophysics and Geophysical Exploration
• /
• v.10 no.4
• /
• pp.268-274
• /
• 2007

Traditional two-dimensional (2D) interpretation of magnetotelluric (MT) data utilizes only transverse magnetic (TM)-mode data, because 2D inversion of transverse electric (TE)-mode data results in spurious features when 3D structures exist in the subsurface. The application of a 3D inversion algorithm to a single MT profile can reduce contamination due to off-profile anomalies and help us to incorporate TE-mode data in the interpretation. In this study, we conduct 2D and 3D inversions of MT data observed along two lines in Jeju Island. First, we invert apparent resistivities and phases in the TM and TE modes separately. Then, we perform 2D joint inversion of both TM- and TE-mode data and 3D inversion of both Zxy- and Zyx-mode data corresponding to TE- and TM-mode data in 2D. The resistivity images derived from all four data show that the geoelectrical structure in Jeju Island is a three-layered earth with the resistive-conductive-resistive stratigraphy within a depth of 5 km. The 3D inversion does not produce clear anomalies in the reconstructed profile image, while all of 2D do. This attributed to the possibility that 2D inversion results are distorted by exiting off-profile 3D anomalies in Jeju. With 3D inversion of 2D profile MT data, we can deduce more reliable results that are not seriously distorted by off-profile 3D anomalies.

• Geophysics and Geophysical Exploration
• /
• v.4 no.2
• /
• pp.25-33
• /
• 2001

Loop-loop electromagnetic (EM) survey in frequency domain has been carried out in order to provide basic solution to geotechnical applications. Source and receiver configuration may be horizontal co-planar (HCP) and/or vertical co-planar (VCP). Three quadrature components of mutual impedance ratio for each configuration are used to construct the subsurface image. For the purpose of obtaining the model response and validating the reasonable performance of the inversion, we obtained each responses of two-layered and three-layered earth models and two-dimensional (2-D) isolated anomalous body. The response of 2-D isolated anomalous body has been calculated using extended Born approximation for the solution of 2.5-D integral equation describing EM scattering problem. As a result of the least-squares inversion with variable Lagrangian multiplier, we could construct more resolvable image from HCP data than VCP data. Furthermore, joint inversion of HCP and VCP data made better stability and resolution of the inversion. Resistivity values, however, did not exactly match the true ones. Loop-loop EM field data was obtained with EM34-3XL system manufactured by Geonics Ltd. (Canada). Electrical resistivity survey was conducted on the same line for the comparison in advance. Since the constructed image from loop-loop EM data by 2-D inversion algorithm showed almost similar resistivity distribution to that from electrical resistivity one, we expect the developed 2.5-D loop-loop EM inversion program can be applied for the reconnaissance site survey.

• Geophysics and Geophysical Exploration
• /
• v.18 no.1
• /
• pp.14-20
• /
• 2015

Three-dimensional (3D) resistivity method is an effective tool in the engineering site survey because it can provide a 3D resistivity distribution of the site. In this study, we tried to find out faults, fractures and coal seams that can cause the collapse of the tunnel. We carried out 2D resistivity survey along 5 parallel lines and 11 cross lines and merged all the apparent resistivity data for 3D inversion. Finally, from the 3D resistivity image and drilling data we presented the 3D distribution of faults, fractures and coal seams that are considered the main cause of the tunnel collapse.

• Geophysics and Geophysical Exploration
• /
• v.10 no.2
• /
• pp.138-146
• /
• 2007

This paper describes 2.5D induced polarization (IP) modeling and inversion algorithms using complex resistivity. The complex resistivity method has merits for acquiring more valuable information about hydraulic parameters and pore fluid than the conventional IP methods. The IP modeling and inversion algorithms are developed by allowing complex arithmetic in existing DC modeling and inversion algorithms. The IP modeling and inversion algorithms use a 2.5D DC finite-element algorithm and a damped least-squares method with smoothness constraints, respectively. The accuracy of the IP modeling algorithm is verified by comparing its responses of two synthetic models with two different approaches: linear filtering for a three-layer model and an integral equation method for a 3D model. Results from these methods are well matched to each other. The inversion algorithm is validated by a synthetic example which has two anomalous bodies, one is more conductive but non-polarizable than the background, and the other is polarizable but has the same resistivity as the background. From the inverted section, we can cleary identify each anomalous body with different locations. Furthermore, in order to verify its efficiency to the real filed example, we apply the inversion algorithm to another three-layer model which includes phase anomaly in the second layer.

• Geophysics and Geophysical Exploration
• /
• v.5 no.4
• /
• pp.291-298
• /
• 2002

We have developed a two-dimensional (2-D) magnetotelluric (MT) inversion algorithm, which can include topographic effects in inversion. We use the finite element method (FEM) to incorporate topography into forward calculation. Topography is implemented simply by moving nodes of rectangular elements in z-direction according to the elevation of air-earth interface. In the inversion process, we adopt a spatially variable Lagrangian multiplier algorithm in the smoothness-constrained least-squares inversion. The inversion algorithm developed in this study reconstructs subsurface resistivity structure quite well when topography variation exists. Also, it turns out to be effective in both resolution and stability from a model study and field data application.

• Geophysics and Geophysical Exploration
• /
• v.11 no.1
• /
• pp.68-77
• /
• 2008

We have developed an inversion algorithm for loop-loop electromagnetic (EM) data, based on the localised non-linear or extended Born approximation to the solution of the 2.5D integral equation describing an EM scattering problem. Source and receiver configuration may be horizontal co-planar (HCP) or vertical co-planar (VCP). Both multi-frequency and multi-separation data can be incorporated. Our inversion code runs on a PC platform without heavy computational load. For the sake of stable and high-resolution performance of the inversion, we implemented an algorithm determining an optimum spatially varying Lagrangian multiplier as a function of sensitivity distribution, through parameter resolution matrix and Backus-Gilbert spread function analysis. Considering that the different source-receiver orientation characteristics cause inconsistent sensitivities to the resistivity structure in simultaneous inversion of HCP and VCP data, which affects the stability and resolution of the inversion result, we adapted a weighting scheme based on the variances of misfits between the measured and calculated datasets. The accuracy of the modelling code that we have developed has been proven over the frequency, conductivity, and geometric ranges typically used in a loop-loop EM system through comparison with 2.5D finite-element modelling results. We first applied the inversion to synthetic data, from a model with resistive as well as conductive inhomogeneities embedded in a homogeneous half-space, to validate its performance. Applying the inversion to field data and comparing the result with that of dc resistivity data, we conclude that the newly developed algorithm provides a reasonable image of the subsurface.

• Proceedings of the KSEEG Conference
• /
• 2001.04a
• /
• pp.53-55
• /
• 2001

• Geophysics and Geophysical Exploration
• /
• v.2 no.1
• /
• pp.43-53
• /
• 1999

Numerical modeling and inversion for electromagnetic exploration methods are essential to understand behaviour of electromagnetic fields in complex subsurface. In this study, a finite element method was adopted as a numerical scheme for the 2.5-dimensional forward problem. And a finite element equation considering linear conductivity variation was proposed, when 2.5-dimensional differential equation to couple eletric and magnetic field was implemented. Model parameters were investigated for near-field with large source effects and far-field with responses dominantly by homogeneous half-space. Numerical responses by this study were compared with analytic solutions in homogeneous half-space. Blocky inversion model was modified to be applied to the forward calculation in this study and it was also adopted in the inversion algorithm. Resolution for isolated bodies were investigated to confirm possibility and limitation of inversion for electromagnetic exploration data.

• 한국신재생에너지학회:학술대회논문집
• /
• 2010.11a
• /
• pp.121.2-121.2
• /
• 2010

제주도는 지질학적으로 제4기에 형성된 화산섬으로 지금까지 고온의 지열징후는 보고된 바 없으나, 남한에서 가장 최근까지 화산활동이 있었던 것으로 기록되어 있어 화산활동과 관련된 심부 지열자원 부존 가능성은 아직 열려있다고 할 수 있다. 본 연구에서는 제주도에서 지열부존 가능성을 타진하고 제주도 심부 지질구조 파악을 목적으로 2차원 및 3차원 자기지전류 (MT) 탐사를 수행하였다. 탐사는 중산간지역에서 한라산을 중심으로 동, 서, 남, 북의 4방향 4측선과 제주 서부지역에 남북방향의 1측선을의 총 5개 측선에 대해 수행하였으며, 이에 대한 MT 탐사자료의 2차원 및 3차원 역산을 통하여 한라산 하부 및 주변의 심부 지질구조를 파악하고자 하였다. 역산 해석 결과는 천부 구조는 기존 시추조사 결과 밝혀진 층서구조의 형태를 잘 나타내어 획득된 자료의 신뢰도가 높음을 지시하였다. 즉, 제주도 최 상부를 피복하고 있는 현무암 등의 화산암류는 고비저항(수백 ohm-m)으로, 그 하부의 해성 미고결퇴적층(U층 및 서귀포층)은 저비저항으로, 그리고 최하부의 응회암이나 화강암으로 구성된 기반암은 1,000 ohm-m 이상의 고비저항 층으로 잘 구분되어 나타났다. 특히, 제주도에서 특징적으로 해수면 하부 수십 ~ 수백 m에 존재하는 것으로 알려진 미고결퇴적층이 10 ohm-m 내외로 측선 전반에 걸쳐 나타났다. 이는 기존의 시추결과에서 미고결 퇴적층이 제주도 전역에 걸쳐 해수면 하부 100 m 내외의 심도에서 관찰되는 것과 일치하는 결과이다. 기반암 하부에서는 특징적으로 모든 측선의 중앙부에서 저비저항 이상대가 영상화되었으며 이는 2차원 역산과 3차원 역산해석에서 공통적으로 나타났다. 특히, 3차원 해석에서는 이러한 저비저항 이상대가 한라산 정상에서 서북쪽 부근에 나타나는데 이는 과거의 화산활동과 관련된 지질학적인 구조에 의한 영향일 가능성과 측선의 양단과 중앙에서 주변 바다의 영향이 다르게 나타나기 때문일 가능성으로 볼 수 있다. 즉, 전자는 심부에 발단된 각각의 파쇄대가 모든 측선의 중앙부에서 교차하거나 이를 통한 한라산의 생성과정과 연관된 지질학적인 구조일 가능성을 의미한다. 만약 한라산을 형성한 화성활동의 영향이 아직 지하 심부에 남아있다면 지열수의 부존 혹은 마그마의 통로가 되었을 단층의 영향으로 한라산 하부에 저비저항 이상대로 나타날 가능성이 높다. 그러나 후자에 의한 가능성도 배제할 수는 없으므로 향후 주변바다에 대한 영향을 고려한 3차원 역산해석이나 심부시추 등을 통한 상세한 지질조사 등 추가적인 연구가 이루어져야 할 것으로 판단된다.

• Geophysics and Geophysical Exploration
• /
• v.12 no.1
• /
• pp.99-104
• /
• 2009

We propose a waveform inversion method for SH-wave data obtained in a shallow seismic refraction survey, to determine a 2D inhomogeneous S-wave profile of shallow soils. In this method, a 2.5D equation is used to simulate SH-wave propagation in 2D media. The equation is solved with the staggered grid finite-difference approximation to the 4th-order in space and 2nd-order in time, to compute a synthetic wave. The misfit, defined using differences between calculated and observed waveforms, is minimised with a hybrid heuristic search method. We parameterise a 2D subsurface structural model with blocks with different depth boundaries, and S-wave velocities in each block. Numerical experiments were conducted using synthetic SH-wave data with white noise for a model having a blind layer and irregular interfaces. We could reconstruct a structure including a blind layer with reasonable computation time from surface seismic refraction data.