• Journal of the Korean earth science society
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• v.32 no.6
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• pp.654-664
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• 2011

A field campaign of magnetotelluric (MT) and audio-frequency MT (AMT) survey was done at 36 measurement points as a complementary for the previous 44 MT measurements completed during the period of 2005-2006. The purpose of additional MT survey is to investigate the possible fracture system in Seokmo Island, which is conceived to be crucial in accumulation and migration of geothermal hot spring in this area. We have done 2D and 3D inversions of overall MT and AMT data distributed on a grid to interpret subsurface of extended area. The inversion results reveal that at least two major faults are imaged in the inversion results, one of which is in NNE-SWW with steep dip, and another is in E-W direction.

• 한국지구물리탐사학회:학술대회논문집
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• 2005.05a
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• pp.21-26
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• 2005

Two-dimensional (2-D) inversion of magnetotelluric (MT) data for two survey lines having south-north direction from Jeiu Island has been carried out. The 2-D models show a thick layer having around 10 ohm-m in the depth of a few hundred meters throughout the survey area, which can be considered as the unconsolidated sedimentary layer. And they also show a conductive anomaly at the central part of each survey lines. But unfortunately by now, we do not have any further information about the anomaly. Comparison of the 2-D inversion model using MT band only and that using both AMT and MT bands said that it is helpful for us to include AMT band as well as MT band in the inversion to interpret not only the shallow part but also the deep structures.

• Geophysics and Geophysical Exploration
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• v.17 no.2
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• pp.95-103
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• 2014

This paper presents a practical inversion method for recovering a three-dimensional (3D) resistivity model and static shifts simultaneously. Although this method is based on a Gauss-Newton approach that requires a sensitivity matrix, the computer time can be greatly reduced by implementing a simple and effective procedure for updating the sensitivity matrix using the Broyden's algorithm. In this research, we examine the approximate inversion procedure and the weighting factor ${\beta}$ for static shifts through inversion experiments using synthetic MT data. In methods using the full sensitivity matrix constructed only once in the iteration process, a procedure using the full sensitivity in the earlier stage is useful to produce the smallest rms data misfit. The choice of ${\beta}$ is not critical below some threshold value. Synthetic examples demonstrate that the method proposed in this paper is effective in reconstructing a 3D resistivity structure from static-shifted MT data.

• Geophysics and Geophysical Exploration
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• v.10 no.4
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• pp.268-274
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• 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
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• v.9 no.4
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• pp.261-270
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• 2006

Two-dimensional (2-D) inversion of magnetotelluric (MT) data for two survey lines having south-north direction from Jeju Island has been carried out. Broad band MT sounding curves with good quality could be gathered by performing audio-frequency magnetotelluric (AMT) survey during the MT survey and by operating the remote reference in Kyushu Island, Japan. Comparison of the 2-D inversion model using MT band only and that using both AMT and MT bands for the field data as well as for the data from numerical 2-D modeling said that high frequency information from AMT survey can be useful for interpreting not only the shallow part but also the deep structures, especially when the formation is resistive. The 2-D inversion models of field data show a thick layer having around 10 ohm-m in the depth of a few hundred meters throughout the survey area, which can be considered as the unconsolidated sedimentary layer. And they also show a conductive anomaly at the central part of each survey lines. It can be either the effect of the surrounding sea water, or the structures due to ancient volcanic events. But unfortunately by now, we do not have any further information about the anomaly.

• Geophysics and Geophysical Exploration
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• v.22 no.2
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• pp.62-71
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• 2019

We conducted DC resistivity and MT survey to obtain the resistivity structure of the central Myanmar basin. We tried to analyze the underground structure through the resistivity variation of Myanmar by performing representative geophysical survey methods because researches on the electrical resistivity structure are insufficient in Myanmar. The electrical resistivity is expected to be low considering the marine sedimentary rocks composed of shale and sandstone in this area. The DC resistivity and MT survey were carried out using SmartRho of Geolux Co., Ltd. and MTU-5A of Phoenix geophysics Ltd., respectively, to visualize the electrical resistivity structure of study area. DC resistivity and MT survey showed an electrical resistivity less than dozens of ohm-m within the depth of 100 m. In particular, MT survey data were almost similar to TM and TE modes in the frequency range above 1 Hz. The two-dimensional inversion of MT data showed a subsurface structure with low resistivity below 150 ohm-m divided into east-west direction. We confirmed that the inversions of DC resisitivity and MT data along an overlapped survey line represented similar results. In the future, considering the high electrical conductivity, it would be effective to perform DC resistivity and MT survey simultaneously to study the electrical resistivity structure of the central Myanmar basin.

• Journal of the Korean earth science society
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• v.32 no.6
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• pp.548-559
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• 2011

3-D Multi-geophysical surveys were carried out around the Hwasan caldera at the Euisung Sub-basin. To overcome the limitations of resolutions in previous studies, dense gravity data and magnetotelluric (MT) data were obtained and analyzed. In this study, the independent inversion models from gravity and MT data were integrated using correlation and classification approaches for 3-D imaging of the geologic structures. A Structure Index (SI) method was proposed and applied to the integration and classification analyses. This method consists of Type Angle (TA) and Type Intensity (TI) values, which are estimated by the spatial correlation and abnormality of the physical properties. The SI method allowed the classification analysis to be effectively performed. Major findings are as follows: 1) pyroclastic rocks around the central area of the Hwasan caldera with lower density and resistivity than those of neighboring regions extended to a depth of around 1 km, 2) intrusive igneous rocks with high resistivity and density were imaged around the ring fault boundary, and 3) a basement structure with low resistivity and high density, at a depth of 3-5 km, was inferred by the SI analysis.

• Journal of the Korean earth science society
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• v.32 no.6
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• pp.602-612
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• 2011

Jeju Island, a volcanic island located in South Korea, has been one of the main targets of geophysical and/or geological studies because of its tectonic importance related to the volcanism and tectonic link to the southern part of the Korean Peninsula. Recently, as a number of broad-band magnetotelluric (MT) measurements were made, we have examined the deep part of the island. In such an insular setting, it is not easy to properly recover the deep structures such as the lower crust and the upper crust using MT data, because their low-frequency components are strongly affected by the surrounding sea of the island. In this study, we apply the sea-effect correction to the existing MT data collected at a total of 102 sites in Jeju Island. The sea-effect correction makes remarkable changes in the observed MT data at frequencies below 1 Hz, clearly indicating the existence of a conductive lower crust. The 2-D inversion results for both Jeju Southern Line (JSL) and Jeju Northern Line (JNL) show that the transition zone separating the resistive upper crust and conductive lower crust exists at a depth of 20 km on average.

• Journal of the Korean earth science society
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• v.30 no.7
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• pp.834-844
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• 2009

Two-dimensional magnetotelluric (MT) inversion algorithm using minimum support (MS) stabilizer functional was implemented in this study to enhance the contrast of inverted images. For this implementation, this study derived a formula in discrete form for creeping model updates in the least-squares linearized inversion. A spatially varying regularization parameter determination algorithm, which is known as ACB (Active Constraint Balancing), was also adopted to stabilize the inversion process when using MS stabilizer as a model constraint. Inversion experiments for a simple isolated body model show well the feature of MS stabilizer in concentrating the anomalous body compared with the second-order derivative model constraint. This study also compared MS stabilizer and the second-order derivative model constraints for a model having multiple anomalous bodies to show the applicability of the algorithm into field data.

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