포항 저온 지열개발 지역에서 지하구조 규명 및 파쇄대 탐지를 목적으로 3 차원 자기지전류 (MT)탐사를 실시하였다. 현장에서 약 480 km 떨어진 일본 큐슈 지역에 원거리 기준점을 설치하여 양질의 자료를 획득하였다. 대상지역이 바다에 인접한 관계로 바다의 효과를 고려한 3 차원 모델링과 역산을 수행하여 측정된 MT 탐사자료에 포함된 바다효과를 고찰하였다. 그 결과 포항지역에 인접한 바다는 해안선으로부터의 거리에 따라 $1\;Hz{\sim}0.2\;Hz$ 이하의 주파수대에 주로 영향을 미치며, 특히 영일만에 인접한 남동쪽의 측점들이 바다의 영향을 가장 크게 받은 것으로 나타났다. 약 2km 심도 이하의 천부에서는 인접한 바다를 3 차원 역산에 포함시킨 경우와 그렇지 않은 경우가 매우 비슷한 결과를 보였으나, 이보다 심부의 경우는 바다를 포함시키지 않은 3 차원 역산에서 대상지역의 남동쪽에 저비저항 구조가 나타나는데 이는 인접 바다의 영향이 투영되어 나타난 것으로 보인다. 시추 결과 및 역산에 의한 전기비저항 구조를 비교한 결과 대상지역의 지하를 크게 다섯 개의 층으로 구분할 수 있었다. 즉: 1) 10 ohm-m 이하의 전기비저항을 보이는 반고결이암층이 대상지역 북쪽에서는 지표로부터 약 300 m, 남쪽에서는 약 600 m 의 두께로 분포하고; 2) 이암층 내에 수십 ohm-m의 전기비저항을 보이는 화산력응회암 및 조면현무암이 교대로 나타나며; 3) 수백 ohm-m의 전기비저항을 갖는 유문암이 약 400 m 의 두께로; 4) 이후 약 1.5 km 까지는 약 100 ohm-m 의 전기비저항을 보이는 사암 및 이암층이 분포하며; 5) 약 3 km 심도에서 저비저항 구조가 나타나는데 이 층에 대해서는 추가적인 지질학적, 지구물리학적인 조사가 필요할 것으로 생각된다.
포항 저온 지열개발 지역에서 지하구조 규명 및 파쇄대 탐지를 목적으로 3 차원 자기지전류 (MT)탐사를 실시하였다. 현장에서 약 480 km 떨어진 일본 큐슈 지역에 원거리 기준점을 설치하여 양질의 자료를 획득하였다. 대상지역이 바다에 인접한 관계로 바다의 효과를 고려한 3 차원 모델링과 역산을 수행하여 측정된 MT 탐사자료에 포함된 바다효과를 고찰하였다. 그 결과 포항지역에 인접한 바다는 해안선으로부터의 거리에 따라 $1\;Hz{\sim}0.2\;Hz$ 이하의 주파수대에 주로 영향을 미치며, 특히 영일만에 인접한 남동쪽의 측점들이 바다의 영향을 가장 크게 받은 것으로 나타났다. 약 2km 심도 이하의 천부에서는 인접한 바다를 3 차원 역산에 포함시킨 경우와 그렇지 않은 경우가 매우 비슷한 결과를 보였으나, 이보다 심부의 경우는 바다를 포함시키지 않은 3 차원 역산에서 대상지역의 남동쪽에 저비저항 구조가 나타나는데 이는 인접 바다의 영향이 투영되어 나타난 것으로 보인다. 시추 결과 및 역산에 의한 전기비저항 구조를 비교한 결과 대상지역의 지하를 크게 다섯 개의 층으로 구분할 수 있었다. 즉: 1) 10 ohm-m 이하의 전기비저항을 보이는 반고결이암층이 대상지역 북쪽에서는 지표로부터 약 300 m, 남쪽에서는 약 600 m 의 두께로 분포하고; 2) 이암층 내에 수십 ohm-m의 전기비저항을 보이는 화산력응회암 및 조면현무암이 교대로 나타나며; 3) 수백 ohm-m의 전기비저항을 갖는 유문암이 약 400 m 의 두께로; 4) 이후 약 1.5 km 까지는 약 100 ohm-m 의 전기비저항을 보이는 사암 및 이암층이 분포하며; 5) 약 3 km 심도에서 저비저항 구조가 나타나는데 이 층에 대해서는 추가적인 지질학적, 지구물리학적인 조사가 필요할 것으로 생각된다.
A three-dimensional (3D) magnetotelluric (MT) survey has been carried out to delineate subsurface structures and possible fractures, for development of low-temperature geothermal resources in Pohang, Korea. Quite good quality MT data could be obtained throughout the survey region by locating the rem...
A three-dimensional (3D) magnetotelluric (MT) survey has been carried out to delineate subsurface structures and possible fractures, for development of low-temperature geothermal resources in Pohang, Korea. Quite good quality MT data could be obtained throughout the survey region by locating the remote reference in Kyushu, Japan, which is ${\sim}480\;km$ from the centre of the field site. 3D modelling and inversion are performed taking into account the sea effect in MT measurements near the seashore. The nearby sea in the Pohang area affects MT data at frequencies below $1\;Hz{\sim}0.2\;Hz$, depending on the distance from the seashore. The most severe sea effects were observed in the south-east parts of the survey area, closer to Youngil Bay. 3D inversion with and without the seawater constraint showed very similar results at shallow depths, roughly down to 2 km. At greater depths, however, a strong sea effect seems to form a fictitious conductive structure in ordinary 3D inversion, especially in the south-eastern part of the survey region. Comparison between drilling results and the resistivity profiles from inversions showed that five layered structures can be distinguished the subsurface beneath the target area. They are: (a) semi-consolidated mudstones with resistivity less than $10\;{\Omega}m$, which are ${\sim}300\;m$ thick in the northern part and ${\sim}600\;m$ thick in the southern part of the survey area; (b) occasional occurrence of trachybasalt and lapilli tuff within the mudstone layer has resistivity of a few tens of${\Omega}m$, (c) intrusive rhyolite ${\sim}400\;m$ thick has resistivity of several hundreds of ${\Omega}m$, (d) alternating sandstone and mudstone down to 1.5 km depth shows resistivity of ${\sim}100\;{\Omega}m$, (e) a conductive structure was found at a depth of ${\sim}3\;km$, but more geological and geophysical study should be carried out to identify this structure.
A three-dimensional (3D) magnetotelluric (MT) survey has been carried out to delineate subsurface structures and possible fractures, for development of low-temperature geothermal resources in Pohang, Korea. Quite good quality MT data could be obtained throughout the survey region by locating the remote reference in Kyushu, Japan, which is ${\sim}480\;km$ from the centre of the field site. 3D modelling and inversion are performed taking into account the sea effect in MT measurements near the seashore. The nearby sea in the Pohang area affects MT data at frequencies below $1\;Hz{\sim}0.2\;Hz$, depending on the distance from the seashore. The most severe sea effects were observed in the south-east parts of the survey area, closer to Youngil Bay. 3D inversion with and without the seawater constraint showed very similar results at shallow depths, roughly down to 2 km. At greater depths, however, a strong sea effect seems to form a fictitious conductive structure in ordinary 3D inversion, especially in the south-eastern part of the survey region. Comparison between drilling results and the resistivity profiles from inversions showed that five layered structures can be distinguished the subsurface beneath the target area. They are: (a) semi-consolidated mudstones with resistivity less than $10\;{\Omega}m$, which are ${\sim}300\;m$ thick in the northern part and ${\sim}600\;m$ thick in the southern part of the survey area; (b) occasional occurrence of trachybasalt and lapilli tuff within the mudstone layer has resistivity of a few tens of${\Omega}m$, (c) intrusive rhyolite ${\sim}400\;m$ thick has resistivity of several hundreds of ${\Omega}m$, (d) alternating sandstone and mudstone down to 1.5 km depth shows resistivity of ${\sim}100\;{\Omega}m$, (e) a conductive structure was found at a depth of ${\sim}3\;km$, but more geological and geophysical study should be carried out to identify this structure.
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문제 정의
More geological and geophysical study will be needed to identify this structure. Also, our study 나ses very simplified coastlines and sea floor bathymetry. Further studies should follow which include detailed seashore and seafloor descriptions as constraints, as well as specifying the resistivity of each layer beneath the sea bottom.
제안 방법
5. A 3D model to investigate the sea effect in MT responses. The white box indicates the region under study.
9. A sketch of the layer structure including the two test boreholes (modified from Song, 2004) and a comparison between the resistivity profiles at the location of BH-2 from the resistivity log and from the resistivity model derived by 3D inversion.
In this study, we first perform 3D modelling for a sea model to infer the sea effect in the observed MT data from our Pohang site. The 3D model is set up to include simplified coastline nearby, layering of subsurface structure, and average resistivity of each layer down to 1.
Measurements at each site were made during two nights for 15h each, from 17:00 to 08;00 next day in local time, using a MTU-5A system by Phoenix Geophysics. The MTU-5A system is designed for tensor measurements, including two electric and three magnetic components at the surface, with 24-bit resolution for both AMT and MT bands.
The 3D model is set up to include simplified coastline nearby, layering of subsurface structure, and average resistivity of each layer down to 1.5 km, based on results from test drilling, and assumes average depth for the bathymetry of the East Sea and average conductivity of the sea water. We then perform a 3D inversion with such geological settings as constraints and compare the results with the ordinary 3D inversion results.
To investigate the sea effect which may be contained in the observed MT data, a 3D model was set up, which included simplified coast lines and a layered structure inferred from the drilling results as shown in Figure 5. The resistivities of the layers down to 1.
대상 데이터
We could obtain quite good quality MT data throughout the survey region by locating the remote reference in Kyushu, Japan. The site is located in the southeastern part of Korean peninsula and is very close to the eastern seashore. The survey area lies 3-8 km away from the seashore, which is aligned in the north-south direction.
The survey area is located north of Pohang City, south-eastern Korea. Figure 1 shows the general geology and a lineament distribution from a Landsat image of the survey area.
The site is located in the southeastern part of Korean peninsula and is very close to the eastern seashore. The survey area lies 3-8 km away from the seashore, which is aligned in the north-south direction. MT data are definitely affected by the nearby sea water, because of its extremely high conductivity compared to that of formation beneath the survey sites.
The 3D interpretation in this study is based on the data from the 44 sites within the rectangle in Figure 2. Two test boreholes, 165 m apart, were drilled in the year 2003-2004, one of them to a depth of 1.5km. Various well logs including caliper, gamma ray, and resistivity have been acquired, so that the layered structures, resistivity of each layer, and fractures intersecting the boreholes can be investigated.
이론/모형
5 km depth, and (b) the oceancontinent transition is taken to be a sharp boundary following the simplified coastline, but neglecting the topography of the oceanic platform. 3D modelling for this model was performed using the staggered-grid finite difference code of Mackie et al. (1993). The total number of nodes was 70 (x) by 55 (y) by 23 (z, inoltiding 10 air layers).
A linearised least-squares inversion with optimum regularisation and static shift parameterisation (Sasaki, 2004) was used for the inversion, in which forward modelling was done by the finite-difference method. Some modifications were added to the code to implement seawater constraints.
후속연구
Also, our study 나ses very simplified coastlines and sea floor bathymetry. Further studies should follow which include detailed seashore and seafloor descriptions as constraints, as well as specifying the resistivity of each layer beneath the sea bottom.
참고문헌 (8)
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Mackie, R. L., Madden, T. R., and Wannamaker, P. E., 1993, Threedimensional magnetotelluric modeling using difference equations - Theory and comparisons to integral equation solutions: Geophysics 58, 215-226. doi: 10.1190/1.1443407
Monteiro Santos, F. A., Nolasco, M., Almeida, E., Pous, J., Marcuello, A., and Queralt, P., 1999, Correction of the ocean and coast effects on the magnetotelluric impedance tensor: Second International Symposium on 3-D EM (3DEM-2), 309-312
Monteiro Santos, F. A., Trota, A., Soares, A., Luzio, R., Lourenco, N., Matos, L., Almeida, E., Gaspar, J., and Miranda, J. M., 2006, An audio-magnetotelluric investigation in Terceria Island (Azores): Journal of Applied Geophysics 59, 314-323. doi: 10.1016/j.jappgeo. 2005.12.001
Oh, S., Yang, J., Lee, D. K., Kim, S.-K., Mogi, T., Nakada,M., and Song, Y., 2003, Study on deep structure of the Korean peninsula by GDS and teleseismic data: Proceedings, 2003 Korea-Japan Joint Seminar on Geophysical Techniques for Geothermal Exploration and Subsurface Imaging, 68-76
Rikitake, T., and Honkura, Y., 1985, Solid Earth Geomagnetism, Terra Scientific Publishing
Sasaki, Y., 2004, Three-dimensional inversion of static-shifted magnetotelluric data: Earth. Planets and Space 56, 239-248
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