Nuree Han

Magnetotelluric survey applied to geothermal exploration: An example at Seokmo Island, Korea*

A magnetotelluric (MT) survey has been performed to delineate deeply extended fracture systems at the geothermal field in Seokmo Island, Korea. To assist interpretation of the MT data, geological surveying and well logging of existing wells were also performed. The surface geology of the island shows Cretaceous and Jurassic granite in the north and Precambrian schist in the south. The geothermal regime has been found along the boundary between the schist and Cretaceous granite. Because of the deep circulation along the fracture system, geothermal gradient of the target area exceeds 45°C/km, which is much higher than the average geothermal gradient in Korea. 2D and 3D inversions of MT data clearly showed a very conductive anomaly, which is interpreted as a fracture system bearing saline water that extends at least down to 1.5 km depth and is inclined eastwards. After drilling down to the depth of 1280 m, more than 4000 tons/day of geothermal water overflowed with temperature higher than 70°C. This water showed very similar chemical composition and temperature to those from another existing well, so that they can be considered to have the same origin; i.e. from the same fracture system. A new geothermal project for combined heat and power generation was launched in 2009 in Seokmo Island, based on the survey. Additional geophysical investigations including MT surveys to cover a wider area, seismic reflection surveys, borehole surveys, and well logging of more than 20 existing boreholes will be conducted.

A comparison of accuracy and computation time of three-dimensional magnetotelluric modelling algorithms

This paper presents a comparison of four different three-dimensional (3D) magnetotelluric (MT) modelling algorithms in terms of accuracy and computation time. Three of them use the finite difference method while the last one uses the edge finite-element method. The modelling algorithms are analysed with respect to governing equations, boundary conditions, solvers, preconditioners and static divergence correction. The accuracy of the algorithms is examined by simulating MT responses for two different models, both of which have anomalous bodies in simple background media. Although the simulated responses generally show good agreement, small discrepancies are observed over the anomalous bodies. These discrepancies are caused by the difference between the electric- and magnetic-field formulations used by the algorithms. The three algorithms using the finite difference method are adequate for 3D inversion of MT data since they produce solutions in a short computation time. Although the algorithm using the edge finite-element method requires longer computation time than the others, it can provide more accurate solutions and model surface topography.

Three-dimensional Modeling of Marine Controlled-source Electromagnetic Surveys Based on Finite Difference Method

요약: 이 연구에서는 다양한 환경에서 인공송신원 해양전자탐사 자료의 정확한 해석을 위해 3차원 모델링 알고리듬을개발하고, 복잡한 탄화수소자원 개발 환경에서 인공송신원 해양전자탐사의 적용 가능성을 파악하고자 암염에 인접한 저류층 모형을 구성하고 그 반응을 계산 및 분석하였다. 모델링 알고리듬은 유한차분법에 기초하고 있으며 이차 전기장 정식화를 도입함으로써 빠르고 정확한 반응 계산이 가능하도록 하였다. 이 알고리듬의 정확성은 2층구조 모형에 대해 해석해 및 기존의 3차원 모델링 알고리듬의 결과와의 비교를 통해 검증되었다. 암염에 인접한 저류층 모형에서는 암염과 저류층 및 배경 매질의 경계에 존재하는 세 군데 송신원에 대한 전기장 및 자기장 반응을 파악하고, 배경매질의 반응으로정규화한 전기장 및 자기장 반응도 분석하였다. 그 결과 세 군데 송신원에 대한 반응을 종합하면 정규화된 전자기장 반응으로부터 지하의 고비저항 이상체의 개수와 각각의 위치를 파악할 수 있었으며 탄성파탐사를 통해 암염의 구조가 사전정보로 알려졌다면 저류층의 존재를 밝힐 수 있었다. 이 연구에서 개발한 알고리듬은 향후 다양한 환경에서 인공송신원해양전자탐사 자료의 반응 분석은 물론 역산 알고리듬 개발에 유용할 것으로 기대된다.주요어: 해양전자탐사, 모델링, 암염, 저류층, 정규화된 전기장, 정규화된 자기장Abstract: This paper presents development of a three-dimensional marine controlled-source electromagnetic (mCSEM)modeling algorithm and its application to a salt and reservoir model to examine detectability of mCSEM for a reservoirunder complex subsurface structures. The algorithm is based on the finite difference method, and employs the secondaryfield formulation for an accurate and fast calculation of modeling responses. The algorithm is verified for a two-layermodel by comparing solutions not only with analytic solutions but also with those from other 3D modeling algorithm.We calculate and analyze electric and magnetic fields and their normalized responses for a salt and reservoir model dueto three sources located at boundaries between a salt, a reservoir, and background. Numbers and positions of resistiveanomalies are informed by normalized responses for three sources, and types of resistive anomalies can be informed whenthere is a priori information about a salt by seismic exploration. Keywords: marine CSEM, modeling, salt, reservoir, normalized electric fields, normalized magnetic fields