| 摘要: | 大地電磁法(Magnetotellurics)是利用天然的電磁場來研究地下深部地殼電阻率變化，其長週期資料將強烈的受到海岸效應的影響。本研究使用三維順推模擬來估計海岸效應對台灣地區大地電磁法資料的影響，利用各種參數，包括 Bahr， WAL， rotational invariants， phase tensor 和induction arrow等，來評估海岸效應和地電模形維度，並進一步應用相減法去除海岸效應。 模擬的結果顯示，週期大於100秒的大地電磁資料強烈的受到海岸效應影響，台灣東部大於台灣西部，台灣北部大於台灣中部和南部，值得注意的是海岸效應最低出現在中南部山區，這可能與來自台灣東方和西南方的兩處深海效應有關。長週期資料的海岸效應取決於圍岩位置，頻率和電阻率。由於海岸效應影響到大地電磁法資料的維度分析以及電性走向，如果沒有修正海岸效應，資料解釋可能會造成錯誤。 大地電磁法應用在台灣北部宜蘭地區的清水地熱場，估算海岸效應的影響從週期10秒開始，大於探測目標的週期，因此逆推之前不需要作海岸效應修正，逆推結果指出，深度6-12公里處存在和地熱有關的低阻帶。 The Magnetotellurics (MT) utilizes naturally occurring electromagnetic field to determine electrical resistivity variations within the sub-surface. Using MT to investigate deeper parts of the crust beneath Taiwan Island acquires longer period data that is also strongly influenced by the coast effect (CE) around Taiwan. In this study, the models were created by using three dimension (3D) forward modeling to evaluate the CE on MT data from Taiwan Island. Parameters Bahr’s, WAL rotational invariants, phase tensor (PT), together with induction arrow (IA) are used to figure out the CE and the dimensionality as well. Moreover, the subtraction method is also applied to both synthetic and real data to remove the CE on the IA. The modeling results show that the 3D CE are generally significant for periods longer than 100s, higher in eastern Taiwan than in western one, pronounce in northern Taiwan than in the southern and middle one. It worth noting that the presence of the lowest value of 3D indicators zone between south-middle Taiwan give an evident of the lowest CE in Taiwan. It may relate to the balance of the CE between two deepest ocean parts, east and south-west-south, surround Taiwan. More detailed study for those longer periods show that the CE depends on location, frequency, and resistivity of host media. Therefore, the CE should inference the dimensional analysis, and the geoelectrical strike determination; those are the routine processing of the MT data reduction; without having correction of the CE will lead to misinterpretation. Other than the CE assessment, the removal of the CE is also applied to a real data, which were carried out in the Chingshui geothermal field, Yilan, Northeast Taiwan. As the CE beginning at period about 10s, longer than the exploration target, it is not necessary to do the CE correction before data inversion. The inversion results point out the low resistivity zone relating geothermal reservoir at depth of 6-12 km. |
