| dc.description.abstract | 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.
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