Sea-effect correction of 3D magnetotelluric responses in Pohang, Korea

In order to investigate geothermal energy resources, Korea Institute of Geoscience and Mineral Resources (KIGAM) have conducted magnetotellurics (MT) surveys for years in Pohang, Korea. Since the survey site is close to sea, low-frequency MT data measured in Pohang are not free from sea effects. Consequently, inversion of MT data may result in misinterpretation of the subsurface if sea effects are not considered properly. The sea effect can be treated in the interpretation of MT data in two different ways: incorporating sea into an earth model in inversion and compensating sea effects before conducting inversion. The first approach has numerical difficulties due to the presence of electrically conductive sea, while the second method allows conventional inversion algorithms for a model without sea since sea effects are removed from MT data. This paper presents the method to compensate sea effects on MT data by correcting both electric and magnetic fields distorted by sea effects. Using a synthetic model with one or two-layered hosts, we validate the correction method. Nonlinear least-squares inversion of corrected MT data successfully recovers major features of the synthetic model. Corrected MT data observed in Pohang is also inverted and compared with former results from 3D inversion including sea effects. Approach of sea-effect removal from MT data helps to improve the geological comprehension of deep-seated structure in Pohang.