Sea effect correction in magnetotelluric (MT) data and its application to MT soundings carried out in Jeju Island, Korea

Abstract

S U M M A R YWhen magnetotelluric (MT) data are obtained in the vicinity of the coast, the surrounding sea makes it difficult to interpret subsurface structures, especially at the deep parts of the subsurface. We apply an iterative method to remove the sea effect. The iterative method was originally developed to remove the distortion due to topographic changes from MT data recorded on the seafloor. The iterative sea-effect correction method is carried out in two steps. The first corrects the sea effect, whereas the second inverts the sea-effect-corrected responses. The two steps are alternatively carried out, until the criterion for either the inversion or the seaeffect correction is satisfied. Because the surrounding 3-D sea bathymetry is only incorporated into forward modelling for the sea-effect correction, it can be more robust than the method that directly incorporates 3-D sea bathymetry into a model space for inversion. The synthetic examples show that the sea-effect correction method yields an inverted model comparable to the true model. By applying the sea-effect correction method to real field data acquired in Jeju Island, Korea, we also demonstrate that the sea-effect correction method effectively removes the sea effects from the 1-D and 2-D real field data, which helps enhance the inversion results. On the basis of these results, it may be concluded that the iterative sea-effect correction method can be used as a promising technique for recovering the true response of the subsurface in MT data suffering from sea effects.

1-D crustal resistivity structure revealed by sea effect corrected magnetotelluric (MT) data obtained at Jeju Island, Korea

“…For the deeper structures such as the lower crust and the upper mantle, however, it is not easy to properly describe geoelectrical structures, because the observed low-frequency MT data seem to be strongly affected by the sea surrounding the island Nam et al, 2009). This is in general called "sea effect", which is mainly attributed to electrically sharp contrast between the sea and the land (Nolasco et al, 1998;Santos et al, 2001;Yang et al, 2010). Recently, Yang et al (2010) introduced an effective sea effect correction method for island MT data, and applied the method to MT data acquired in Jeju Island.…”

“…This is in general called "sea effect", which is mainly attributed to electrically sharp contrast between the sea and the land (Nolasco et al, 1998;Santos et al, 2001;Yang et al, 2010). Recently, Yang et al (2010) introduced an effective sea effect correction method for island MT data, and applied the method to MT data acquired in Jeju Island. However, since they only focused on showing the effectiveness of the sea effect correction method, they did not provide detailed geophysical interpretation for the subsurface structure of Jeju Island.…”

Two-dimensional Inversion of Sea-effect-corrected Magnetotelluric (MT) Data in Jeju Island

“…일반적으로 심부지각 구조를 조사하기 위해서는, 자연송신원 자기지전류(MT; Magnetotelluric) 탐사가 폭넓게 활용된다 (Jones, 1999;Jones and Ferguson, 2001 (Nolasco et al, 1998;Santos et al, 2001;Yang et al, 2010 …”

Distribution of Electrically Conductive Sedimentary Layer in Jeju Island Derived from Magnetotelluric Measurements

“…제주도의 심부 구조를 이해하기 위한 지구물리 탐사도 많이 수행되었는데, 중·자력탐사 (Lee, 1994;Kwon et al, 1995) 외에도 자기지전류(MagnetoTelluric, MT) 탐사가 수행되어 왔 다 (Lee et al, 2008;Lee et al, 2009;Nam et al, 2009;Yang et al, 2010;Yang et al, 2012 (Oh et al, 2000). 서귀포층은 연 체동물화석을 비롯하여 석회질 초미화석, 유공충, 완족류, 성 게, 개형충 등 다량의 화석을 포함하고 있어 해안~외대륙붕의 천해환경에서 퇴적된 것으로 알려져 있다 (Oh et al, 2000).…”