Application of an iterative sea effect correction method to MT soundings carried out in Jeju Island, Korea

When magnetotelluric (MT) data are obtained in the vicinity of the coast, the surrounding seas make it difficult to interpret subsurface structures, in particular for the deep parts of the subsurface. We apply an iterative method to remove the sea effects. 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 performed in two steps. One is to correct the sea effect, and the other is the inversion of the sea-effect corrected responses. The two steps are alternatively carried out, until a criterion for either the inversion or the sea-effect correction is satisfied. Since the 3D surrounding sea bathymetry is only incorporated into forward modeling for the sea-effect correction, it can be more robust than the method that incorporates the 3D 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 contributes to enhance the inversion results. From these results, we can conclude that the iterative sea-effect correction method can be promising to recover the true response of the subsurface more precisely. (a) Observed (uncorrected) (b) sea-effect corrected sounding curves of XY- and YX-mode with those of determinant average (DET) at the site JSL12. (c) 1-D resistivity models obtained by Occam inversion of determinant average (DET) at each iteration stage for the site JSL12. (d) RMS misfit between Z and Zo at each iteration stage. Note that the inverted model at the initial stage (0th) was obtained without sea effect correction.