Application of the Rayleigh-FFT technique to magnetotelluric modeling and correction

Details Application of the Rayleigh-FFT technique to magnetotelluric modeling and correction Application of the Rayleigh-FFT technique to magnetotelluric modeling and correction

Mar 1989

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989pepi...53..365j&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 53, Issue 3-4, p. 365-375.

Physics

12

Scientific paper

A modification of the original 1896 Rayleigh scattering theory has been implemented using the fast Fourier transform (FFT) to calculate magnetotelluric (MT) responses for complex Earth structures including topography. The method has been tested for three-dimensional Earth models with arbitrary irregular layering and topography, although results are presented for two-dimensional (2-D) Earth models only. As layers can be pinched-out, isolated surface and sub-surface bodies can be included.
An inherent error in the Rayleigh method limits the maximum surface and sub-surface slopes that can be accurately modeled. Valid 2-D results are obtained for surface slopes of 53° in the TE mode and 26° in the TM case; however, sub-surface interface slopes can exceed 60° for both modes.
The Rayleigh-FFT approach enables simultaneous calculation of many surface points, typically 32 or 64 in 2-D. Model input requires only the digitized values of surface and sub-surface interfaces and the electrical properties of the various media. The point TM topographic distortions are slope dependent and are nearly height independent, e.g., maximum TM distortions for topography of 1 m and 1 km are nearly equal if the surface slopes are equal. However, since measured electric fields are averaged over finite length dipoles, the point-by-point small-scale effects are greatly smoothed in practice. TE topographic distortions are typical frequency-dependent inductive effects which are much smaller than TM effects as manifested in the apparent resistivity calculations. Use of the Rayleigh-FFT algorithm to remove the unwanted effects of topography and near-surface inhomogeneities is demonstrated by 2-D distortion tensor stripping of both theoretical and actual field MT results.
Present address: Earth Sciences Associates, Palo Alto, CA, U.S.A.

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