FAST APPROXIMATE INVERSION OF MAGNETOTELLURIC(FAIMT) DATA FOR A HORIZONTALLY LAYERED EARTH WITH NEAR-SURFACE INHOMOGENEITIES

IwrRODUcrIONNearsurface2-Dand3-DinhomogeneousdistortionsinMTdatamakeitdimculttointerpretMTdataevenwhentheregionalorbasinstruCturescanbetreatalas1-dfornsionalnyohmann,l975,Weide1t,l975,Wannamakeretal.,l984,LaTormcaetal.,l986,Zhang,l987,TorresWhrdinetal.,l992).Inthelasttwentyyears,2-Dand3-DmodelinghavebeenaanmPlishedbyusingnumricalmethods.Afteranalyringthesefor-wardmodelingresultS,wenowdeariyunderstandthemaorphySica1meCanisrnsasultingfromthesedistortionsofnear6urfaceinhomogeneities(Pa…

Fast approximate inversion of magnetotelluric (FAIMT) data for a horizontally layered earth with near-surface inhomogeneities

This paper discusses use of approximations and the Integral Mean Value Theorem to show that 6 coefficients approximately describe the distortions of near surface inhomogeneities on the MT field of a horizontally layered earth model. When these 6 coefficients are considered together with those of the magnetic field of a horizontally layered earth model,the analytic and approximate wave impedance equations can be derived for the MT response of a horizontally layered earth model with near-surface 2-D and 3-D inhomogeneities. These approximate wave impedance equations are used with inverted MT data for 2-D and 3-D forward modelling. Although these 6 coefficients cannot be determined before inversion,initial estimates can be used. The 6 coefficients and the asistivity and thickness of each layer of a horizontally layered earth can be obtained by using published inversion methods. The 6 coefficients give important informaion (depths and resistivities) on the near-surface inhomogenelties.The authors inverted 2-D and 3-D theoretical models for Fast Approximate Inversion of Magnetotelluric (FAIMT) data for a horizontally layered earth with near-surface inhomogeneities compares favorably with traditional invrsion methods, especially for inverting regional or basin structures. This method simplifies computation and gives a reasonable 1 -D geological model with fewer nonuniquenas problems.