Automatic 1-D inversion of magnetotelluric data by the method of modeling, Geophys

Two popular methods for obtaining a preliminary 1-D geo-electric model from magnetotelluric data are the simple inversion algorithms due to Niblett, Bostick or Schmucker, and the modelling scheme of Fischer & LeQuang. The former provides resistivity values at different depths, while the latter uses an optimizing program to obtain the resistivities and thicknesses of a prescribed number of layers which give a best-fitting response to the given data. The present scheme combines features of both methods by using Niblett inversion to provide an initial resistivity profile from which, successively, 2, 3, 4, ... layer models are constructed automatically. The algorithm provides a response with minimum misfit for each layered model according to the method of Fischer & LeQuang, and uses a statistical F-test to determine when the inclusion of additional layers is no longer justified by the improvement of fit. The method therefore provides a best-fitting ‘least-layered’ model consistent with the given data without the human intervention required by the Fischer-LeQuang scheme. Some examples of its application to both synthetic and real data are presented. Key words: conductivity, geo-electric structures, geo-electromagnetic induction, inversion, magnetotellurics.