AUTOMATIC 1-D INVERSION OF MAGNETOTELLURIC DATA BY THE METHOD OF MODELING

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 re-al data are presented.