#Toau appraisal verification

We test the algorithms on a notoriously difficult synthetic 5-layer structure with two conductors situated at different depths, as well as on the experimental COPROD1 data set standardly used to benchmark 1D magnetotelluric inversion codes. We present a comparison of three common global optimization and stochastic approaches to the solution of a magnetotelluric inverse problem for thick layer structures, specifically the controlled random search algorithm, the stochastic sampling by the Monte Carlo method with Markov chains and its newly suggested approximate, but largely accelerated, version, the neighbourhood algorithm.

#Toau appraisal verification

Global optimization and stochastic approaches to the interpretation of measured data have recently gained particular attraction as tools for directed search for and/or verification of characteristic structural details and quantitative parameters of the deep structure, which is a task often arising when interpreting geoelectrical induction data in seismoactive and volcanic areas. Some examples of its application to both synthetic and real data are presented. 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.

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 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 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. 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.