What to trust in a Magnetotelluric Model? - IGU

K.K.Roy et al.interpretation. We cannot choose one of these modelsas a true representative of the subsurface structure.Mathematical ModeL EXPERIMENTModel 1: Fig.4(a) shows a 3D model of a 100 km. cubeof resistivity 5000 ohm-m. with 3 graben type ofstructures. The dimensions of these three structuresare respectively given by (i) left graben : width 2km.,depth 40 km., resistivity of the rock filling the graben50 ohm.m, (ii) central graben : width 20 km. anddepth 25 km., resistivity of the rocks fiing the grabenis assumed to be 200 ohm-m. and (iii) right graben :width 1 km. and depth 30 km., resistivity of thematerial filling the graben is 10 ohm-m.Figs 4(b) and (c) show the TM and TE modeapparent resistivity pseudosections obtained from 3Dfinite difference source code of Mackie, Madden &Wannamaker (1993). The ordinate, time period, is inlog to the base 10 scale. The abscissa is distance inkilometer. The two apparent resistivity pseudosectionsshow that the TM mode apparent resistivities aresuperior to the TE mode apparent resistivities inresolving vertical faults, dykes etc.Model 2: Fig.5 (a) shows a model of a 100 km. cubeof resistivity 5000 ohm-m. Four dykes of width 100meters, depth extent 10 km. and mutual separationof 2km. between these dykes are assumed. Thesedykes are assumed at a depth of 500 meters from thesurface. Resistivity of the rocks filling the grabenstructures is assumed to be 20 ohm-m.Figs 5(b) and (c) show respectively the TE modeapparent resistivity and phase profile across thesedykes. Figs. 5(d) and (e) show the TM mode apparentresistivity and phase profiles across these dykes. Figs5(d) and (e) show that both TM mode apparentresistivity and phase profiles could resolve all the dykesseparately. TE mode apparent resistivity and phaseprofiles failed to resolve the presence of the four dykes.Model 3: Fig.6 (a) shows a 100 km. cube ofresistivity 5000 ohm-m. in which a conducting dykeof width 15 meter is assumed. Resistivity of thematerials filling the dyke is taken to be 10 ohm-m.and the depth of the dyke was varied from 15 m. to 1km. Figs 6(b) and (c) show the TM and TE modeprofiles across the dyke. The two figures clearly showthat TM mode MT has immensely superior resolvingpower than the TE mode MT. TE mode MT sees thetarget from a greater distance. As a result biggervolume of earth materials are involved in generatingthe TE mode response.These three sets of model experiments clearlyshow that the resolving power of the TM moderesponse is very much different from that of the TEmode response. Electric and magnetic fields paralleland perpendicular to the strike direction of thestructures reflect very much different physicalproperties of the earth. Discrepancies in these modelspartly come from theory and partly come from theinhomogeneities and anisotropies of the subsurface.Common features in the modelsStrong data with minimum noise content will alwaysshow some common features in all the models evenafter using different inversion source codes anddifferent MT parameters. Geometrical shapes of thesemajor features may be different. Some of the minorsignatures may be different in different modelsobtained from different magnetotelluric parameters. 2Dquantitative models should be accepted in asemiquantitative way and the broad features, whichare common in all the models, should be taken withgreater degree of confidence. If several earth modelshave something common, then earth must have thatproperty (Bachus & Gilbert 1967). All the three TE,TM and (TE+TM) models presented in the Figs 3b,c and d show the blackish high conductivity zone. Thegeometrical shapes of these zones are different indifferent models. All the three models show that thecontinent is rifted in this area. These parts arecommon in all the three models and can be acceptedFigure 3(a). Shows the location map of the Khandwa-Ujjain traverse.160