What to trust in a Magnetotelluric Model? - IGU

K.K.Roy et al.features in all the models with different geometricalshapes should be accepted with greater level ofconfidence. Minor features which are not common inTE, TM and TE+TM mode models should preferablybe ignored or attached some weightage after crossverification from other auxiliary geophysical orgeological tools.b) From model experiment it has been shown veryclearly that for vertical inhomogeneities, viz., grabens,dykes, etc., TM mode MT anomalies are much moresharper than TE mode MT anomalies. Therefore,greater weightage can be attached to the TM modemodels for mapping shear zones, suture zones, faults,fractures, lineaments, etc.c) Rotation invariant apparent resistivities and theirphases are very close to each other. Moreover, theinformation content will be more in (ρ D,φ D), (ρ C,φ C)and (ρ B,φ B) pairs. Therefore, models generated fromrotation invariant parameters will be very close andmore trustworthy. For 1D and 3D problems we canuse these pairs as such for modeling. For 2D we haveto make 2D stiched in models from 1D-invertedmodels because TE and TM mode electromagneticwave equations cannot be used for rotation invariantparameters. Models generated using RITs and TE modeof RRI are presented just to show the readers how dothey look like.d) Routine do’s and don’ts in MT, which are wellknown to the MT community, must be followedreligiously to improve the trustworthiness of themodels. These guidelines are (i) avoid cultural noiseas far as practicable (ii) avoid solar quiet days as faras practicable for field measurements (iii) summermonths are preferable for MT field data acquisition(iv) for deeper probing, hard rock terrain should bechosen so that high frequency MT signals also canpenetrate to a considerable depth.e) For interpretation of MT data collected over anArchaean or Proterozoic terrain, if we go for 1Dinversion, we should over parameterize the models toincrease the trustworthiness of the models. 3 to 4layered earth model for a granite batholith or ahighgrade granulitic terrain is not acceptable to thegeologists. For 2 dimensional interpretation if wechoose square or rectangular blocks of bigger size infinite difference or finite element forward models,square elements or blocks of different colors appearin the 2D models. Geologists are reluctant to acceptthe square blocks in the models. Either very fine finitedifference cells should be taken with some increasein computation time or finite element method withisoparametric elements should be chosen to map thecomplicated subsurface geology.ACKNOWLEDGEMENTSAuthors are grateful to DST for sanctioning theproject ESS/16/103/98. Thanks are due to the DirectorGeneral of the Geological Survey of India for providinginfrastructural facilities for carrying out fieldwork nearKhandwa, Madhya Pradesh. Thanks are due to Mackiefor providing us their softwares for model experiment.Thanks are due to Mr.S.P.Hazra of IIT, Kharagpur fordrafting the diagrams nicely. Thanks are due to JohnBooker for allowing us to work with their softwares.REFERENCESBachus,G.E.& Gilbert, F., 1967. NumericalApplication of formalism for geophysical inverseproblems, Geophys. J.R. Astr. Soc., 13, 247-276.Berdichevskii, M.N. & Dmitriev, V.I.,1976. Basicprinciples of interpretation of magnetotelluriccurves in geoelectric and geothermal studies,KAPG geophysical monograph, pp.165-221, ed.Adam, A., Academic Kiado, Budapest.D’ Erceville, L. & Kunetz, G., 1962. The effect of afault on the earth’s natural electromagnetic Field,Geophysics, 27, 666-676.Eggers, D.E.,1982. An eigen state formulation of themagnetotelluric impedance tensor, Geophysics,47,1204-1214.Lilley, F.E.M., 1993. Magnetotelluric analysis usingMohr Circle. Geophysics, 58, 1498-1506.Mackie,R.L., Madden,T.R. & Wannamaker,T.E., 1993,Three Dimensional Magnetotelluric Modellingusing difference equations-Theory andComparison to Integral Equation Solution,Geophysics, 58, 215-226.Rankin, D., 1962. The magnetotelluric effect on aDike, Geophysics, 27, 651-665.Roy, K. K., Singh, A. K. & Rao, C. K., 1998a. Twodimensional Magnetotelluric model of theSinghbhum granite batholith. in DeepElectromagnetic Exploration, pp.120.-151, Eds.Roy, K.K., Verma, S.K. & Mallick, K., SpringerVerlag, Germany.Roy, K. K., Srivastava, S. & Singh, A. K., 1998b.Rotation invariant magnetotelluric impedancetensor: a case study from West Singhbhum,Bihar, in Deep Electromagnetic Exploration, pp.99-119, Eds. Roy, K.K., Verma, S.K. & Mallick,K., Springer Verlag, Germany.Roy, K.K., Das, L. K., Mukherjee, K. K. & Naskar,D. C.,1996. Some upper crustal information ofthe Singhbhum craton based on direct currentgeoelectric survey. Indian Journal of Geology170