Numerical Simulation And Analysis Of Magnetotelluric Sounding For Underground Geologic Structures

Magnetotelluric sounding(MT) method is an electromagnetic geophysical methodof imaging the earth’s subsurface by measuring natural variations of electrical andmagnetic fields.Using magnetotelluric sounding method to detect buried geologicalstructure has important practical significance on studies of tectonic, mineral resourcesand construction.With the in-depth theory research and wide practical application,many new problems are needed to be solved or to be further studied and consummated,these problems come down to the technique of numerical forward modeling. In theMT method, electromagnetic fields are used to investigate the electrical conductivitystructure of the earth. The phase, and directional relationships between electric andmagnetic fields on the surface depend on the distribution of electrical conductivity inthe subsurface. By modeling some geological structure models, the characteristic ofelectromagnetic response can be studied, and this has important guiding significancefor the data interpretation.Based on the theoretical characteristic magnetotelluric fields, this paper hasmodeled common geological structure models with finite element method(FEM). Theirregular meshing method is used in the modeling process. In order to improve themodeling efficiency and calculation accuracy, multi-grid method is used to solve thelarge liner equation formed in the modeling process. In order to make up the shortfallof the Cagniard definition of apparent resistivity on accuracy, an improved definitionis applied in this paper. By comparing the apparent resistivity results respectively bythe two definitions, it proves that the improved definition proforms better on reducingthe oscillating interference on the electrical interfaces of the apparent resistivitycurves, and can speed up the convergence rate to the true resistivity.For comprehensive analysis the characteristics of electromagnetic response, theforward modeling results of apparent resistivity and impedance phase are both studied.They show that: the apparent resistivity results can intuitively reflect the electricalcharacteristic and distribution of the geological structures; the impedance phase ismore sensitive to electrical changes in subsurface, and with the impedance phaseresults, the electrical relations in subsurface can be got. So the results of apparentresistivity and impedance phase can paly complementary roles for each other.By studding the forwarded modeling results of apparent resistivity andimpedance phase,this paper has reached the characteristics of electromagneticresponse of common folds and faults. From these characteristics,the correspondingtype of geological structure can be accurately identified. At last, the effectiveness offorward modeling work is well verified by a practical example.