Study Of Three Dimentional Magnetotelluric Forward Modeling Using Integral Equation Method

With the further development of oil exploration, the difficulty of prospecting isbecoming so large that the geophysicists gradually put emphasis on theMagnetotelluric Sounding Method (MT). Numerical modeling about electromagneticresponse of 3-D bodies has been studied by scholars of western countries since middle1970s and a lot progress has been made. However, the same study in our countrystarted lately. Though there are some experimental models and numerical modelingresults of simple models with regard to 3-D resistivity modeling, we haven't seensystemic, mature and practical results so far.Recently, American Utah university has developed a 3-D forward modelingsoftware named INTEM3D,which is based on integral equation method. It can beused to model 3-D geoelectric bodies in horizontal layered earth in frequencydomain. Based on the software, elementary probing about variation rules of resistivityand depth of the anomalous body is done in order to give a comparative standard.Electromagnetic response of simple geometrical bodies in homogeneous half spaceand horizontal three-layered earth has been modeled. We have also modeled theresponse of two bodies in homogeneous half space, static shift and low-resistivitylayer effect. Some elementary conclusions have been drawn.In the case of homogeneous half space, the apparent resistivity contourpseudosection map figures out the top border and the side borders of the anomalousbody but it can't figure out the bottom border of the anomalous body. However, thephase contour pseudosection map shows the bottom border of the anomalous body.The result figures of xy mode have false anomaly but the yx mode doesn't have.In the case of horizontal three-layered earth, xy mode is better than yx mode andthe effective detecting depth is much smaller than that of the homogeneous half space.Resolution to vertical slab is not very large. When the bottom is conductive, instead ofthe anomalous body, the top interface of the bottom can be easily observed on ourresult figures;when the bottom' resistivity is high, the prospecting depth is larger thanthe former.In the case of vertical contact plane in homogeneous half space, xy mode is betterthan yx mode while in the case of horizontal contact plane the thing is quite theopposite. When the resistivity of the anomalous body becomes larger, the results comenearer to the actual condition. Our study about static shift shows that a subsurfaceconductive body has great effect to our numerical modeling results, the apparentresistivity contour pseudosection map of both modes have side anomaly and thatwhen there is a high resistivity subsurface body the numerical modeling results gonearer to the actual geological condition.