Application Of 3-D Geophysical Data In Geological Research Of Zhenba Area

Undulating terrain and steep hill make it difficult to explore the submountainous Zhenba area, which leads to incomplete seismic and geologic data, and the structure pattern is therefore hard to define. However,portable instrument and low price make electrical prospecting much easier and more economic to conduct.Due to its advantage, electrical prospecting is mainly conducted in research area,combining with gravity,magnetic and seismic data.Based on these data, symthetical interpretation is accomplished of stratigraphic features, structural patterns, source-reservoir-cap assemblage features and reservoir characterization description.Gravity data is processed by unified standard formula with four types of correction:normal field correction, terrain correction, interlayercorrection and elevation item correction. Variable density method is applied during Terrain correction and interlayer correction. During inversion process of gravity data, triple integral method is used in forward modelling/while genetic algorithms is used in inversion process.Magnetic data are processed with normal field correction and diurnal variation correction. Three-dimensional constrained inversion of magnetic data is a problem of three dimensional linear vector. Therefor, triple integral method is used in forward modelling,while simulated annealing method is used in inversion process. The magnitude and direction of remanent magnetization can be obtained from laboratory magnetic measurement. Well logging and seismic data are mainly used to difine the boundaries of special layer.Electrical data processing is the major work of this paper, which involves three-dimensional static correction and three-dimensional terrain simulation. Impedance tensor decomposition is used in static displacement correction. Terrain processing method involves terrain during inversion.It is science-based and rational to perform three-dimensional inversion because three-dimensional structures are quite developed in study area.Three-dimensional forward calculation is a volume integral equation method based on Tensor Green’s function,while dimensionality reduction approach inversion process. Besides,Terrain factor is considered during the process of inversion.Here is the inversion principle of dimensionality reduction approximation method. First, perform 3D forward calculation of initial model.Then obtain one-dimensional measured apparent resistivity by combining field apparent resistivity. Next, perform one-dimensional formal inversionto form a new model and then move to next iteration.Characteristics of each electrical abnormalities plane are analyzed from comprehensive gological, seismic, gravity and magnetic and electrical data. Plane interpretation of Zhemba area is performed based on a deeply study of the relationship between thrust belt structures, fracture development status and its distribution characteristics. Five potential reservoirs and six fractures are recognized. Meanwhile,three first-ordertectonic units are marked out, which include northern Dabashan nappe tectonic belt,Micanshan-southern Dabashan thrust-fault tectonic belt and Sichuan Basin. Five secondary-order tectonic units also are marked out, including Ziyang nappe,Zhenping nappe,Gaotan nappe, Dabashan southern edge thrust-fault belt,Southern Dabashan fault-fold structural belt. Through comprehensive interpretation, four local structures in study area are comfirmed, two of which are confirmed favorable structures based on resistivity feature and structural evaluation.