The Study Of Magnetotelluric Static Shift Correction And Its Application To Oil And Gas Exploration

Magnetotelluric is a kind of electrical sounding, working in frequency field, using the magnetotelluric field as source, used to study the earth’s interior electrical structure. There will be inducted electromagnetic field in the space due to the electromagnetic induction, when electromagnetic wave propagates in the underground medium, and information of the medium is carried by the data measured on the surface. The spatial distribution of the resistivity of the underground medium can be learned by processing and analyzing the data. Electromagnetic wave of different frequency travels to a different depth, based on the Skin effect, so electrical information of different depth can be detected by measuring and processing data of different frequency. Magnetotelluric can explore to a great depth, since the frequency of magnetotelluric field very wide, which can be as low as1000s and even lower, and it can be used to study structure of the earth’s crust and upper mantle. It is widely used in the research of geological structure in the earth’s crust and upper mantle, oil and gas prospection, geothermal exploration and the earthquake prediction, etc. because of its portable equipment and the advantages that the field is not to be established artificially, and that it is able to explore to a greater depth.However, if there is electrical inhomogeneity in the shallow ground, charge will be accumulated on the interface when current flows through the inhomogeneity. So, an electrical field is added to the magnetotelluric filed, and the additional field is not related to frequency. As a consequence, the curve of measured resistivity will move parallel, and this is the so-called static shift. Static shift is an unpredictable and inevitable phenomenon, which usually lead to false anomaly, and even results in wrong outcome.Both domestic and foreign researchers have do a lot study on static shift, and a number of results have been achieved. Results are summed briefly in this paper, and measured data are influenced by the static shift in upper Yangtze region, based on its complicated geological conditions. The significance of study of static shift is discussed. Then, basic principles of magnetotelluric are introduced, and the generation mechanism of static shift is stated by mathematical derivation based on the basic principles of electromagnetic field. characteristics of static shift are concluded by establishing theoretical model include homogeneous half-space model and horizontally-layered medium model, forward modeling, analyzing resistivity curve, phase curve, f-ps section and f-ω section. Two digital recognition methods of static shift are introduced, include correlation coefficient and wavelet analysis, which is used to guide practical work. And then, a description of several correction methods is given, include graphic shifting method, EMAP filtering, phase transformation, medium filter, registration of first part and wavelet analysis. Advantages and disadvantages of the methods are analyzed. Corrections of different methods are compared, based on a model that heterogeneity is contained in horizontally-layered medium. At last, corrections are used into oil and gas prospection in upper Yangtze region. The influence of static shift is suppressed effectively, and satisfactory results are achieved.