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2D Forward Modeling and Joint Inversion of the CSAMT Signal Incorporation IP Effect

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Tutor: LiuHongFu
School: Taiyuan University of Technology
Course: Mineral prospecting and exploration
Keywords: CSAMT,Cole-Cole models,Cagniard resistivity,IP effect,joint inversion
CLC: P631.325
Type: PhD thesis
Year:  2012
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In the exploration of controlled source audio-frequency magnetotellurics method (CSAMT),the artificial established alternative electric field, as the excitation source, will make hypogeal polarization medium produce induced polarization effect, owing to which the current earth resistivity in fact is a plural complex number to the line frequency. However, the earth resistivity is simply considered as a real number that is irrelated to the frequency while people collect and deal with the data in practice. Obviously, the change of real numbers to complex one will bring huge error to the data.In order to solve the problem, starting with the induced polarization characteristic (IPC) of rock ores, this thesis makes a thorough analysis on IPC in the field of stable electric current and alternating electric current in the first place. Then based on the Cole-Cole model of IPE, it explains the relationship between complex resistivity and the parameter of Cole-Cole model. Also, it expatiates the characteristic of real component, imaginary component, amplitude and phase for the complex resistivity. A conclusion can be drawed as follows: The frequency spectrum characteristic of complex resistivity amplitude is similar with that of real component. Both decrease with increasing frequency. They will tend towards direct current resistivity ¦Ñ0when frequency is low;(1-¦Ç)¦Ñ0, when high. Likewise, the frequency spectrum characteristic of phase and that of imaginary component resemble with each other. Both increase with the increasing charging rate values. The numerical value of them will incline to zero when the frequency is both high and low, and an extreme will be obtained in a certain middle frequency. The extreme of imaginary component is only related to time constant, inversely proportional to time constant (¦Ó). Nevertheless, the extreme of phase is related to charging rate (m), time constant (¦Ó) and frequency exponent (c).The regulations of the impact of symmetrical semi-space Cole-Cole model parameter change on various component, apparent resistivity and apparent phase in CSAMT magnetic field imply some enlightenments as follows:Firstly, the data of the real part and imaginary part of direct gravimetric in electric field Ex are both negative numbers. The laws of curve are the same. Secondly, the value of the real part of magnetic field Hy maintained fixed. By contraries, the values of imaginary part vary with the change of frequency. Among the four parameters of Cole-Cole model, the impact of charging rate on forward result is the most intense one, then the frequency, the time constant ranks in the end.As for three layers of resistivity models, this thesis also probes deeply into the abnormal feature of specific value for the curve of apparent resistivity, apparent phase, and the ratio of them. In a conclusion, both apparent resistivity and apparent phase of different resistivity models present out of the way intuitionally. Moreover, the configurations of these two curves resemble with each other. And the frequency of apparent resistivity ratio extreme is a bit higher than that of apparent phase ratio. As is educed from the above passage, it is the depth of polarization layer and the scale of polarization ratio, that have something to do with the ratio of apparent resistivity and apparent phase, but have nothing to do with the resistivity models.This thesis also analyzes the CSAMT2D forward modeling of IPE for single low resistivity medium model, single high resistivity medium model, dual resistance medium one and fault¡¯s. It is noticed that the consideration of IPE of medium makes the abnormity of forward modeling data more distinct than that showed without consideration of IPE. And likewise, apparent resistivity, apparent phase, and the ratio of them can intuitively reflect the characteristic of abnormity. The abnormity of apparent resistivity is more frequent than that of apparent phase. Similarly, the same thing happens on the ratio of apparent resistivity and apparent phase.A thought of joint inversion between CSAMT and induced polarization (IP) sounding has been put forward. It is discovered that whenever the medium is characterized by IPE, the data of joint inversion can fix the position and scope more exactly than those of CSAMT, which proves the superiority of joint inversion. The theory of joint inversion has been put into practice in the joint inversion of a Zhejiang cuprum polymetallic ore CSAMT and IP sounding. The position and scale of metallogenic belt have been marked off; the spots of fault of F5and F6have also been deduced. The experiment on ZK002drill hole discloses that ¢ñ¡¢¢ò¡¢¢ó¡¢¢ô copper ore bodies are indigenous to crypto explosive breccia chimney and the F5and F6faults certainly exist. In a word, the joint inversion method is effectively feasible through the practical engineering.
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