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The Study on Electromagnetic Detection of Small Aplitud Low Gradient Magnetic Anomaly

Author: ChengXiaoShun
Tutor: HuXiangYun
School: China University of Geosciences
Course: Geological Engineering
Keywords: magnetic anomaly distinguishing magnetization curve secondary potentialattenuation electricity dispersion rate the magnetic dispersion rate the impedancespectrum
CLC: P631.325
Type: Master's thesis
Year: 2013
Downloads: 15
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Abstract


In a new round of geological prospecting work, exploration for iron ore resources is one of the important tasks of mineral resource exploration in china. In order to meet the market demand for iron ore resources, geological exploration expenses of investment is increased year by year, broke through2billion yuan [1]in2008. Generally, magnetic anomaly is still the main basis of iron ore resources exploration. The new prospecting experience show that the geological exploration has achieved better prospecting results according to the magnetic anomalies, but there are also borehole tend to see rock mass instead of ore body by using magnetic anomalies(geomagnetic/aeromagnetic), resulting in waste of exploration funds. How to reduce the risk of prospecting funds? This would require the geophysical workers study on magnetic anomalies, and find out appropriate means and methods to distinguish the different types of magnetic anomaly, guide the prospecting work, save money.Research on distinguishing the different types of the magnetic anomaly is basic synchronization with magnetic prospecting.60-70’s of the last century, the main research are as follows:(1) using the magnetic prospecting data and geological data analysis to distinguish the different types of the magnetic anomaly;(2)research on the magnetic properties of rocks, ores in magnetic anomaly zone through comparing the forward calculation with the test results, to distinguish the different types of the magnetic anomaly;(3) Analysis and summary of known mineral anomalies and known non-mineral anomalies, by comparing the unknown abnormal and the known mineral anomaly characteristics, distinguish ore and non-ore magnetic anomaly;(4) using physical properties of ore body to be different from those of the surrounding rock, distinguish the different types of the anomaly by comprehensive geophysical prospecting method. This method has certain application conditions, such as (2) in the coverage area is invalid. In the late1970s-80s began to study magnetic susceptibility κ value or Q values, to distinguish the nature of the magnetic anomaly. Method is "geomagnetic diurnal variation method ","artificial magnetization","low frequency electromagnetic induction method" and "magnetic sounding method". Limited by the instrument sensitivity, precision and power at that time, have not been able to get popularization and application.With the progress of science and technology, electromagnetic instruments have rapid development, has realized the high resolution, low noise, large dynamic range and other characteristics, make use of multi-parameter measurement of electromagnetic field to distinguish the magnetic properties of the anomalous source possible. Based on the two kinds of four different magnetic anomalies, this study try to find out different electromagnetic response and characteristics of magnetic anomaly, by sorting, induction and analysis of various electromagnetic measurement results, and achieve the purpose of distinguishing magnetic anomaly with different properties by these characteristics. At the same time, summarized a set of optimized and feasible field work method and the technical scheme, scientifically evaluate the unknown nature of the magnetic anomaly.Magnetic anomalies are caused by geological body containing magnetic minerals, magnetic minerals concentration to a certain extent become ore body, and the magnetic ore body is a good electrical conductor. Therefore,studying on the properties of the anomalous source must tightly around magnetic conductivity and electrical conductivity of the magnetic anomalies. We studied the electromagnetic properties of the magnetic anomaly of the different area from eight aspects.1. Magnetic relaxation effect:TEM method still observe the stronger attenuation slower abnormal later in iron mine area, observation of the weaker, attenuation fast in non-mine, it may be observed magnetic relaxation effect under natural conditions.2. The relative magnetic permeability:we calculated relative permeability ur by TEM observation results of the magnetic dipole source and CSAMT observation results of the electric dipole source in different measurement area respectively, ur of the different types of the magnetic anomaly area have obvious difference.3. Magnetization curve:material according to its magnetic characteristics can be divided into paramagnetism and diamagnetism and ferromagnetism, the difference is that magnetization curve to lie in the two former is linear, the latter is non-linear. Studies have found that there is a similar relationship between artificial excitation field intensity and the response of vertical secondary field.4. The impedance spectrum:phase anomaly were observed better by impedance phase induced polarization method in mine area, Measurement results show that relation between frequency and the impedance is obviously different in the different magnetic anomaly area.5. Impedance phase compound spectrum method:using results of the impedance phase of induced polarization, make impedance phase spectrum diagram, in the mine area and the non-mine area had no obvious difference between impedance phase spectrum.6. Secondary potential attenuation:we have studied on secondary potential attenuation characteristics by time domain induced polarization, secondary potential decay is slow in ore region, no attenuation is quickly in non-mine area.7. Charging time characteristics of charge rate:in different magnetic anomaly zone, difference of the charging time characteristics of the charge rate is very small.8. Electricity the dispersion rate, magnetic dispersion rate:Using the different excitation source,measure the electromagnetic response of high frequency and low frequency respectively, calculate electric dispersion rate and the magnetic dispersion rate, there is obvious abnormal of electric or magnetic dispersion rate in mineralization anomaly areas, without abnormal of electricity or magnetic dispersion rate in not mineralization anomaly areas.Research thinks through eight aspects, magnetic relaxation effect, relative permeability, magnetization curve, impedance spectrum, secondary potential attenuation, dispersion rate, etc., in which two or more parameters combination can distinguish different nature of the source of magnetic anomaly, phase spectrum method, the complex impedance charge rate of charging time characteristics can not.With parameter combination of electromagnetic prospecting method as a field work technology solutions, is able to distinguish between the nature of the different magnetic anomaly source, guide the arrangement of exploration.

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CLC: > Astronomy,Earth Sciences > Geology > Geology, mineral prospecting and exploration > Geophysical exploration > Electrical prospecting > Method > Electromagnetic Method
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