Research On Method Of Electromagnetic Sounding With Low Induction Number In Frequency Domain

With the sharp increase in the demand for deep resource exploration,conventional electromagnetic methods widely used in resource exploration have been diffcult to meet the practical requirements.Therefore,developing a large-depth electromagnetic exploration method with high working efficiency,high precision,and strong anti-interference ability in the field is imperative.The main reason for restricting the exploration depth of frequency domain EM method is that the weak signal caused by the sharp attenuation of electromagnetic field with the increase of the offset.Whether the electromagnetic field could be used to frequency sounding under low induction number(ie,near source)remains to be discussed.This paper systematically studies the sounding mechanism and method under the low induction number,which mainly includes four aspects:1.A high-precision method based on Weighted Averages algorithm to get electromagnetic field of layered modle was developed.Firstly,magnetic vector potential is used to deduce the electromagnetic field for any dipole source.A new weighted average extrapolation technique is introduced for the Hankel transformation.The influence of the number of Gaussian points and the number of integral intervals on the accuracy is studied,and the comparison with the analytical solution demenstrate the higher efficiency over existing algorithms.Then,the sensitivities and Poynting vectors were calculated,which provided technical tools for calculation of electromagnetic field value apparent resistivity and sensitivity with high precision,under low induction numbers zone.2.Aiming at get a full knowledge of the mechanism of near source,characteristics and feasibility of frequency sounding for electric dipole source and magnetic dipole source near source are studied in detail by single-component equivalent apparent resistivity,approximate expression,and sensitivity analysis.The single-component equivalent apparent resistivity and the sensitivity of the field component are calculated,the approximate expressions are analyzed,and the regularity of the frequency sounding with low induction number are clarified.The theoretical calculation results show that for magnetic dipole source,the quadrature component of the electric field and the in-phase component of the magnetic field have the capability of frequency sounding with small induction numbers,while the in-phase component of the electric field and the quadrature component of the magnetic field are mainly expressed as geometric soundings.For the electric dipole source,it is pointed out that the in-phase and quadrature components of the horizontal magnetic field and the in-phase component of the vertical magnetic field are dominated by frequency sounding,while the quadrature of the vertical magnetic field and electric fields are dominated by geometric sounding.Finally,through practical feasibility analysis,it is clear that the primary field is the main factor hindering the feasibility and application winth low induction numbers.3.A technical scheme of whole-area apparent resistivity based on multi-componen was proposed to enhance the sounding performance of near-field sources.The response characteristics of different schemes are compared in detail.A series of multi-component and analytical calculation solutions for whole-area apparent resistivity was designed,numerical simulations and field experiments were carried out,preliminary data processing procedures were developed,and an understanding of the sounding performance of apparent resistivity under near-field sources was obtained.The theoretical calculation results show that the whole-area apparent resistivity is not distorted and has a certain ability of frequency sounding near source.The field test results show that when the azimuth angle is 90 degrees,the whole-area apparent resistivity and Ex apparent resistivity are comparable,and the phase information can be obtained effectively.When the azimuth angle is 45°,the whole-area apparent resistivity can still obtain the measured curve without distortion,revealing that the proposed apparent resistivity has a wider measurable area.4.the methods of secondary field measurement in frequency domain based on an electric dipole source was proposed.The meaning of each integral in the formula of EM fields for the layered medium is described in detail,and the relationship between the primary field and the secondary field is clarified.On this basis,two methods to cancle the primary field are designed,and the formulas of apparent resistivity were deduced.The results of the layered and three-dimensional model show that the current instrument can measure the secondary field which is at lest strongger than1 p T,and the apparent resistivity can correctly reflect the basic shape and properties of the anomaly.The one-dimensional inversion results further show that the method of measure the secondary field can obtain better inversion results at the offset smaller than the detection depth.These results show that the secondary field measurement methods designed in this paper can significantly improve the detection ability near source and has practical application value.Through the above work,this paper clarifies the relationship between the primary field and the secondary field near source and clarifies that the electromagnetic field near-source still has the ability of frequency sounding theoretically.An innovative multi-component combined wavenumber apparent resistivity and two new device to mearsure the secondary field is proposed to extract the information of frequency sounding near source.The feasibility of frequency sounding with small indution number is clarified.