Study On Frequency Domain Electromagnetic Sounding Method Of Measuring Magnetic Field Vertical Components In The Center Of Loop

Frequency domain electromagnetic sounding is one of the commonly-used electromagnetic exploration methods. In theory, existing artificial sources in frequency domain electromagnetic sounding (for example, CSAMT or MELOS method) are based on the source formulas of electric dipole or magnetic dipole source, requiring to make the measurement at a distance that the dipole formula can be established, even the distance as far as where the approximate formulas can be established. The measured signal source is very weak and inversely proportional to the three power (or more) of the distance between the observation point and the source center. Hitherto there is no near-region frequency domain electromagnetic sounding method that conducts the observation at the source center.This paper studies the frequency domain electromagnetic sounding method that conducts the observation at coil center (that is, field source center), observing the magnetic field vertical component, or the virtual reality component or components of the model at the electromagnetic coil center in a certain carrier frequency range. This study can make up for the insufficiency of far-region artificial source in frequency domain electromagnetic sounding and puts an end to the history that frequency domain electromagnetic sounding can not conduct observation in near region.This paper puts forward a method to observe electromagnetic field vertical component artificial sources in the current-carrying coil center, including three kinds of apparent resistivity measurement and extraction means. The first is the observation of vertical component of the magnetic field apparent resistivity model extraction in the artificial source electromagnetic coil center, the second the observation of the magnetic field vertical component of the imaginary component extraction resistivity in the artificial source electromagnetic coil center, and the third is the observation of magnetic field perpendicular to the artificial source Real component extraction component apparent resistivity in the observed electromagnetic coil center.Compared with the existing frequency domain electromagnetic sounding methods, this paper has the innovations points as below:1. This papers studies the feasibility of measuring the carrier component of magnetic field perpendicular in the coil center to measure the frequency domain electromagnetic sounding and puts forward three methods for sounding:(1) the electromagnetic sounding of measuring the magnetic field vertical component; (2) the electromagnetic sounding of measuring the vertical magnetic field component of the virtual component; (3) the electromagnetic sounding of measuring the magnetic field vertical component of the real component.2. The measurement of the magnetic field vertical component of the coil center model adopts the use of multiple iterations, successive approximation method to make the extraction of the apparent resistivity, breaking the limitation that frequency domain electromagnetic sounding can not be observed in the near regions; the accuracy of iteration can be set in advance, hence to realize human control and high accuracy.3. Through two different frequency components of the magnetic field, that is the imaginary or real component of the vertical component, the frequency difference (dispersion ratio) of apparent resistivity can be extracted, hence cleverly eliminating the impact of primary magnetic field and realizing frequency domain electromagnetic sounding in the near area. In particular, the apparent resistivity obtained by measuring the imaginary component is sensitive to the change of the underground geometric structure and electrical distribution and can show large anomalies of the amplitude and width, especially the significant electrical characteristics reflected in the low-frequency asymptotes.4. The study methods of this paper adopts the pseudo-random multi-frequency current combination put forward by Mr. He Jishan, to simultaneously send n-potential frequency excitation current and receive n-potential difference of the frequency response, hence saving time and realizing high efficiency and anti-interference ability.5. Strong signal and high precision can be realized in near-region frequency domain measurements in the power supply coil center. For other existing frequency domain electromagnetic sounding methods (such as CSAMT or MELOS), the measurement can be realized only when the far dipole approximation formula or dipole formula can be established far from the coil center. Due to the fact that the intensity of the field is inversely proportional to the three power (or more) of the distance between the observation points and the source center, the field far from the coil center is very weak and difficult to be measured accurately. The method put forward in this paper directly conducts the measurement in the coil center, and it is easy to realize accurate measurement because there is strong signal at the perpendicular magnetic field. It can reduce the readings superimposed, save the reading time and improve efficiency.This paper includes five chapters. Chapter I is Introduction, Chapter II the basic theory of coil center frequency of electromagnetic field, Chapter III the basic principle of coil center frequency domain electromagnetic sounding, Chapter IV the numerical simulation and analysis on the apparent resistivity extracted from measuring the magnetic field vertical component of the coil center, and Chapter V the conclusion and suggestions.