Computation Of Magnetic Fields And Study Of Investigation Depth By Vertical Magnetic Dipole In Layered Earths

It is necessary to solve electromagnetic field of the dipole source in artificial field source electrical survey and induction logging. ground surface electrical survey mainly calculates the layer's influence to ground surface electromagnetic field while the source is on the ground surface. With the development of the exploration technology, well-well, well-surface exploration is attended more and more. And when the emitter is in the conduction medium (especially to multi-layer and multi tilted stratum), the calculation of the electromagnetic field must be solved.The paper give a kind of forward simulation method that vertical magnetic dipole is in layered medium. It starts from Maxwell equation, in draft a Hertzian complexor, and then get Helmholtz equation. The Hertzian complexor is get by solving the equation, then the undetermined coefficient in any layer is get by using terminal condition, and at last the magnetic field expression.When programming and calculating, two question are mainly considered: one is the algorithm of the undetermined coefficient in electromagnetic field ; anther one is calculation method of Bessel equation. Any layer's undetermined coefficient, will be deduced from the terminal condition of the interface. For high concussion Bessel equation, continued fraction numerical algorithm of the immediate integration is adopted, and the result is good.It is calculating to three layers (A,Q,H,K) while numerical simulate. The paper discussed from two situations. One is the situation that the R-T unit are in the same layer, and another one is the situation that the R-T unit are in the different layers. We found that while numerical simulating to A, Q, H, K four models, no matter what model, it is linear between low-frequency stage imaginary quantity and frequency, and the real quantity in low-frequency stage almost maintain a value, however in high-frequency stage, the imaginary and real quantity concuss with frequency. And then the paper discussed the factor influencing magnetic dipole source magnetic field response. The magnetic field response of magnetic dipole source and dipole moment, R-T distance, layer thickness have close relations. While the dipole moment increases, the field value increases proportionally. While R-T distance increases, the magnetic field response value reduces, and it's magnetic response starts concussion earlier. While the layer thickness changes, it's response value also changes, but the magnetic field real quantity'change is not obvious.The depth of investigation in electromagnetic soundings is a maximum depth at which a given target in a given host can be detected by a given sensor. The depth of investigation is related to many factors, such as sensor sensitivity, precision, operating frequencies, ambient noise level, target and interpretation. Quantitative understanding of the relationships between the depth of investigation and these factors will help users meet their geologic objectives, avoid unnecessary survey expenses, and display meaningful geologic features. Many authors deal with a layered half-space under a magnetic dipole source excitation at a distance above the surface. The analysis may use the anomalous in-phase and quadrature components or the amplitude and phase of the measured secondary magnetic field.At last we analyses the factor influencing the magnetic dipole exploration depth. We mainly discuss the influence of the layer resistivity, observation accuracy, R-T distance and frequency. The contrast of resistivity effects the determination of the exploration depth. The bigger the contrast of resistivity is, the deeper the exploration depth is. The higher the exploration accuracy is, the exploration depth is; the longer the R-T distance is, the wider the exploration range is; while the frequency changes, the exploration depth will be influenced, the higher the frequency is, the more shallow it detects. And the resistivity of the source layer also influences the exploration depth. while the resistivity of the source layer increases, the exploration depth reduces correspondingly, but after the resistivity of the source layer increases to some level, the influence to the exploration depth is very small.So it is very important to solve the question for us studying the physical structure underground and selecting the experiment parameter.