| 1Electromagnetic (EM) prospecting method is one of important sounding ways in the applied geophysics because of its many advantages such as flexible source, various mode, high efficiency and stable performance compared with other ways. It is widely used in the geophysical engineering, oil and gas exploration, evaluation of geothermal reserves, metal mineral exploration and deep geological research, etc. Recently, although many more flexible and reliable new digital instruments have continuously invented, lagging interpretation theories and methods restrict obviously further development and more application of EM method. As to my knowledge, 1-D numerical modeling has already become quite mature and 2-D numerical simulation is also gradually maturing, however the 3-D modeling is just still in the start-up and developing stage. With the continuous improvement in accuracy of EM measurement and the rapid development of computer technology, 3-D EM forward modeling has become a very important subject and attracted more and more scholars to engage in such study. 3-D EM numerical modeling has three main algorithms: the finite difference (FD) method, the finite element (FE) method and the integral equation (IE) method. Different from the full-space discrete FD method and FE method, IE method only requires to discrete the abnormal region on background formation to solve Maxwell's equation, so smaller number of grids is required and higher accuracy of approximation can be obtained. IE method has in fact become a very important method to solve 3-D EM field and has been deeply and wildely studied. However, it must be pointed out that currently IE method is chiefly applied to solve the 3-D isotropically geoelectric problem only. In fact, many theoretical analyse and field observations show that the upper crusts and sedimentary rocks often behave obvious anisotropy. The effect of anisotropy must be taken into account to correct analyze and interprete the EM datum, otherwise neglect of anisotropy will propably lead to incorrect result. Because of its complexity, the study about 3-D EM numerical simulation in anisotropic media is still not much executed up to now. In this paper, it will try to be developed the 3-D EM IE method in layered anisotropic earth and the corresponding key techniques, and to be analyzed the influence of anisotropy on the 3-D EM responses through numerical results obtained by the IE method in various environments so as to supply some theoretical basises for correct and new methods to interpretate EM datum in complex formation.Because dyadic Green's function plays a key role in the the IE algorithm, an efficient algorithm on it in the horizontally layered anisotropic formation has been studied. Through Fourier transform, the EM field is decomposed into two independent transverse magnetic wave(TM) and transverse electric wave(TE) in frequency domain and their solutions are analytically gotten by using transmission line and propagation factor so that it finally produces four analytical EM dyadic Green's functions in frequency domain. Then, the dyadic Green's functions in space domain are expressed in form of Sommerfeld integrals by using inverse Fourier transform. Value of the Sommerfeld integral is usually computed by a digital filtering algorithm, and the high order window function and continued fraction expansion technique is advanced to further enhance efficiency of Sommerfeld integral.Chapter 3 mainly discusses the basic principles of IE method for 3-D EM modeling in layered anisotropic earth. To discretize integral equation and obtain an approximation system, the electrical current dyadic Green's function is decomposed into two parts: one is the singular primary wave directly from the source and another a nonsingular waves reflected and transmitted from the interfaces.Value of the singular kernel of IE each node is computed by using equivalent volume element and surface integral methods. Then Bi-Conjugate Gradient (BICGSTAB) iterative method is introduced to reduce computer memory and improve computational efficiency based on Krylov subspace because the approximation system of the IE is complex and dense. Besides, in order to efficiently solve 3-D EM responses produced by several different anomalous bodies far from each other, a block over-relaxation iterative method is applied because it require smaller computer resource only. Furthermore, the contraction IE in layered anisotropic earth is derived by EM energy inequality so that the convergence of numerical solution can be always assured for any formation. Finally, numerical examples validate the efficient algorithms.Frequency sounding is an EM method using artificial source, it detects the change of rock resistivity with depth to understand geological structure by altering frequency of alternating EM field. This method has been applied for more than thirty years in our country and played a very important role in the geological research and national economy. In order to improve 3-D forward efficiency in the case of multi-frequency, a technique is introduced to optimize the initial solution for each frequency. Chapter 4 gives a lot of 3-D EM numerical results to investigate frequency sounding responses in layered anisotropic earth. The results show that anisotropy has obvious effects on apparent resistivity of frequency sounding and the intensitivity of both axial and equatorial system responses to 3-D underground anormal body is reduced with the increase of anisotropic coefficients.Controlled source audio-frequency magnetotelluric(CSAMT) measurement is commonly used artificial source such as grounding electric dipole(electric source) or non-grounded loop(magnetic source) to excite EM field, and two orthogonal components of electric and magnetic fields far enough from the source is observed to determine apparent resistivity so that it can still used the interpretation method similar to that of MT and AMT. However, unlike to the response of MT or AMT approach with natural source, that of CSAMT has often such many additional effects as non-plane wave effects, shadow effects, transmitter overprint effects and static shift effects even if the distance between the transmitter and receiver might be enough far. In Chapter 5 some numerical results in 3-D buried bodies in layered anisotropic earth are given to investigate the influence of anisotropy on the field source effects and static shift effects. Numerical results show that the anisotropy of formation will reduce the far field extent and enhance shadow effects and static shift effects of CSAMT so that the ability for CSAMT to detect abnormal information in the pseudo-sections will be reduced.In recent years, offshore oil and gas exploration has been become a new research subject with the increasing demand for oil and gas resources. Because of strong capacity to detect high resistance of the seabed strata, marine controlled-source electromagnetic (MCSEM) method has developed into an important means of exploration marine oil and gas resources. Chapter 6 establish an efficient algorithm to simulate 3-D MCSEM response in layered anisotropic seabed through the sub-domain multi-grid and quasi-linear approximation technique. Numerical results show that MCSEM responses are very sensitive to change in the horizontal and vertical resistivity of sedimentary strata undersea surrounding oil and gas reservoir. Horizontal resistivity of 3-D finite oil and gas reservoir has some effects on MCSEM responses, but vertical resistivity hardly influences MCSEM responses.In conclusion, the 3-D responses of EM measurements in anisotropic formation have been better understood through deep study on 3-D IE numerical modeling of EM and systematically numerical results in various complex environments. So it provides essentially theoretical basis for processing and interpretation of EM datum in complex formation. Thus it is theoretically and practically very usefull.
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