| Recently, MCSEM (Marine Controlled Source Electromagnetic Method) hasbecome research focus in sea energy exploration, resource exploration, and marinetarget detection. Three-Dimensional Numerical Simulation of time domain is animportant branch of MCSEM and a challenging scientific problem. In this paper, FDTD(Finite-Difference Time-Domain) simulation for mcsem is carried out with the supportof a major national project.On the basis of predecessors’ study of the basic theory and key technology to theland csem exploration with FDTD method, a comprehensive study of the applicationin MCSEM has been made in this article, including:1. As a testing standard and basis of the validity of numerical simulation resultsof fdtd, for the characteristics of MCSEM, source inspired in conducting space, paperfirst exports analysis formulas of step current response, truncated current response,pulse current response and time-domain response of arbitrary current type of electricdipole in the whole space, half-space, studies the relationship of the step currentresponse of electric dipole, with the resistivity of the media, propagation time andrange in the entire space. Rather more, draws spatial distribution pattern of each fieldcomponent of electric dipole in Cartesian system.2. Numerical solution of electric dipole field in the layered conductive space isgiven and compared with analytical solution in whole space and half space to verifythe correctness of the program.3. The control equation of FDTD method suitable for marine CSEM, time andspace division method, electromagnetic iterative difference formula is studied, anddiscussed the key technology of various boundary conditions, stability, numericaldispersion and current source,3D Du Fort-Frankel derived format of diffusionequation,and expression of virtual dielectric constant are derived. Finally, the program is compiled and numerical experiments are carried out.4. The validity and reliability of the marine FDTD method was conducted tovalidate and analyze. Field distribution of electric dipole in space is calculated withGauss pulse of transmission signal. The results show that the field is ellipsoid-shapedin the early stages, later is spherical, and the numerical results are similar to thepatterns of analytical ones; Gauss impulse responses in the whole space werecompared with analytical solution of pulse response, Timing is matched very well;Gauss pulse response of marine conductive half space was calculated and comparedwith analytical solution of impulse response in half space, Timing is matched verywell; Electromagnetic field of the ocean three space is calculated, for the situation ofcontaining high resistance layer and low resistance layer. Resistivity anomalyinterface is clear, and the results are consistent with prior knowledge. All these resultsindicated that parameters of FDTD is tunned appropriately, virtual velocity iscontrolled well, that verify the correctness and effectiveness of FDTD method inMCSEM.5. According to the electromagnetic exploration model of towed MCSEM inshallow sea,3D numerical simulations have been done with the FDTD method for themarine high resistance body as oil and gas and other low resistivity reservoirs as metalmineral resources of one-dimensional, two-dimensional and three-dimensionalstructure. Anomaly analysis of forward data using a variety of methods, to illustratethe effectiveness of the FDTD method in the numerical simulation of MCSEM..6. As for the electromagnetic exploration model of combination of air and sea,firstly, frequency domain numerical simulation of one dimensional and2.5dimensional is done to verify the effectiveness of the model. Then,3D modeling withFDTD is carried out respectively on high resistivity anomaly of the marine oil and gas,and low resistance anomaly of and metal ore. Last, abnormal analysis are done toforward data of3d modeling using various methods, and the results were comparedwith that of one-dimensional2.5dimensional frequency domain calculation. Twoconclusions can be maken, first of all, the joint exploration model of sea and air iseffective, and in addition, the numerical simulation method of FDTD is effective forthe joint exploration of sea and air.This paper has obtained the following innovative results:1. Export analytical formula of time-domain response of arbitrary electric currentof electric dipole in the whole conducting space and in a half space of conducting.Numerical algorithm of time-domain response of arbitrary electric current of electric dipole in layered conductive space is given.2. The FDTD method is first applied in marine controlled source electromagneticexploration successfully. The key parameters such as equation, stability, numericaldispersion, boundary conditions, virtual dielectric constant, and the initial time arediscussed; much numerical experiments have been done, and the experimental resultshave a good agreement with analytical results, and verify its effectiveness. Fill a gapin the time domain numerical simulation of MCSEM. The paper use the Gauss pulseas transmission signal, and add it to the iterative equation directly, avoidingcomplexity and limitation of the previous method about source, in which analyticalresults are added to electromagnetic fields iterations as initial conditions. Unlike thelinear current signal, Gauss pulse has more important research value and practicalvalue. First of all, it is a pulse, with broadband characteristics; secondly, the risingedge of it can be used to simulate the step current, and falling edge of it can be used tosimulate the truncation current. While strong numerical dispersion can not be causedusing Gauss pulse rather than using step current and truncated current, which willgenerate high-frequency oscillations at early and late time; also, Low frequencyspectrum of the Gauss pulse is rich and easy to be controled and adjusted. Therefore,Gauss pulse is very suitable to be used as signal source of marine electromagneticnumerical simulation.3. Using FDTD method, simulate the propagation of electromagnetic waves andthe interaction with different resistivity anomaly of one-dimensional, two-dimensional,three-dimensional structure in the ocean. Space-time distribution of the whole spacefull time marine electromagnetic field is given, which has a certain reference value forthe marine electromagnetic research and data interpretation.4. One-dimensional,2.5dimensional frequency domain methods are combinedwith3D FDTD method to verify the validity of the air-sea electromagneticprospecting model.The research results of this paper have reference value to the marineelectromagnetic exploration, design, numerical simulation and detection of marinetargets. More over, they have the theory significance and the important guiding role ofCSEM and MCSEM. |
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