Research On CSAMT2.5-D Forward Algorithm And Response Anomaly

Controlled source audio magnetotelluric method which had manually controlled source ofelectromagnetic sounding method was based on the theory of magnetotelluric method, bychanging the frequency of transmitting and receiving electromagnetic waves to detectunderground typical geological bodies at different depths characteristics. In practical work,artificial electromagnetic emission source was three-dimensional, and ground geologicalstructure of the target along strike often had small changes, so it could be regarded astwo-dimensional. This three-dimensional electromagnetic field source, the two-dimensionalnumerical simulation of ground CSAMT structure, called CSAMT2.5dimension forward.Numerical simulation process starting from the Maxwell equations, combined with theequation of state, in the y-direction conductivity constant use of Fourier transform spatial domainwas transformed into wave-number domain. In the wave-number domain, used the finite elementmethod to obtain the various components of the wave-number domain electromagnetic field. Thewave-number domain electromagnetic value of each component was converted into thefrequency domain by cosine Fourier transform, so as to achieve the purpose of CSAMT2.5dimensional forward.When solving Maxwell’s equations in the frequency domain electromagnetic field wasdecomposed into a field and the secondary field, so that the three-dimensional field source wasreduced to the two-dimensional field source. In solving electromagnetic wave-number domainby the finite element method, the boundary at infinity convergence and wave number selectionare the two issues could not be avoided. This paper introduced an unlimited truncation unitreplaced the boundary conditions at infinity such that the value of the electromagnetic fielddecay rapidly to zero, so that the range of reducing the mesh also meet the requirements ofprecision, improved forward speed. Wave number selection was based on the curve characteristicvalue of real and imaginary parts of the electromagnetic field, according to the number ofintervals selected to minimize the number of selected wave number, in order to reduce theamount of the corresponding calculations, numerical simulation to ensure accuracy, improvework efficiency.The design of the three typical geoelectric models, application of these research CSAMT2.5-dimensional numerical simulations carried forward algorithm. CSAMT2.5-dimensionalforward to a high-impedance anomaly reflected a more sensitive response anomaly wasrelatively obvious and clear analog position。Of low resistivity anomalies can have a goodreflection of the position but obviously extends down。Numerical simulations of individual anomalies and anomalies combination of the two models had similar response characteristics andabnormal variation. Thus confirming this study CSAMT2.5-D algorithms were not only able toeffectively simulate a simple geological geoelectric model, the more complex model for groundgeological body could be well simulated, and obtain a relatively reliable response anomalies.