| As an efficient and economical geophysical method, airborne electromagnetic method is widely applied in the fields including geological mapping, mineral exploration, hydrologic geology, engineering geological prospecting and environment monitor etc. Towed bird helicopter transient electromagnetic system (HTEM) has many characters of good horizontal resolution, convenient for explanation, low cost and high efficient, big effective exploration depth, so it is obtained sustainable development. Because of the calculation amount in2D problem is smaller than that in3D problem, and also it is more similar with true geologic condition than1D problem, so this thesis mainly focuses on theoretical study, combining sub-project of the funding by the national863high-tech project "Helicopter time domain airborne electromagnetic survey system practical research", we analyzed and researched the2.5D forward response curves for towed bird helicopter transient electromagnetic method, and the studied contents were shown in the following:1. Expanded on the background and source in this thesis, and introduced the development history and application state of airborne electromagnetic, described the development and research current state of ground and airborne transient electromagnetic2.5dimensional forward modeling, and indicated study meaning of airborne transient electromagnetic2.5dimensional forward algorithm.2. Researched the forward modeling of airborne transient electromagnetic method. Be using the Laplace-Fourier transformation, we constructed the dual second order differential equations along the strike of geologic body according to the time domain Maxwell equations, and detailed deduced the boundary conditions in the Laplace-Fourier domain and compiled variational equations of HTEM finite element forward modeling. In the curved boundary element, by using double quadratic interpolation and element analysis we got extreme value equation of functional for one single element. Summing each element integral we can obtain whole extreme value equation set of functional in Laplace-Fourier domain. In finally, the results of equations should be processed by inverse Laplace-Fourier transformation, and then we can get the electromagnetic components for each of nodes in2D spatial of time domain.3. Discussed some key problem of HTEM2.5dimensional finite element forward algorithm, such as calculation of background field, inverse Laplace-Fourier transformation and wave-front method of linear equation set. In which, background field model is layered earth; with the electromagnetic boundary conditions, we have deduced the Laplace-Fourier resolution of spatial magnetic dipole source excitation. For the Laplace-Fourier transformation, in frequency domain,28frequency points are calculated from1Hz to100kHz, each frequency point can be decomposed into21wave-numbers of the wave-number domain. Wave-front principle is an enhanced Gaussian elimination method, in the processing elimination and integration are conducted parallel. So in the processing of solving linear equation set, it occupies less inner memory of computer.4.Simply studied response curves of1D layer model. Also comparing the2.5D response curves of K-type and H-type geoelectric model, the results shown that2.5D algorithm used in this thesis is right.5. Detailed analyzing response curves of2.5finite element forward modeling. For different device types, device parameters and earth’s model, we compared and analyzed their theoretical response curves, and studied changing discipline of HTEM response curves under certain condition. According to the forward modeling results, they indicated that helicopter airborne transient electromagnetic system has higher horizontal resolution, good responses for the anomalies with different inclinations and bigger exploration depth. |
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