Time-domain Airborne Electromagnetic Simulation And Key Technologies

Airborne Electromagnetic Method(AEM) is a kind of inductive electromagnetic(EM) prospecting method. Due to its moving platform, compared to conventional ground EM methods it is is convenient, fast, without needing ground accessment etc, especially suitable for exploration of oil and gas, minerals, groundwater and geothermal resources and environment and engineering in complex terrain conditions, like mountains, deserts, swamps, lakes, forests, etc. At present, time- domain airborne EM data processing like imaging and inversion for one dimensional(1D) model has been very mature, but it can only handle very simple geology and mostly work on off-time data, losing abundant information from the on-time data. The imaging or inversion based on 1D model cannot accurately resolve the underground three dimensional(3D) targets. In addition, the actual formations often shows the characteristics of anisotropy, the interpretation of anisotropic data using isotropic models can produce large deviations. To improve the airborne EM data processing, in this thesis I carried out 3D anisotropic modeling for full-time airborne EM systems in addition to the forward modeling of 1D full wave and investigated key techniques of airborne EM method in time domain.First, I carry out researches on 1D modeling of full-time responses for a time-domain AEM system. The frequency-domain responses for a vertical magnetic dipole are calculated with the fast Hankel transformation algorithm and via a Fourier transform, I obtain the time-domain full-time responses. Considering the instability of impulse response at early time channels, I propose a method of convolution between step responses and transmitting waveforms to achieve the full-time responses for arbitrary transmitting waveforms. The theoretical results show that this algorithm is stable and accurate, the responses of B and d B/dt are well consistent in the format of integration/derivative relations. This establishes a good basis for full-time modeling of complex 3D underground electric structures.Further, I present in this thesis a 3D time-domain AEM full-time modeling technique in an arbitrarily anisotropic medium. The conductivity tensors are obtained by an Euler rotation. The governing equations for the frequency-domain secondary EM field in an arbitrarily anisotropic medium have been derived based on generalized variation principle. Then, I discretize the objective region with hexahedrons and use vector linear interpolation functions to make finite element(FE) analysis to achieve the discretization of the governing equations. I further combine all the elements to form a large sparse matrix equations and solve the equations system with an efficient Multi-frontal Massively Parallel Solver(MUMPS). Finally, I take the transformation algorithm from frequency-domain to time-domain and the convolution to get 3D time-domain AEM full-time responses for arbitrary transmitting waveforms. By comparing the results with analytical solutions and analyzing results for typical 3D structures, I demonstrate the accuracy and feasibility of the algorithm. Finally, I analyze the influence of anisotropy on 3D time-domain AEM responses.Some key techniques in time-domain AEM modeling have also been studied. I make numerical simulations on 3D time-domain AEM full-time responses for different transmitting waves(half-sine, trapezoid, triangular, square and multi-pulses) and compare the EM response for each single transmitting waveform. The exploration capability of a multi-pulse system has also been analyzed. I further study the exploration capability of different AEM systems, different field components and different transmitting waveforms under two base frequencies of 30 and 90 Hz. Then I study the influence of flight height on time-domain AEM responses by analyzing the errors for different flight heights and comparing the canopy and deciduous pseudo-layers under uniform half-space model and the abnormal body model. Finally, I simulate the effect for arbitrary attitude changes of transmitter and receiver over a homogeneous half-space model. Results show that we need to reasonably choose waveforms and set up the measurement parameters to improve the exploration capability in AEM practical survey. It is a good choice to use multiple pulses as transmitting wave and to sample both on-time and off-time channel signal and make the accurate measurement of altitudes of transmitter and receiver.I hope that the research in this thesis can provide reference and theoretical basis for time-domain airborne EM system design and data processing and interpretation.