Study On Conductivity-Depth Imaging Methods Of Airborne Transient Electromagnetics

Because airborne transient electromagnetic(ATEM) has some advantages such as lower cost, wider detection, greater probing depth, etc., so it was widely used in mineral resources exploration, geological mapping, prospecting of water resources and environmental monitoring and other fields. Although ATEM inversion has gained a lot of research, the data volume of ATEM is large and the inversion calculation is very time-consuming, so the inversion interpretation has hindered the promotion of this method. Therefore, as a kind of fast imaging method, conductivity-depth imaging(CDI) method has become one of the key technologies of making ATEM practical. This paper studies on the two CDI methods of lookup tables and horizontal thin-sheet model and programs implementation, at the same time, the characteristics of these methods were analyzed. Specifically, the main contents are described as follows:(1) Airborne transient electromagnetic 1-D forward method. First, using the Maxwell’s equations, we can get the diffusion equation, and carried out the forward calculation formula of central loop’s airborne transient electromagnetic; then, using G-S transform and Hankel integral transform calculate the ATEM forward modeling. In my thesis, 12-point filters are taken in G-S transform, 120-point J0 and 140-point J1 in Hankel integral transform. The forward calculation formula establishes a theoretical foundation for the establishment of lookup tables.(2) The conductivity-depth imaging based on the lookup table of one dimensional forward. First, we established pseudolayer half-space’s lookup tables and time series’ lookup table; then, using cubic spline interpolation method interpolates in the lookup tables, we can implement conductivity query from the electromagnetic response value; finally, we can get the conductivity-depth imaging by using the different calculation formulas of apparent depth. At the same time, we analyzed the imaging results of the two lookup table methods in the 1-D and 2-D geoelectric models.(3) The conductivity-depth imaging based on horizontal thin-sheet model. First, we analyzed the electromagnetic forward response of the horizontal thin-sheet model, and carried out the theory of the conductivity-depth imaging by the forward response; then, we used the 2.5D ATEM forward program of Maxwell software to acquire the electromagnetic simulation data; finally, using the horizontal thin-sheet model conductivity-depth imaging method processes the electromagnetic simulation data, and analyzes the imaging results.(4) Imaging and analyzing for the measured data of ATEM. First, using our Helicopter ATEM data processing and interpretation software system processes the measured data to get the ATEM section data; then, we used the pseudolayer half-space CDI method and horizontal thin-sheet model CDI method to image the ATEM section data; finally, we analyzed the imaging results to confirm the effectiveness and correctness of the two imaging methods.