Research On Airborne Transient Electromagnetic 2.5-D Forward Modeling

Airborne Transient Electromagnetic (ATEM for short), is one rapid exploration method which is developed in recent years and has characteristics such as lower-level flight, deeper exploration and wider airworthiness range. This method is widely applied in the areas such as forests, mountains, gobis, marshes, lakes and shallow seas and is a promising one. At present, ATEM only can do 1-D data processing and interpretation, which seriously hamper the development of this method. Therefore, funding by the national 863 high-tech porject 'ATEM fast forward modeling and invertion', this paper aims at the research on ATEM 2.5-D finite element forward modeling in order to offer theoretical base for people to understand the transient anomalous features of the two or three-dimensional geological target.First, starting from the time-domain electromagnetic theory, the author constructs the laplace-fourier domain variational question of anomalous field due to step current loop in the air, and uses structured triangular mesh to solve the finite element equation. Then the author adopts Gaver-Stehfest transform and sine transform digital filtering algorithm to realize the transition from laplace-fourier domain to time domain, and gets induced voltage with high precision by improving the numerical derivation way. By the derivation of laplace-fourier domain background electromagnetic field from transmitter loop in the air, the author also calculates vector of right hand of finite element equation to realize magnetic source in the air. In the process of calculation of background field and anomalous field, different numeric algorithms overlap, thus accumulated errors are easily made. As a result, the author analyzes the calculation process and gets the conclusion that the relative error in the calculation meets the accuracy requirement.Moreover, the author does numerical modeling on the typical 1-D and 2-D geoelectric models and analyzes their response characteristics respectively. The results of typical 2-D models such as equiaxued block conductor, vertical plate-like conductor model show that integral anomaly characteristics are in accordance with 1-D response characteristics. Their early-time amplitude all become small along with the gradual increase height of the coil leaving from the earth surface and the decay rate becomes slow. The scale and the attitude of the local anomalous body can be realized from the response, but the laws are complicated and needs more analysis and research.At last, the author does optimization on the calculation of ATEM 2.5- D finite element method forward modeling. Besides, by designing coarsness parallel algorithm in MPI+Fortran programming environment, the author preliminarily solves the problem of too-long calculation time.