Study On Inverse Synthetic Aperture Imaging Of Ground-airborne Transient Electromagnetic Method

Ground-airborne transient electromagnetic method is still in the research stage at present. Although some equipments are available and have come into practice for investigating volcanic structures, metal deposits, saltwater intrusion and water resources, most of their interpretation methods are based on ground transient electromagnetic method. The lack of corresponding interpretation technique restricts the development of this method. In this paper, we aim at realizing three-dimensional interpretation of ground-airborne transient electromagnetic method and do research on three scientific problems, they are full field apparent resistivity, Kirchhoff surface migration and inverse synthetic aperture imaging.Firstly, apparent resistivity is studied in our research to promote ground-airborne transient electromagnetic method as this parameter can intuitively and clearly show the underground electrical variation. Full field apparent resistivity iterative algorithm of magnetic induction intensity, which has no limitation in time gate, offset or flight height, is proposed based on the inverse function theorem. That is, it applies to full time zone and full space zone apparent resistivity calculation of both single-source and multi-source ground-airborne transient electromagnetic method. Results of layered model show that full field apparent resistivity can completely, smoothly and gradually tell the electrical variation of the designed model. And the early time and late time curve can reach the resistivity of the first and last layer, respectively. As thickness and resistivity of the layered model change, apparent resistivity curvres have obvious differentiation and can easily recognize underground electrical layer. Parameter offset and flight height have little influence on full field apparent resistivity.Secondly, considering the difficulties in three-dimensional electromagnetic inversion, such as its slow computation speed, Kirchhoff surface migration is chosen for three-dimensional interpretation of ground-airborne transient electromagnetic method. New algorithm of velocity model construction, which is the key problem in minute imaging, is put forward on the basis of floating plate theory. With this algorithm, higher precision velocity analysis can be achieved with no limitation in survey point position or source shape. Migration results of three-dimensional models show that wave field signal is strong where interface has large vertical inhomogeneity. And it has good indication of the geological abnormity body.Thirdly, inverse synthetic aperture imaging is built up with the Inverse Synthetic Radar(ISAR) to improve the deep weak abnomormity separation capability of ground-airborne transient electromagnetic method. Based on the correlation stack method, multi-survey point synthesis in a synthetic aperture is realized, which spreads traditional single-survey point data processing to pointwise and multi-coverage data processing. Results of three-dimensional models indicate that inverse synthetic aperture imaging can narrow down the range of anomalous body and more clearly depict the anomaly boundary. Inverse synthetic aperture imaging is verified that it can strengthen useful signal, improve signal-to-noise ratio and enhance resolution.We concentrate on the three parts mentioned above in this paper. Our work provides a new way to spread ground-airborne transient electromagnetic method in practice and enriches the three-dimensional interpretation system of ground-airborne transient electromagnetic method. Part of our research production gives a new idea for ground-airborne transient electromagnetic inversion and lays the foundation of its three-dimenional inversion.