Study on multi-resolution imaging method of urban underground space based on high performance transient electromagnetic source

High-precision and multi-scale detection of urban underground space is the prerequisite for the rational development of urban underground. Due to the particularity of the urban detection environment, the detection method needs to have the characteristics of strong anti-interference ability, good resolution ability and differentiation of multi-scale underground targets. The existing geophysical methods cannot meet the needs of urban underground space detection. In this paper, the scales of underground targets vary greatly in urban underground space, and traditional transient electromagnetic methods (TEM) are difficult to distinguish the multi-scale targets. As an effective forward modeling method, the three-dimensional vector finite element method can analyze the emission characteristics of TEM. In the previous article, the author discussed in detail the emission characteristics of transient electromagnetic high-performance radiation sources, and used transient electromagnetic differential pulses for pulse scanning detection, and demonstrated the effectiveness of differential pulse scanning detection by comparison. This paper performs the wave-field transformation by multi-windows scanning on the detection data after the differential pulse-scanning detection. The wave-field transformation by window scanning is an important method to improve the resolution of detection. The window scanning can highlight the response of the corresponding depth and scale. By changing the size of the window, the response of different depths and scales can be highlighted, and multi-scale information extraction can be realized. In order to improve the resolution of the wave-field, the multiple coverage technology is used to process the wave-field. Finally, pseudo-seismic migration imaging is performed on the wave-field after multiple superposition to realize high-precision detection of urban underground space. In summary, with the assistance of high-performance transient electromagnetic radiation sources and multi-resolution pulse scanning technology, the wave-field conversion is improved accordingly. While enhancing the detection information, the multi-resolution response information is extracted to improve the resolution. However, the structure and material of the urban underground are complicated, the corresponding electrical parameters and electromagnetic field velocity are relatively complicated, and further research is needed for the velocity analysis of the wave-field ; secondly, the calculation stability of the wave field transformation needs to be further strengthened. The solution method of the wave-field transformation equation needs to be further improved to make sure the stability of the calculation ; finally, the imaging method using the combination of apparent resistivity and migration imaging is the future research after this article.