Comments on pulse broadening phenomena of wavefield transformation in transient electromagnetic exploration
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摘要: 波场变换技术使瞬变电磁测深反演应用层析成像技术成为可能,但变换后得到的电磁场量波形随偏移距和地层电导率的增大而展宽,严重影响着反演的空间分辨率。本文通过导出的大回线圈在均匀无耗媒质中的阶跃响应之解析表达式,分析了大回线圈的辐射场波形宽度与场点坐标的关系。应用这一关系和大地感应电流空间分布的特点,讨论了波形展宽的原因,并指出,波场变换中波形展宽是由于大地中感应涡旋电流在空间上呈体分布的特性所造成的。Abstract: It is possible using ray tomography to explain the response data of transient electromagnetic (TEM) exploration when we use wavefield transformation technology.But the pulse shape broadening phenomena of transformed wave field with increasing the receiver offset distance and the earth conductivity reduce the space resolution of the interpretation significantly.In this paper, we obtained the expression of the electrical vector potential of the large loop antenna which is excited by the step current in uniform losses space.By this expression, we discussed the relationship of the response pulse width with the receiver offset distance and the media permittivity.By using that relationship and the distribution characteristic of the induction eddy current in the earth, we explained the pulse broadening phenomena.We find the pulse broadening phenomena in the fictitious loss media resulting from the space distribution characteristics of the induced eddy current in the earth.Because the propagation velocity of the eddy current is very slow than the light velocity, the broadening phenomena in the fictitious losses media is also very evident than that in the physical losses media such as free space.
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Keywords:
- TEM /
- inversion /
- wavefield transformation /
- pulse width /
- ray tomography technology
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