层状介质中地下瞬变电磁场全空间效应

Whole-space Effect on Underground Transient Electromagnetic Field in Layered Media

全空间效应是矿井瞬变电磁方法固有的特殊理论问题之一.建立了3种典型的层状地质模型,采用时域有限差分法求解Maxwell方程,并引入电磁波衰减因子EA(dB),来分析全空间效应的影响特征及影响因素.研究结果表明,地下瞬变电磁法工作装置中,分离回线装置对低阻目标层的探测效果优于重叠回线装置;相对于半空间低阻层瞬变响应,全空间低阻层瞬变响应的时间范围更宽,因而需要更长的观测时间;全空间效应影响因素为顶、底板内低阻层相对电阻率差异大小和目标层深度与厚度,当目标层电阻率大于顶板内低阻层电阻率时,所观测响应主要为低阻层响应,此时将会增大资料解释的难度.此外,研究还显示,瞬变电磁法对低阻层的纵向分辨率与低阻层厚度相关,厚度越小分辨率越高,当厚度较大时,其纵向分辨能力较差.研究成果为实际工作参数选择及资料处理与解释提供了参考.

Whole-space effect is an inherent attribute among many special theoretical problems in Mine transient electromagnetic method （TEM）. Three typical layered models were built and their Maxwell equations were solved by FDTD method, and electromagnetic attenuation factor EA（dB） was introduced to analyze the whole-space effect. The results indicate that, among the common underground TEM configurations, the availability of detecting conductive layer using $1ingram configuration is better than using coincident configuration. Compared with half- space transient response of conductive layer, the time range of the response in whole-space is much wider, so it needs to choose much longer acquisition time in actual measurement. The influence factors of whole-space effect are related to the resistivity contrast of conductive layer in roof and floor, the depth and thick of the objective layer. When the resistivity of objector is higher than that of roof media, the response of the later will become the main response, and this will cause great difficulty for data interpretation. Besides, vertical resolution to conductive layer has a closed relationship with the thickness of the layer, it will get satisfied vertical resolution when the thickness of layer is small but it will achieve terrible vertical resolution when the thickness of layer is big. These results are helpful reference for the design of measurement parameters and data processing and interpretation.