The transient electromagnetic method will be influenced by shielding effect of the low-resistivity layer during its execution in the area of low-resistivity overlaid layers. Although the value of observing data, such as the induced electromotive force, is relatively large, the exploring depth is greatly reduced actually. In case of an overlying low-resistivity layer, longer exploring time is required to explore the same depth, which is because as a conductive medium, the ground is dispersive, and the electromagnetic wave transmits faster in high-resistivity bodies but slower in low-resistivity ones. What's worse, the electromagnetic wave will consume more energy in the low-resistivity layers. By adopting the smooth pseudo Wigner-Ville distribution and Gabor expansion, this article expands the attenuation curves of induced electromotive force respectively of the typical two-layer (D- and G-type)geo-electricity models and three-layer (H-, A-, K- and Q-type) geo-electricity models to the two-dimensional time-frequency plane. It is indicated by the study through comparing the analysis on time-frequency energy spectrum with the model that energy of the transient electromagnetic field really aggregates and is consumed in the low-resistivity layers; especially with respect to the A-type geo-electricity model (with earth's surface being the low-resistivity layer) describing the North China type coalfield of our country, energy of the transient electromagnetic field has been depleted when arriving at the bottom interface (interfaces of Ordovician limestone and coal series) of the greatest observing significance due to aggregation and consumption in the low-resistivity layers above the base during the process of its diffusion downwards from the ground surface. Therefore, during the transient electromagnetic exploration in areas of strata in A-type model, influence of the low resistive shielding layer shall be taken into full consideration, and relatively longer observing time window shall be selected to ensure the exploring depth and high-power instruments shall be adopted to increase the signal-noise ratio during construction design.
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