Time-frequency analysis on influence of low resistive shielding layer in the exploration of transient electromagnetic field

The transient electromagnetic exploration method is often influenced by low resistive shielding layers,resuting in actual observation depth shallower and more time required.By adopting the smooth pseudo Wigner-Ville distribution and Gabor expansion,the paper expands the attenuation curves of induced electromotive force 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;energy of the transient electromagnetic field is depleted when arriving at the bottom interface(interfaces of Ordovician limestone and coal measure) of the greatest observation significance due to aggregation and consumption in the low-resistivity layers above the base during the process of its propagation 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 ra-tio during the design.