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Attempts to use discrete sferics for low-frequency impedance mapping have relied on Fourier analysis to derive time-independent frequency profiles of the earth. A one-dimensional transmission line model of the earth above a perfectly conducting layer is used to demonstrate that the impedance is time dependent. Assuming a sferic-type transient for the applied magnetic field, the electric field is changed significantly corresponding to the time of flight of the radiation in the lossy earth. In an attempt to maintain this timing information and its link with the frequency components, the wavelet transform was implemented to analyze the data. The associated surface impedance may be approximated as the input impedance of the earth during excitation but reverts to the impedance of free space once the source field returns to zero.
Date of Publication: April 2005