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The depth of penetration into a conducting medium, where sinusoidal surface excitations generate peak magnetic fields or derivatives equal to those of an impulse-type surface excitation, are calculated. Finite pulses generate fields similar to those of a surface impulse provided they are sufficiently short. The maximum permissible pulse duration is proportional to the square of the specified penetration depth. In order to exhibit minimum attenuation, the excitation fields must be unipolar over a time period comparable with the transient duration. Such fields may be generated within the induction field of the source.