Transient surface impedance measurements using spherics — Source identification and characterisation

Transient recordings of the electric and magnetic fields over the audio-band display significant transient pulses (spherics) which can be used for surface impedance measurements providing the lighting strike is distant and predominantly vertical. Horizontal lighting strikes, other atmospheric noise and instrument noise creates incorrect surface impedance data. Automated spheric selection can be made by calculating the sliding window correlation coefficient, and observing a period of elevated values followed by a rapid transition to a negative value. Data selected in this manner yield coherent surface impedance values.