Limitations in the use of spherical harmonic methods for deep conductivity determinations

Diurnal variations supply only four frequencies with sufficient power to be useful, giving an overall frequency ratio of only 4. Also only permanent observatories supply sufficient accurate data, and their distribution on the Earth is very irregular. More importantly, the assumption implicit in deep conductivity determinations, that the conductivity is a function of depth only, breaks down because of near surface conductivity anomalies.