The earliest estimates of the conductivity of the earth were based on the analysis of Sq. The picture that emerged from analyses performed up to 1940 was that of a shallow conducting layer at the surface underlain by a few hundred kms in which the conductivity is low followed by a region of very rapid or discontinuous increase of conductivity. The overall pattern now considered most likely is not essentially different, but is now realised that the period range spanned by Sq is too small. Furthermore, when trying to match a radial model of conductivity, Sq data is biased by conductivity anomalies. Longer period variations seem to be less subject to surface conductivity irregularities.
Important progress has been made towards estimating the conductivity structure beneath oceans; it seems to be very different from that under continents. The work of Larsen on Oahu indicates a distribution in which the conductivity is an order of magnitude greater for the top 300km. This, and similar work off the coast of California, have done much to elucidate the behaviour of the geomagnetic field at the edge of oceans.
Sq periods have proved valuable in investigating conductivity anomalies also explored by shorter period variations. Generally anomalies that influence the field at sub-storm periods also appear at diurnal frequencies, but “dead-Z” anomalies of the Tuscon type often do not.