The Schumann resonance waveforms in the lower ELF band (5-100 Hz) are produced within the Earth-ionosphere cavity by distant lightning discharges; they provide a useful source field for shallow audiomagnetotclluric (AMT) crustal sounding. In this study we investigate their waveform characteristics that are important to the assumptions of AMT sounding. A time-domain polarization analysis technique is applied to a variety of examples of Schumann resonance waveforms. The multiplicity of worldwide thunderstorm centres provides a background activity which is generally incoherent and, accordingly, displays incoherent polarization characteristics. Superimposed on the background are larger-amplitude transient events (sferics) from individual thunderstorm centres; they represent the response of the Earth-ionosphere cavity to very large lightning discharges and are generally linearly polarized at a given azimuth. The analysis indicates that the Schuman resonance waveforms provide a plane-wave source field, as required for electromagnetic crustal sounding. The differences in the "received" characteristics between the two waveform types prompted an investigation of the extent to which the waveform type and its particular polarization characteristics influence the determination of a geoelectric sounding curve. A detailed study, carried out with a multivariate maximum entropy spectral analysis algorithm, indicates that the two types of Schumann resonance waveform provide repeatable and consistent results at the 95% confidence level and that the linear polarizations associated with the sferics do not influence the estimation of the Earth response
