Is ‘tectonic interpretation of conductivity anomalies’ a useful concept? Probably yes, if over-simplifications are avoided. For example, there are cases where a high-conductivity layer in the mid crust also exhibits anomalously high reflectivity, and the combination electromagnetics and seismics has been advocated as a tool for ‘detecting’ water in the crust. However, application of the techniques described in Chapter 8 and petrological considerations suggest that, in old, stable regions, the lower crust is generally dry.

The combination of long-period MT and surface-wave seismology have revealed concomitant directions of seismic and electrical anisotropy, apparently supporting a hypothesis involving alignment of olivine crystals. How is such an alignment realised and maintained?

A collaborative project involving geodynamics, seismology and long-period MT sounding has been established in search for an intra-plate plume under Central Europe. The seismological part of the experiment suggests some evidence for a 150–200 K temperature increase in the uppermost 200 km within a 100 km by 100 km region. However, not only is the electrical conductivity within that volume only moderately increased, but also the strongest conductivity increase occurs outside the velocity anomaly. In addition, the electrical anomaly exhibits strong electrical anisotropy. Is this another hint that if melt is present, then this melt is not the dominant conductor, and that the bulk conductivity of the mantle is more strongly influenced by another conductive phase?

Few ocean-bottom MT studies have been performed and investigations into possible differences between the physical compositions of oceanic and continental mantle have begun only recently.