The interfacial E-fields produced by S-waves emanating from a vertical seismic source are, however, expected to be about an order of magnitude smaller than those produced by the P-waves emanating from the same source (Warden et al. 2013). In addition, we note that magnetic measurements would suffer interference from seismic shaking of sensors in the earth‘s magnetic field. For those reasons, field trials have focussed on the use of P-waves and electric field receivers, although magnetic fields associated with S-waves have been observed during an experiment in an ultra low-noise laboratory (Bordes et al. 2008).