On the behaviour of the induction arrows over a buried conductive plate — a numerical model study

Two-dimensional numerical model calculations, employing a finite difference technique, are used to study the behaviour of the induction arrows, for a range of periods, for a conductive plate of (i) semi-infinite and (ii) finite width in uniform and layered resistive hosts. The results for the conductive plate at the surface of the host have application to a uniform-depth ocean, while the results for the plate buried at some depth in the resistive host have application to a conductive sill in a resistive Earth. The numerical results indicate that for a profile over the plate-host vertical interface the in-phase arrows for all periods and locations point towards the conductive plate, while the quadrature arrows at periods near the characteristic period of the model are oppositely directed on either side of the interface so as to point towards each other and towards the interface for nearby locations, both over the conductive plate and the resistive host. Further, the quadrature arrow undergoes a second reversal over the resistive host at a distance from the interface that is somewhat dependent on the period. Thus, at either side of the location of this second reversal, the quadrature induction arrows are again oppositely directed, but pointing away from each other, with the arrows near the interface pointing towards, and the more distant arrows pointing away from the conductive plate. The period range for the quadrature-arrow reversal is characteristic of conductivities and layer depths. The features of the quadrature-arrow sign reversals at and near the interface are in accordance with the earlier laboratory analogue model results of Hebert et al. for the Newfoundland coastal region and Nienaber et al. for a conductive plate in a resistive host.

It is suggested that in practice the sign reversal of the quadrature arrow may aid in locating a conductor-host interface, and that if the conductivity of the host is known, the maximum in the anomalous vertical magnetic field response may permit an approximate determination of the conductive-layer depth.