Magnetovariational soundings across the South Island of New Zealand: difference induction arrows and the Southern Alps conductor

Abstract

Magnetovariational (MV) data from 14 sites on a transect of the South Island of New Zealand are presented. The induction arrows clearly show the effects of induction in the surrounding oceans. Comparison with the responses of an analogue model of the effect of the oceans on induction in the New Zealand region indicates that the induction arrows also contain information on anomalous on-land conductivity structure. The use of difference induction arrows to remove the effect of induction in the oceans from the observed responses is validated using both two- and three-dimensional numerical models pertinent to the New Zealand situation. Difference induction arrows calculated by subtracting the analogue model results from the observed data show a clear reversal, in both real and quadrature arrows, across the South Island. Two-dimensional numerical modeling of the projections of the difference arrows on to the transect shows that the response is consistent with the existence beneath the Southern Alps of an anomalously conducting region previously identified by magnetotelluric (MT) sounding. Use of the difference arrow technique is seen to be valid over a large period band because of the reduction in the coast effect response range as a result of the island situation.