Magnetotelluric measurements in the Wellington region

Abstract

Magnetotelluric measurements have been made at three sites on a northeast-southwest profile across the Wellington region. Apparent resistivity curves and induction arrows for the three sites have been calculated. Significant differences occur in the amplitude of the apparent resistivity curves between two sites to the east of the Wairarapa Fault and a site just inland from the west coast. Along with a reversal of short period real induction arrows, this suggests the existence of a major boundary in electrical resistivity structure, probably associated with either the Wellington or Wairarapa Faults. One-dimensional modelling of the apparent resistivity curves indicates that the two sites to the east of the Wairarapa Fault have a conducting layer at the surface. At the eastern site, the thickness of this layer agrees with the depth to the top of the subducted Pacific plate ascertained from seismic studies. In contrast, to the west of the fault, the surface layer has a resistivity a factor of 1000 times greater.

Keywords:

• magnetotelluric surveys
• induction
• electrical conductivity
• Wellington