An application of thin-sheet electromagnetic modelling to the Tasman Sea

Abstract

The method of thin-sheet approximation is invoked for the ocean layer to model and interpret magnetotelluric (MT) data observed on the floor of the Tasman Sea, between Australia and New Zealand. A technique to remove 3D distortion from the observed seafloor MT data is developed from the thin-sheet model MT responses; the ‘de-distorted’ data then appear isotropic, and are reinverted using 1D procedures. This approach thus assists interpretation through a combination of 3D forward-modelling procedures and 1D inversion. A seafloor sediment layer is shown to have a strong effect on observed seafloor MT data. Also, an exercise is carried out to compare the results given by the vertical gradient sounding (VGS) method with seafloor MT data. The electrical conductivity structure beneath the Tasman Sea is analysed in terms of an upper-mantle lithosphere and asthenosphere, and a highly conducting lower mantle. The observed anisotropy in seafloor MT data is diagnostic of a lithospheric conductivity of less than 10 −4 S m −1. Conductivity rises by two orders of magnitude below 80 km, to greater than 10 −2 S m −1, and is probably related to an asthenospheric layer in the upper mantle. Seismic interpretations similarly place a low-velocity zone below a depth of 70 km. At a depth of approximately 400 km, the conductivity is approximately 1 S m −1, consistent with global estimates for the lower mantle. There is little evidence for major change with age in the structure across the Tasman Sea.