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The Tasman Project Of Seafloor Magnetotelluric Exploration

  • Author(s): Ferguson, Ian James
  • et al.
Abstract

The Tasman Project of Seafloor Magnetotelluric Exploration was performed between December 1983 and April 1984 in order to investigate the electrical conductivity structure beneath the Tasman Seafloor and the Australian continental margin. Recordings were made at nine seafloor and nine land sites on a line extending from inland Australia to the Lord Howe Rise in the eastern Tasman Sea. Magnetic field recordings were made at all sites and horizontal electric field recordings at seven of the seafloor sites. In addition, oceanographic recordings were made at several of the seafloor sites, as an additional aim of the project was to provide physical oceanographic information on the Tasman Sea. Data return from the experiment was almost complete and the data quality high. All of the raw recordings have been converted into final magnetic horizontal electric field and ­oceanographic time series.

An analysis of the seafloor magnetotelluric (SFMT) data has been completed. The 'results indicate that geomagnetic induction in the Tasman Sea is a three-dimensional process dependent on the large-scale shape of the Tasman Sea. The seafloor impedances have a dominantly two-dimensional form with the B-pol impedance component (the component perpendicular to the trend of the Tasman Sea) being strongly attenuated near the Australian coastline. Evidence for the three-dimensionality includes large impedance skew-angles and consistent differences between impedances estimated using the SFMT and vertical gradient sounding (VGS) methods. The spatial consistency of these results supports the hypothesis of large-scale geomagnetic induction in the Tasman Sea. The induction arrows calculated for land and seafloor sites near the Australian coast exhibit significant components parallel to the coastline, providing further evidence for a three-­dimensional process.

Evidence suggests that the most appropriate data to analyse using one-dimensional MT techniques are the E-pol impedances from three sites, TP3, TP4 and TP5, in the central Tasman Sea. Inversions were therefore performed on these data using delta-function and minimum-structure inversion algorithms. Delta function inversions were performed in order to investigate the one-dimensionality of the data, the depth of resolution of the and the significance of differences between the conductivity models at the different sites. Minimum-structure inversions were performed in order to produce more physically-realistic conductivity profiles, and to examine which features in the profile are definitely required by the data. In addition to these inversions, the B-pol impedance terms from the seafloor sites were used with the MT anisotropy method to determine the minimum integrated crustal resistance of the Tasman Seafloor, 107 [capital omega].m2. The E-pol impedance terms were used to estimate the depth to the good conductor beneath each site with the MT asymptotic method.

The conductivity models obtained for sites TP3 and TP4 appear to a reliable conductivity profile for the Tasman Sea. The Tasman Sea profile includes a high conducting layer occurring at a depth comparable to that for similar age lithosphere in the Pacific Ocean. The differences between the SFMT and VGS impedance estimates cause some ambiguity regarding the depth to the high conducting layer with the VGS and SFMT impedances estimates suggesting depths of 100 and 200 km respectively. Comparison of the conductivity models with other geophysical results suggests the correct depth lies between 120 and 150 km. At shallow depths the TP4 profile is more conductive than the TP3 profile. The higher conductance at TP4 cannot be explained by differences between the sediment layer at the two sites and is tentatively attributed to thermal effects associated with the source of an active seamount chain located near TP4.

The conductivity profile obtained for the third site, TP5, in the Central Tasman is probably only geophysically accurate at depths greater than 200 km. The deep structure at this site is more resistive than at sites TP4 and TP3 to the east, a result supported by the asymptotic-method results for sites near TP5. Confirmation of the conductivity results for the Tasman Sea will be provided by the application of three-dimensional modelling methods. In particular, thin-sheet modelling should indicate the accuracy of the assumptions made during the ID analyses described in this thesis.

In addition to information on the sub-oceanic conductivity structure, the Tasman Project has provided valuable information in other areas. Induction arrows at the land and seafloor sites are currently being analysed for information on the electrical conductivity structure of the Australian continental margin. Physical oceanographic information on processes including tides, eddies, and internal waves has been obtained, and information is also available on geomagnetic source-fields in the Tasman Sea region.

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