The Tasman Project of Seafloor Magnetotelluric Exploration: experiment and observations

Abstract

A major experiment in natural geomagnetic induction has been carried out by making seafloor magnetotelluric observations on a line of sites between Australia and New Zealand; simultaneous observations were made by land magnetometers on the Australian continent. The large volume of data recorded shows a great variety of phenomena of electromagnetic induction in the earth and sea. Parkinson arrows and magnetotelluric impedances have been computed for the seafloor sites. The Parkinson arrows show a strong coast effect near the Australian continent and a weaker coast effect over the submerged Lord Howe Rise. The occurrence of smoothly varying arrows across the Tasman Sea suggests a simple, large-scale pattern for electromagnetic induction in the Tasman Sea. The interpretation of Parkinson arrows at seafloor sites is shown to require special care, due to the attenuation and phase-shift effects caused by the ocean on seafloor horizontal magnetic field fluctuations. The magnetotelluric impedances exhibit a strong anisotropy at all sites in the Tasman Sea. The orientation of the impedance axes is constant across the Sea and the anisotropy may be attributed to the two-dimensional shape of the Tasman Sea. Calculations based on the distance scale of the magnetotelluric anisotropy, interpreted as an ocean-continent boundary effect, give a lower bound for the integrated resistivity of the Tasman Sea oceanic lithosphere of order 10 7 Ω m 2, corresponding to an estimate of 2 × 10 2 Ω m for its average resistivity.