A Seafloor Magnetotelluric Sounding in the Tasman Sea

This paper reports an analysis of electromagnetic data from a site occupied on the floor of the Tasman Sea during an Australia-U.S. research program, the Tasman Project of Seafloor Magnetotelluric Exploration. This site (37ø33'S, 155ø58'E, depth 4450 m) is in the central portion of the Tasman Sea, and near both a fossil spreading ridge and the Tasmantid Seamount Chain. The extensive electric and magnetic data sets, covering 60 days, exhibit the signals of a most energetic oceanic environment. Conspicuous features of the electric field include an unusually large component induced by mesoscale fluid motions, as well as sporadically occurring high-frequency signal probably induced by ocean turbulence. These motional fields contribute a significant background noise to the ionospheric storm and substorm magnetic activity from which the seafloor magnetotelluric impedance tensor is estimated, but permit a usable frequency band of 0.06 to 5.5 cph. Over this band the seafloor impedance is found to exhibit frequency-independent skew and anisotropy. The skew suggests the presence of some nearby lateral conductivity heterogeneity. The anisotropy is characterised by a principal axes ratio of 3.5, the major axis being oriented 24 o clockwise from true north. Tentative inversion of the data implies the presence of an unusually high conductance in the uppermost seafloor. Altogether, information on upper mantle conductivity may reach a 500 km depth. The principal magnetotelluric axes are approximately aligned with both the trend of the seamount chain and with the Australian coastline. The ensemble of data from eight other sites occupied in the project should permit an assessment of the relative contribution of these two features.