The Australian continent: a numerical model of its electrical conductivity structure, and electromagnetic response

Abstract

Electromagnetic induction, commonly associated in exploration geophysics with applied source fields, takes place on continental and global scales. At these scales it is driven by natural magnetic fluctuations which arise external to the Earth. These magnetic fluctuations, and the secondary signals which they induce within the Earth, are measured at permanent magnetic observatories, "roving" temporary magnetic observatories, and at magnetotelluric field sites. Such observed data contain information on the electrical conductivity structure of the Earth, and are a major source of geophysical information. The global induction process is of such a magnitude that it could not be generated using applied source fields. An electrical conductivity model of the Australian continent with its surrounding oceans has been constructed numerically, for the purpose of examining its response to natural magnetic fluctuations. This paper first presents the continental response plotted as Parkinson arrows, for period 1 hour. Parkinson arrows summarize the behaviour of the three components of the Earth's fluctuating magnetic field, and for the model show particularly the coast effect. The coast effect arises most strongly at the continental edge, and penetrates far into the continent. There are also secondary effects within the continent, which arise due to electrical conductivity changes associated with geological boundaries. The present exercise models continental geological structures in a regional sense; their improved definition on a local scale, both in the field and by modelling, is an important and major exercise for the future. The response of the continental model has also been computed in terms of magnetotelluric impedance values, which take into account the electric fields occurring at the Earth's surface. Such impedance values are presented for six particular sites, using a Mohr circle method to display their characteristics. Generally the magnetotelluric data complement the pattern shown by the Parkinson arrows. They also give extra information on the regional distortion which may be expected for data observed on the Australian continent. The character of the Mohr circles indicates that the magnetotelluric "skew" caused regionally is small at the period of 1 hour, and that generally the computed impedances are two-dimensional (2D) in character. Such a predominantly 2D pattern for regional magnetotelluric impedances gives optimism for the interpretation of observed magnetotelluric data. Observed data for higher frequencies may be expected to be more affected by local distortion and more 3D in character; however knowledge that regional effects are 2D allows the use of decomposition techniques which are based upon local 3D effects perturbing a regional 2D pattern.