Geomagnetic fluctuation anomalies across the southeast Australian coast

Abstract

Magnetic surveys are used in both mineral and hydrocarbon exploration to map the characteristic patterns of magnetisation of crustal rocks. However there is a part of the magnetic field of the earth which fluctuates with time and changes over seconds, minutes and hours. This fluctuating part of the magnetic field is due to the flow of electric currents both internal and external to the earth. The fluctuating part is anomalously strong at coastlines, a phenomenon known as the 'Geomagnetic Coast Effect' which extends for distances of order 100 km either side of the coastline. A significant portion of the coast effect can be explained by large scale induction of electric current in the seawater. The electric conductivity structure of the continental margin will also influence the flow of electric current. The south-east Australian coast is an ideal location for investigating this phenomenon due to its simple two-dimensional character and narrow continental shelf. Measurements of the fluctuating magnetic field were made using stationary magnetometers on land and seafloor during the Tasman project of Seafloor Magnetotelluric Exploration and the more recent Continental Slope Experiment. Within the accuracy of the station spacing, the coast effect has a maximum amplitude halfway across the continental slope.